JP6315669B2 - Method for preparing silver fine particles - Google Patents

Description

  The present invention relates to a method for preparing silver fine particles using silver oxide as a starting material.

  Metal fine particles are widely used in various fields such as a wiring material, a magnetic material, a sensor material, and a catalyst by utilizing the characteristics of the metal constituting the metal fine particles. In recent years, it has been found that the particle size of these metal fine particles has a great influence on the performance of final products using such fine particles. From this point of view, for the purpose of enhancing the functionality and miniaturization of the final product, fine particles having an extremely fine particle size, more specifically, metal nanoparticles having an average particle size of a nanometer scale are produced. Has reached.

  In metal fine particles (metal nanoparticles) whose average particle diameter is a nanometer scale, the metal element that appears on the surface of such fine particles is located on a spherical surface composed of fine stepped lattice steps, For example, it exhibits unique properties (nano-size effect) such that the mobility on the surface is significantly increased. In recent years, new applications of metal nanoparticles have been developed more and more with the goal of improving the performance of derived products and adding new functions by utilizing the properties that appear for the first time by using such metal nanoparticles. It is getting popular. Among these, conductive pastes using silver nanoparticles with high thermal conductivity and electrical conductivity are used for forming fine wiring layers, for example.

  Silver nanoparticles on the surface have a significantly high mobility on the surface, and when silver nanoparticles touch each other, the silver atoms on the surface move to each other and tend to cause fusion. Is strong. This fusion proceeds even near room temperature. Therefore, in order to prevent the silver nanoparticles contained in the conductive paste from fusing to each other and forming aggregated particles, the surface of the silver nanoparticles is coated with a coating molecule, and the silver nanoparticles are directly connected to each other. The metal surface is prevented from touching.

  In Patent Document 1, powdered silver oxide is dispersed in a nonpolar solvent, an excessive amount of formic acid is added, and formic acid is allowed to act on the powdered silver oxide to convert it into powdered silver formate (HCOAAg). Then, a primary amine is allowed to act on the powdered silver formate to form a primary amine addition salt of silver formate, and a decomposing reduction reaction of the primary amine addition salt of silver formate is performed at a liquid temperature of about 70 ° C., A method of preparing silver nanoparticles having a surface coating layer composed of a primary amine is described.

  Patent Document 2 discloses a metal oxide nanoparticle having at least rod-shaped metal oxide nanoparticles by heating a mixture containing an amine compound containing a secondary amine, a metal precursor, and a nonpolar solvent to 60 to 300 ° C. Forming an intermediate; adding capping molecules and a reducing agent to the mixture; heating to 90-150 ° C. to form metal nanoparticles; and recovering the metal nanoparticles. A method for producing metal nanoparticles comprising rod-shaped nanoparticles is described.

  Patent Document 3 discloses a metal fine particle dispersion in which a metal fine particle dispersion in which nanometer-scale metal fine particles are dispersed in a solution is obtained by heating and reducing a metal compound in a solution containing a primary amine and a tertiary amine. A method for producing a liquid, in which the tertiary amine contains an alkyl group having 2 or more carbon atoms is described.

  Patent Document 4 discloses a metal nanoparticle obtained by dispersing coated metal nanoparticles in which a protective agent made of an organic compound having a carboxyl group is coated on a metal nanoparticle surface in a polar dispersion solvent containing a polyhydric alcohol ether. A particle dispersion is described.

  Patent Document 5 discloses a conductive ink substantially free of a binder component for forming a conductive pattern by a letterpress reverse printing method, and having a volume average particle size (Mv) of 10 to 700 nm, A release agent, a surface energy regulator, and a solvent component as essential components, the solvent component having a surface energy at 25 ° C. of 27 mN / m or more, and a volatile solvent having a boiling point of 120 ° C. or less at atmospheric pressure A conductive ink characterized in that the surface energy of the ink at 25 ° C. is 10 to 21 mN / m.

JP 2011-153362 A JP 2009-084777 A JP 2008-081828 A JP 2011-038128 A WO2008 / 11484A1

  The conductive paste containing silver fine particles is printed in a desired pattern on the substrate by a printing method such as a reverse printing method or an inkjet method, but the desired silver fine particle size varies depending on the printing method.

  As described above, the desired particle diameter varies depending on the use of the silver fine particles. Therefore, a method for preparing silver fine particles capable of easily controlling the particle diameter of the silver fine particles in a wide range is desired. If silver fine particles having greatly different particle diameters can be prepared by the same process, it becomes possible to obtain silver fine particles having various particle diameters using the same or almost the same production apparatus. It becomes possible to obtain a conductive paste containing silver fine particles having a diameter.

  However, a method for preparing silver fine particles capable of easily controlling the particle diameter of silver fine particles in a wide range has not been known. For example, in the method described in Patent Document 1, only silver nanoparticles having an average particle diameter in the range of 5 to 20 nm can be produced, and silver fine particles having an average particle diameter of several hundred nm to several μm cannot be produced. Patent Documents 2 to 5 do not disclose a method for preparing silver fine particles capable of performing particle size control over a wide range.

  An object of the present invention is to provide a method for preparing silver fine particles in which the particle size of silver fine particles can be easily controlled in a wide range.

  Another object of the present invention is to provide a conductive paste for printing, particularly a conductive paste for reversal printing, containing silver fine particles obtained by such a preparation method.

According to each aspect of the present invention, the following method for preparing silver fine particles, a conductive paste for printing, and a conductive paste for reverse printing are provided. However, the conductive paste for printing according to 10) below and the conductive paste for reverse printing according to 15) to 18) are for reference.

1) A method for preparing silver fine particles, comprising preparing silver fine particles using powdered silver oxide (I) as a raw material,
a) converting powdered silver oxide (I) into silver formate (I) by allowing formic acid to act on powdered silver oxide (I) in a hydrocarbon solvent; and b) in the hydrocarbon solvent, A step of reducing silver cations contained in the silver formate (I) to silver atoms by a reduction reaction with an amine compound to form silver fine particles whose surface is coated with the amine compound;
A method for preparing silver fine particles, wherein both a primary amine and a secondary amine are used as the amine compound.

2) In the case of forming silver fine particles having a relatively small particle size in the step b, the proportion of primary amine in the amine compound is relatively increased to form silver fine particles having a relatively large particle size. The method for preparing silver fine particles according to 1), wherein the particle diameter of the silver fine particles is controlled by relatively reducing the proportion of the primary amine in the amine compound.

3) The hydrocarbon solvent is a straight chain alkane having 6 to 9 carbon atoms or a cycloalkane having 6 to 9 carbon atoms, and has a boiling point of 65 ° C. or more and 155 ° C. or less,
The method for preparing silver fine particles according to 1) or 2), wherein in step a, the hydrocarbon solvent is used in an amount of 350 parts by mass or more and 600 parts by mass or less per 100 parts by mass of the powdered silver oxide (I).

4) The primary amine has a molecular weight of 120 or more and 300 or less,
The method for preparing silver fine particles according to any one of 1) to 3), wherein the primary amine has an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent.

5) The molecular weight of the secondary amine is 100 or more and 190 or less,
The method for preparing silver fine particles according to any one of 1) to 4), wherein the secondary amine has an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent.

6) In step b, a total of 110 mol% to 150 mol% of primary amine and secondary amine are used based on the silver cation contained in the raw powdered silver oxide (I). ) To 5).

  7) In step b, when forming silver fine particles dispersed in a hydrocarbon solvent, a primary amine having a relatively high molecular weight is used, and when forming silver fine particles precipitated in a hydrocarbon solvent, The method for preparing silver fine particles according to any one of 1) to 6), wherein the dispersion state of the silver fine particles is controlled by using a primary amine having a small molecular weight.

8) forming fine silver particles dispersed in a hydrocarbon solvent in step b;
After step b,
c) a step of recovering a residue containing silver fine particles by removing the hydrocarbon solvent used in steps a and b by evaporating under reduced pressure from the reaction solution obtained from step b;
d) washing the residue obtained from step c with alcohol; and
e) Dispersing the alcohol-washed residue obtained from step d in a hydrocarbon solvent that is the same as or different from the hydrocarbon solvent used in steps a and b to obtain a dispersion containing silver fine particles. The method for preparing silver fine particles according to any one of 1) to 7).

9) forming fine silver particles precipitated in a hydrocarbon solvent in step b;
After step b
f) removing the hydrocarbon solvent used in steps a and b from the reaction solution obtained from step b, and collecting a residue containing silver fine particles;
g) washing the residue obtained from step f with alcohol, and
h) The method for preparing silver fine particles according to any one of 1) to 7), including a step of collecting the residue obtained from step g to obtain silver fine particles.

10) A conductive paste for printing, comprising silver fine particles obtained by the method described in 8) or 9) above and a solvent.

11) forming fine silver particles dispersed in a hydrocarbon solvent in step b;
After step b,
c) a step of recovering a residue containing silver fine particles by removing the hydrocarbon solvent used in steps a and b by evaporating under reduced pressure from the reaction solution obtained from step b;
d) washing the residue obtained from step c with alcohol; and
i) In alcohol, the amine compound covering the surface of the silver fine particles contained in the residue obtained from step d is replaced with a protective agent having affinity for a polar solvent, and the surface is covered by the protective agent. Forming a coated silver fine particle;
j) Step of performing alcohol removal and washing with a hydrocarbon solvent on the mixture containing silver fine particles and alcohol obtained from step i.
k) The process according to any one of 1) to 7), comprising the step of dispersing the residue washed with the hydrocarbon solvent obtained from step j in a polar solvent to obtain a dispersion containing silver fine particles. A method for preparing the silver fine particles as described.

12) forming fine silver particles precipitated in a hydrocarbon solvent in step b;
After step b,
f) removing the hydrocarbon solvent used in steps a and b from the reaction solution obtained from step b, and collecting a residue containing silver fine particles;
g) washing the residue obtained from step f with alcohol, and
l) In an alcohol, the amine compound covering the surface of the silver fine particles contained in the residue obtained from step g is replaced with a protective agent having affinity for a polar solvent, and the surface is covered by the protective agent. Forming a coated silver fine particle;
m) removing alcohol from the mixture containing silver fine particles and alcohol obtained from step l, washing the resulting residue with a hydrocarbon solvent, and n) collecting the residue obtained from step m, The method for preparing silver fine particles according to any one of 1) to 7), comprising a step of obtaining silver fine particles.

13) The method for preparing silver fine particles according to 11), wherein the protective agent is any one of organic acids and polyamines having two or more amino groups, or a combination thereof.

14) The method for preparing silver fine particles according to 12), wherein the protective agent is any one of organic acids and polyamines having two or more amino groups, or a combination thereof.

15) A conductive paste for printing, comprising silver fine particles obtained by the method according to any one of 11) to 14) and a solvent.

16) A silver fine particle which does not contain a binder resin component and is obtained by the method described in 13) and whose surface is coated with a protective agent having affinity for a polar solvent, and a solvent,
The solvent comprises a low boiling point solvent having a boiling point of 50 ° C. or more and 120 ° C. or less, and a high boiling point solvent having a boiling point of 150 ° C. or more and 300 ° C. or less,
Inversion characterized by containing a fluorine-based solvent having a surface tension (25 ° C.) of 12 mN / m or more and 18 mN / m or less as the low-boiling solvent in an amount of 10 to 50% by mass with respect to the total amount of the solvent Conductive paste for printing.

  17) The conductive paste for reversal printing according to 16), wherein the fluorine-based solvent is a hydrofluoroether.

  18) The conductive paste for reversal printing according to 16) or 17), wherein the protective agent covering the silver fine particles is an organic acid.

  19) The method for preparing silver fine particles according to 8), 9), 11) or 12), wherein all the alcohols are methanol.

  ADVANTAGE OF THE INVENTION According to this invention, the preparation method of the silver fine particle which can control the particle diameter of a silver fine particle easily in a wide range is provided.

  Moreover, according to this invention, the electrically conductive paste for printing containing the silver fine particles obtained by such a preparation method, especially the electrically conductive paste for reverse printing are provided.

  The inventors of the present invention made silver formate (I) by reacting powdered silver oxide (I) and formic acid in a hydrocarbon solvent, and then converting the silver cation contained in the silver formate (I) with an amine compound. When producing silver fine particles whose surface is coated with an amine compound after being reduced to silver atoms, a primary amine and a secondary amine are used in combination as an amine compound, thereby reducing the reaction rate of the reduction reaction and the fine particle generation. It was found that the progress of particle aggregation can be controlled. And it discovered that the particle diameter of the silver fine particles obtained was controllable by changing the compounding ratio of a primary amine and a secondary amine. The present invention has been made based on such knowledge.

  The present invention relates to a method for preparing silver fine particles using powdered silver (I) oxide as a starting material.

  According to the present invention, silver fine particles having an average particle diameter of nanometer order can be produced, and silver fine particles having an average particle diameter of micrometer order can be produced. For example, silver fine particles having an average particle diameter in the range of 5 nm to 10 μm can be produced. In addition, an average particle diameter points out the particle diameter of 50% of integrated values in the particle size distribution measured by the laser diffraction method.

In the present invention, steps a and b are performed. And as a amine compound used at the process b, both a primary amine and a secondary amine are used.
a) A step of converting powdered silver oxide (I) into silver formate (I) by allowing formic acid to act on the powdered silver oxide (I) in a hydrocarbon solvent.
b) A step of reducing silver cations contained in the silver formate (I) to silver atoms in the hydrocarbon solvent by a reduction reaction with an amine compound to form silver fine particles whose surface is coated with the amine compound. .

  Step b can be performed in the hydrocarbon solvent used in step a following step a. Therefore, step b can be carried out by adding an amine compound (primary amine and secondary amine) to the reaction solution obtained from step a and stirring as necessary. A mixture obtained by previously mixing the primary amine and the secondary amine may be added to the reaction solution obtained from the step a, or the primary amine and the secondary amine may be added separately. When these are added separately, they may be added simultaneously or sequentially. Preferably, a mixture of primary and secondary amines is added.

  In step b, the same hydrocarbon solvent used in step a may be added. Thereby, an amine compound can be diluted with a hydrocarbon solvent and the density | concentration of an amine compound can be adjusted. For example, the concentration of the amine compound can be adjusted (diluted) by adding a hydrocarbon solvent of 50 to 150 parts by mass per 100 parts by mass of powdered silver oxide (I).

  By performing steps a and b, silver fine particles can be prepared in a hydrocarbon solvent. Depending on the dispersion state of the silver fine particles, the silver fine particles can be obtained in the form of a dispersion in which the silver fine particles are dispersed in the hydrocarbon solvent after the steps a and b, or the silver fine particles are precipitated in the hydrocarbon solvent. In some cases, silver fine particles can be obtained in a form where silver fine particles in a dispersed form and silver fine particles in a precipitated form coexist in a hydrocarbon solvent.

  After steps a and b, if necessary, silver fine particles can be separated from the hydrocarbon solvent. An appropriate method can be used for the separation. For example, the hydrocarbon solvent can be vaporized and removed under reduced pressure to obtain silver fine particles (a state not included in the hydrocarbon solvent).

  There is a technique for controlling copper fine particles into a rod shape by mixing primary amines and secondary amines in a predetermined ratio (see Patent Document 2). However, in this technique, it is necessary to separately add a reducing agent for reducing copper cations to copper fine particles and a capping molecule for improving the dispersibility of the copper fine particles.

  On the other hand, the present invention can reduce silver cations to silver fine particles only by using a primary amine and a secondary amine, and form silver fine particles whose surfaces are coated. That is, in the present invention, it is not necessary to add a reducing agent or a capping molecule separately. Therefore, the present invention is a technique that can control the particle size of silver fine particles by a simple process, and is also a useful technique from the viewpoint of green chemistry.

[Hydrocarbon solvent used in steps a and b]
Hydrocarbon solvents are generally nonpolar solvents that have no polarity or very low polarity.

  The hydrocarbon solvent is used as a dispersion medium for powdered silver oxide (I), and is also used as a solvent for dissolving formic acid and amine compounds (primary amine and secondary amine).

  In addition, silver fine particles may be separated and recovered from the reaction liquid (liquid containing silver fine particles obtained in step b). At this time, by removing the hydrocarbon solvent contained in the reaction liquid under reduced pressure, Can be removed. Therefore, a hydrocarbon solvent showing transpiration that can be distilled off under reduced pressure is preferred.

  Furthermore, when a hydrocarbon solvent having a boiling point similar to or lower than the boiling point of formic acid (100.75 ° C.) is used, when the liquid temperature of the reaction liquid rises due to heat generation in the silver (I) formate production reaction, Since the temperature does not exceed the boiling point of the hydrocarbon solvent, transpiration of formic acid can be suppressed.

  Considering the above points, the hydrocarbon solvent is preferably a straight chain alkane having 6 to 9 carbon atoms or a cycloalkane having 6 to 9 carbon atoms. The boiling point of the hydrocarbon solvent is preferably 65 ° C to 155 ° C, more preferably 80 ° C to 130 ° C, and further preferably 80 ° C to 101 ° C.

  Examples of the hydrocarbon solvent include methylcyclohexane (boiling point 101 ° C.) and heptane (boiling point: 98.42 ° C.).

  In addition, from the viewpoint of preventing the reaction liquid temperature from rising due to heat generation of the silver formate (I) formation reaction, a hydrocarbon solvent is used in the step a at 350 parts by mass or more and 600 parts by mass or less per 100 parts by mass of the powdered silver oxide (I). It is preferable.

[Powdered silver oxide (I)]
As the starting material, powdered silver oxide (I) (Ag ₂ O; formula weight: 231.74, density: 7.22 g / cm ³ ) is used. The particle diameter of the powdered silver oxide (I) can be appropriately selected. From the viewpoint of uniform dispersion in a hydrocarbon solvent, the particle diameter distribution of the powdered silver oxide (I) is in the range of 200 mesh or less (75 μm or less). Those that fall within the range are preferably used.

[Formation of silver formate (I)]
In step a, formic acid (HCOOH) is allowed to act on powdered silver oxide (I) in a hydrocarbon solvent to convert the powdered silver oxide (I) into silver formate (I). For this purpose, powdered silver (I) oxide can be added and dispersed in a hydrocarbon solvent, and formic acid can be added to the dispersion.

  In the reaction, stirring can be appropriately performed.

  Formic acid (HCOOH) associates by hydrogen bonding to form a dimer (HCOOH: HOOCH). Most hydrocarbon solvents dissolve in a dimer-formed state. Therefore, the dimer of formic acid (HCOOH: HOOCH) acts on the powdered silver oxide (I) in the dispersion to produce silver formate (I) (HCOOAg) by the reaction represented by the following formula i. Is done.

Formula i:
Ag ^I ₂ O + (HCOOH: HOOCH)
→ 2 [(HCOO ⁻ ) (Ag ^I ) ⁺ ] + H ₂ O
It should be noted that most of the water molecules (H ₂ O) by-produced in the reaction of Formula i are incorporated in the form of “crystal water” into the aggregate of [(HCOO ⁻ ) (Ag ^I ) ⁺ ] to be generated. It is estimated that

  The reaction represented by the above formula i corresponds to a “neutralization reaction” between silver (I), which is a basic metal oxide, and a dimer of formic acid, and is an exothermic reaction. By selecting the ratio of the amount of the hydrocarbon solvent to the powdered silver oxide (I) within the aforementioned range (350 parts by mass or more and 600 parts by mass or less per 100 parts by mass of the powdered silver oxide (I)), the temperature of the reaction solution It is easy to suppress the rise to around 40 ° C. That is, it is easy to suppress the side reaction by suppressing the liquid temperature from rising. Examples of the side reaction include a reduction reaction that can be expressed by the following formula A1, and a decomposing reduction reaction of the resulting silver (I) formate (HCOOAg) that can be expressed by the following formula (A2).

Formula A1:
2 [(HCOO ⁻ ) (Ag ^I ) ⁺ ] + HCOOH
→ 2Ag + 2HCOOH + CO ₂ ↑,
Formula A2:
2 [(HCOO ⁻ ) (Ag ^I ) ⁺ ]
→ 2Ag + HCOOH + CO ₂ ↑.

  In order to carry out the reaction of the above formula i, formic acid is preferably 1.02 mol to 1.4 mol, more preferably 1.05 mol per mol of silver cation contained in the raw powdered silver oxide (I). It is preferable to use in the range of -1.2 mol. By adding an excessive amount of formic acid, the total amount of the raw material powdered silver oxide (I) can be converted into silver formate (I).

  When an excessive amount of formic acid is added, unreacted formic acid remains and is dissolved as a dimer of formic acid in the hydrocarbon solvent.

[Reduction reaction with amine compound]
After step a, the silver cation contained in the silver (I) formate is subsequently reduced to silver atoms in the hydrocarbon solvent by a reduction reaction with an amine compound. Thereby, silver fine particles having a surface coated with an amine compound are formed.

Specifically, preferably, when the temperature of the reaction solution obtained from step a drops to 30 ° C., an amine compound, that is, a primary amine (NH ₂ —R ^a ) and ^a secondary amine is added to the solution obtained from step a. (NH—R ^b R ^c ) can be added.

Thus, an amine complex of silver (I) formate, that is, a primary amine complex (HCOOAg: NH ₂ —R ^a ) and a secondary amine complex (HCOOAg: NH—R ^b R ^c ) are generated.

In the formation of the amine complex, water molecules incorporated in the form of “crystal water” in the aggregate of silver formate (I), that is, in the aggregate of [(HCOO ⁻ ) (Ag ^I ) ⁺ ], formic acid anion species ^- the (O-CHO) portion becomes a state of "solvate", to be estimated. Therefore, it is presumed that the amine complex of silver (I) formate to be finally produced is dissolved in the hydrocarbon solvent in a state where the above water molecule (H ₂ O) is “solvated”.

When unreacted formic acid remains in the reaction solution obtained from step a, when the amine compound is added to the reaction solution obtained from step a, the amine compound reacts with the remaining formic acid to form an amine of formic acid. Addition salts (HCOOH: NH ₂ —R ^a , HCOOH: NH—R ^b R ^c ) are produced. The amine addition salt formation reaction of formic acid corresponds to an acid / base “neutralization reaction” and is an exothermic reaction. Therefore, as the reaction proceeds, the temperature of the reaction solution increases. Note that, when the temperature of the reaction solution approaches the boiling point of the hydrocarbon solvent being used, the evaporation of the hydrocarbon solvent starts, so the temperature of the reaction solution does not exceed the boiling point of the hydrocarbon solvent.

  When the liquid temperature rises to, for example, about 55 ° C. to 65 ° C., the silver formate amine complex (primary amine complex and secondary amine complex) starts a degradative reduction reaction represented by the following formula ii or iii. To do. Since these degradative reduction reactions are endothermic reactions, they hardly proceed until the temperature of the reaction solution reaches a certain temperature or higher.

Formula ii:
2 (R ^a —NH ₂ : Ag—OOCH)
→ 2 [R ^a -NH ₂ : Ag] + HCOOH + CO ₂ ↑
Formula iii:
^{2 (R b R c -NH:} Ag-OOCH)
→ 2 [R ^b R ^c —NH: Ag] + HCOOH + CO ₂ ↑
Since carbon dioxide (CO ₂ ) derived from this reaction forms bubbles, foaming is observed in the reaction solution.

  The by-product formic acid (HCOOH) once forms a dimer of formic acid (HCOOH: HOOCH), but reacts with the amine compound dissolved in the reaction solution to form an amine addition salt of formic acid. May be generated.

On the other hand, the metal silver atoms [Ag: NH ₂ —R ^a ] and [Ag: NH—R ^b R ^c ] generated by the degradative reduction reaction aggregate to form an aggregate of metal silver atoms. At that time, ^{a part of the} primary amine (R ^a —NH ₂ ) and secondary amine (R ^b R ^c —NH) coordinated to the metal silver atom accompanying the formation of an aggregate of metal silver atoms. Dissociates thermally. Therefore, the formed aggregate of metallic silver atoms includes a spherical nucleus composed of metal atoms, an amine compound (primary amine (R ^a -NH ₂ ) and secondary amine (R ^b R) covering the surface of the nucleus. ^c- NH)).

Thermally dissociated amines (R ^a —NH ₂ and R ^b R ^c —NH) produce amine complexes of silver formate (HCOOAg: NH ₂ —R ^a and HCOOAg: NH—R ^b R ^c ). It may be involved in the reaction as well as the reaction of formic acid to form an amine addition salt.

  By adjusting the amount of unreacted formic acid remaining when step a is completed, the amount of amine compound to be added, and the total amount of the reaction solution, the temperature of the reaction solution in step b is 70 ° C. or higher. Can be prevented from rising.

  In step b, if an excessive amount of an amine compound is present in the reaction solution, the amine addition salt formation reaction of formic acid proceeds, so that the concentration of dissolved formic acid is maintained at a low level. Accordingly, it is possible to prevent the formic acid having the function of a reducing agent from acting and the reduction reaction (side reaction) represented by the above formula A1 from proceeding.

  Further, if the liquid temperature of the reaction solution is prevented from rising to 70 ° C. or higher in step b, it is easy to preferentially advance the formation reaction of the silver formate amine complex, which is a side reaction. It is easy to suppress the progress of decomposing reaction of silver formate (I) itself.

[Primary amine]
From the viewpoint of easy preparation of silver fine particles, the molecular weight of the primary amine is preferably 120 or more and 300 or less. When silver fine particles are prepared using a primary amine having a molecular weight of 300 or less, when the particles are used as a conductive paste, the elimination and decomposition of the primary amine from silver during the firing process is easy. From the viewpoint of conduction characteristics, it is preferable. On the other hand, when silver fine particles are prepared using a primary amine having a molecular weight of 120 or more, stable silver fine particles can be easily prepared. Specifically, it is easy to avoid the phenomenon that the side surface of the reaction vessel is mirror-finished (silver deposition) during particle production.

Further, from the viewpoint of dispersibility of the silver fine particles, the primary amine preferably has an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent. That is, a primary amine (R a ^-NH ₂₎ constituting the atomic group (other than hydrogen, atom group bonded to the nitrogen atom) R ^a is an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent It is preferable to contain. When ^Ra contains an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent, the prepared silver fine particles can be easily dispersed stably in the hydrocarbon solvent. Specifically, it is preferable to use ^a primary amine (R ^a —NH ₂ ) having 6 to 20 carbon atoms in the molecule. Specific examples include 2-ethylhexylamine, 3- (2-ethylhexyloxy) propylamine, dodecylamine, 3- (lauryloxy) propylamine, oleylamine, stearylamine and the like.

[Secondary amine]
From the viewpoint of reactivity during the production of silver fine particles, the molecular weight of the secondary amine is preferably 100 or more and 190 or less. A secondary amine having a molecular weight of 190 or less is preferable in terms of reducing power, and it is easy to proceed the reaction. An amine having a molecular weight of 100 or more is preferable from the viewpoint of boiling point, and it is easy to prevent rapid evaporation during the reaction.

Further, from the viewpoint of dispersibility of the silver fine particles, the secondary amine preferably has an aliphatic hydrocarbon chain having an affinity for the hydrocarbon solvent. That is, atomic group forming a secondary amine (R b ^{R c -NH)} (other than a hydrogen, atom group bonded to the nitrogen atom) one or both of R ^b and R ^c, relative to the hydrocarbon solvent It is preferable to include an aliphatic hydrocarbon chain having high affinity. When one or both of R ^b and R ^c contain an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent, the prepared silver fine particles may be stably dispersed in the hydrocarbon solvent. Easy. Specifically, it is preferable to use a secondary amine (R ^b R ^c —NH) having 6 to 12 carbon atoms in the molecule.

  Examples of such secondary amines include dipropylamine, diisopropylamine, methylhexylamine, dibutylamine, diisobutylamine, and dicyclohexylamine.

[Particle size control of silver fine particles]
In step b, for example, even if the molar amount of the amine compound (the total molar amount of the primary amine and the secondary amine) is the same as the molar amount of the silver cation contained in the raw material powdered silver oxide (I), By changing the molar ratio of the primary amine and the secondary amine, the average particle diameter of the silver fine particles obtained in the step b can be changed. Specifically, it is obtained in step b by reducing the molar amount of primary amine, that is, by reducing the ratio of primary amine molar amount to secondary amine molar amount (primary amine molar amount / secondary amine molar amount). The average particle diameter of the silver fine particles can be increased. In addition, although the further process can be performed after the process b as described later, the average particle diameter of the silver fine particles obtained after the further process is substantially equal to the average particle diameter of the silver fine particles obtained in the process b. Is the same.

  That is, in the case of forming silver fine particles having a relatively small particle size in step b, the proportion of primary amine in the amine compound is relatively large, and silver fine particles having a relatively large particle size are formed. Further, the particle diameter of the silver fine particles can be controlled by relatively reducing the proportion of the primary amine in the amine compound. In step b, when silver fine particles having a certain average particle diameter D1 are prepared under the condition of a ratio of a certain primary amine (ratio of primary amine in the amine compound) r1, a primary amine larger than r1. When the silver fine particles are prepared under the condition of the ratio r2 (other conditions may be the same), the average particle diameter D2 of the obtained silver fine particles becomes smaller than D1. In other words, by increasing the proportion of primary amine in the amine compound, the particle diameter (average particle diameter) of the silver fine particles formed in step b can be reduced.

  In this way, silver fine particles having different average particle diameters can be prepared by the same process only by changing the proportion of primary amine in the amine compound.

[Control of dispersion state of silver fine particles]
Depending on the average particle diameter of the silver fine particles, the dispersion state of the silver fine particles in the liquid (for example, the reaction liquid obtained from step b) may be different. That is, the smaller the average particle size of the silver fine particles, the better the dispersion state in the liquid. When the average particle size is large, the silver fine particles tend to precipitate in the liquid. Furthermore, by controlling the molecular weight of the primary amine used in step b (and thus changing the type of primary amine used), the dispersion state of the silver fine particles in the reaction solution obtained from step b is controlled. Can do. Specifically, the higher the molecular weight of the primary amine used in step b, the better the dispersion state of the silver fine particles obtained in step b in the hydrocarbon solvent. That is, by using a primary amine having a relatively high molecular weight in step b, silver fine particles can be prepared in the form of a dispersion in which silver fine particles are well dispersed in the liquid, or the molecular weight is relatively small. By using a primary amine, silver fine particles can be prepared in a form in which silver fine particles are precipitated in the liquid. If a primary amine having an intermediate molecular weight is used in step b, the silver fine particles can be prepared in such a form that a part of the silver fine particles is dispersed in the liquid and the remainder is precipitated.

  That is, in the step b, when forming silver fine particles dispersed in a hydrocarbon solvent, a primary amine having a relatively high molecular weight is used, and when forming silver fine particles precipitated in a hydrocarbon solvent, it is relatively high. By using a primary amine having a small molecular weight, the dispersion state of silver fine particles can be controlled. In Step b, when a primary amine having a certain molecular weight (M1) can be used to form precipitated fine silver particles, a primary amine having a molecular weight (M2) larger than M1 is used (other conditions are the same). Good) When silver fine particles are formed, the dispersion of the silver fine particles in a hydrocarbon solvent can be improved.

[Amount of amine compound used in step b]
The amount of the amine compound used in step b (total amount of primary amine and secondary amine) is 110 mol% or more based on the silver cation contained in the raw powdered silver oxide (I) (100 mol%). It is preferable to be 150 mol% or less. That is, the total amount of primary amine and secondary amine is 1.1 mol or more and 1.5 mol or less per mol of silver cation contained in the raw material powdered silver oxide (I). It is preferable to select such a quantitative balance from the viewpoint of forming silver fine particles whose surface is coated with an amine compound.

[Mode 1: Form in which silver fine particles dispersed in a hydrocarbon solvent are formed in step b]
Here, one mode (referred to as Form 1) of the method of the present invention for forming silver fine particles dispersed (not precipitated) in a hydrocarbon solvent in Step b will be described. This form is used when more than 50% by mass, preferably 75% by mass or more, more preferably 90% by mass or more of silver is desired to be dispersed as fine particles in a hydrocarbon solvent based on the silver used as a raw material. Is preferred.

  In order to form dispersed silver fine particles, it is effective to reduce the particle diameter (average particle diameter) of the silver fine particles formed in step b and to increase the molecular weight of the primary amine used in step b.

Although depending on the molecular weight of the primary amine, the average particle diameter is relatively small in the step b by satisfying both of the following two conditions (for example, the average particle diameter is 5 nm to 40 nm, particularly the average particle diameter is Silver fine particles can be formed.
The primary amine is based on the silver cation contained in the raw powdered silver oxide (I) (that is, when the silver cation is 100 mol%), more than 3.0 mol% and 50 mol% or less Use by volume. That is, the primary amine is used in an amount of more than 0.030 mol and not more than 0.50 mol per mol of silver cation contained in the raw powdered silver oxide (I).
Secondary amine is used in such an amount that the total amount of primary amine and secondary amine is 110 mol% or more and 150 mol% or less based on the silver cation contained in the powdered silver oxide (I) as a raw material. That is, the total amount of primary amine and secondary amine is 1.1 mol or more and 1.5 mol or less per mol of silver cation contained in the raw material powdered silver oxide (I).

In the first embodiment, after the step b, the following steps can be performed in this order. Thereby, a dispersion liquid in which silver fine particles are dispersed in a certain hydrocarbon solvent (hydrocarbon solvent used in step e) can be obtained.
c) The hydrocarbon solvent used in steps a and b is removed by evaporation under reduced pressure from the reaction solution obtained from step b (for example, an average particle size of 5 nm to 40 nm, particularly an average particle size of 5 nm to 30 nm). The following step) d) collecting the residue containing silver fine particles d) washing the residue obtained from step c with alcohol, and
e) The alcohol-washed residue obtained from step d is dispersed in a hydrocarbon solvent (which may be the same as or different from the hydrocarbon solvent used in steps a and b) (for example, the average particle size is A step of obtaining a dispersion liquid containing silver fine particles (5 nm to 40 nm, in particular, having an average particle diameter of 5 nm to 30 nm).

  In step c, the hydrocarbon solvent used in steps a and b is removed by evaporation under reduced pressure. This operation can be appropriately performed by a known method such as a method using an evaporator. For example, the pressure in the container is lowered to about 50 hPa or less by an evaporator at a liquid temperature of 40 ° C., and the hydrocarbon solvent is removed. Depending on the amine species used in step b, for example, an amine compound having a boiling point of 155 ° C. or lower may be removed by vaporization under reduced pressure together with a hydrocarbon solvent, but in that case, problems occur in subsequent steps. There is no.

  In addition, examples of the alcohol in the form 1 (alcohol used in step d) include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 2-methyl-1-propanol. Among these, methanol can be suitably used from the viewpoint of particularly high polarity and ease of removal in a subsequent process. In the following description of step d, methanol is described as an example of alcohol.

  In step d, a known method of washing fine particles with methanol can be employed. For example, methanol can be substantially removed from the residue by adding methanol to the residue obtained from step c and stirring, and then removing the methanol by decantation. At this time, the silver fine particles are washed by dissolving a considerable amount of components such as excess formic acid, amine compound, or salt containing them in methanol. In order to perform washing efficiently, use 500 parts by weight or more and 1000 parts by weight or less of methanol per 100 parts by weight of powdered silver oxide (I) used as a raw material, and wash by dividing it into 2 to 4 times. Is preferred.

  The hydrocarbon solvent used in step e may be the same type as or different from the hydrocarbon solvent used in steps a and b. As the hydrocarbon solvent, the hydrocarbon solvents described above in [Hydrocarbon solvent used in steps a and b] can be used. It is preferable to disperse silver fine particles in 100 to 300 parts by mass of a hydrocarbon solvent per 100 parts by mass of powdered silver oxide (I) used as a raw material. Since the silver fine particles obtained in Form 1 are silver fine particles coated with an amine compound having a hydrocarbon chain, they are uniformly dispersed in the hydrocarbon solvent by using a hydrocarbon solvent in step e. Silver fine particles can be obtained.

  Depending on the use of the silver fine particles, the dispersion obtained from step e can be used as it is as a raw material for ink or the like. In particular, a conductive paste can be prepared according to the following procedure.

  A hydrocarbon solvent having a boiling point in the range of 65 ° C. to 155 ° C. is preferably used as a dispersion solvent for the silver fine particle dispersion (dispersion obtained in step e) in Form 1. The hydrocarbon solvent of the dispersion obtained in step e is preferably a high-boiling hydrocarbon solvent having a boiling point in the range of 180 ° C. to 350 ° C., more preferably in the range of a boiling point of 200 ° C. to 310 ° C., further preferably a boiling point of 220 ° C. to 310 ° C. A conductive paste can be prepared by substituting with a high boiling point hydrocarbon solvent in the range of ° C.

  As an example of a high-boiling hydrocarbon solvent that can be used, JX Nippon Mining & Co., Ltd. is an alkane having a carbon number in the range of 12 to 16 such as tetradecane (boiling point: 253.6 ° C.) or a mixed solvent of naphthene / paraffin hydrocarbon. A stone energy AF solvent (trade name) and the like can be mentioned. A mixture of a plurality of high boiling hydrocarbon solvents can also be used.

  Conductive pastes can be used for printing inks. Examples of the printing method include an inkjet printing method and a screen printing method. Other known components (components other than silver fine particles and a solvent) used for the conductive paste for printing can also be added.

[Mode 2: Form in which silver fine particles precipitated in a hydrocarbon solvent are formed in step b]
Here, one form (referred to as Form 2) of the method of the present invention for forming silver fine particles precipitated in a hydrocarbon solvent in Step b will be described. This form is used when more than 50% by mass, preferably 75% by mass or more, more preferably 90% by mass or more of silver is desired to be precipitated as fine silver particles in a hydrocarbon solvent based on the silver used as a raw material. Is preferred.

  In order to form the precipitated silver fine particles, it is effective to increase the particle diameter (average particle diameter) of the silver fine particles formed in step b and to reduce the molecular weight of the primary amine used in step b.

Although depending on the molecular weight of the primary amine, the average particle size is relatively large in the step b by satisfying both of the following two conditions (for example, the average particle size is 50 nm or more and 10 μm or less, in particular, the average particle size is 50 nm or more and 8 μm or less) silver fine particles can be formed.
A primary amine is used in an amount of 0.2 mol% or more and 3.0 mol% or less based on the silver cation contained in the raw powdered silver oxide (I). That is, the primary amine is used in an amount of 0.002 mol or more and 0.030 mol or less per mol of silver cation contained in the powdery silver oxide (I) as a raw material.
Secondary amine is used in such an amount that the total amount of primary amine and secondary amine is 110 mol% or more and 150 mol% or less based on the silver cation contained in the powdered silver oxide (I) as a raw material. That is, the total amount of primary amine and secondary amine is 1.1 mol or more and 1.5 mol or less per mol of silver cation contained in the raw material powdered silver oxide (I).

In the form 2, after the step b, the following steps can be performed in this order. Thereby, silver fine particles can be obtained in a form not included in the liquid.
f) The hydrocarbon solvent used in steps a and b is removed from the reaction solution obtained in step b, and silver fine particles (for example, having an average particle size of 50 nm to 10 μm, particularly an average particle size of 50 nm to 8 μm) are obtained. A step of recovering a residue containing,
g) washing the residue obtained from step f with alcohol, and
h) A step of collecting the residue obtained from step g to obtain silver fine particles (for example, having an average particle size of 50 nm to 10 μm, particularly an average particle size of 50 nm to 8 μm).

  In step f, the hydrocarbon solvent used in steps a and b is removed from the reaction solution obtained in step b, and a residue (for example, silver having an average particle size of 50 nm to 10 μm, particularly an average particle size of 50 nm to 8 μm). (Including fine particles). In Form 2, separation of the hydrocarbon solvent is relatively easy, and step f can be performed by an appropriate separation method such as decantation.

  In addition, the alcohol in Form 2 (alcohol used in the step g) includes, for example, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 2-methyl-1-propanol. Among these, methanol can be suitably used from the viewpoint of particularly high polarity and ease of removal in a subsequent process. In the following description of step g, methanol is described as an example of alcohol.

  In step g, a known method of washing fine particles with methanol can be employed. For example, methanol can be substantially removed from the residue by adding methanol to the residue obtained from step f and stirring, and then removing the methanol by decantation. At this time, the silver fine particles are washed by dissolving a considerable amount of components such as excess formic acid, amine compound, or salt containing them in methanol. In order to perform washing efficiently, use 500 parts by weight or more and 1000 parts by weight or less of methanol per 100 parts by weight of powdered silver oxide (I) used as a raw material, and wash by dividing it into 2 to 4 times. Is preferred.

  By collecting the residues washed with methanol in this manner, silver fine particles (for example, having an average particle size of 50 nm to 10 μm, particularly an average particle size of 50 nm to 8 μm) can be obtained without being included in the liquid.

  The silver fine particles (silver fine particles obtained in step h) in Form 2 are preferably a high-boiling hydrocarbon solvent having a boiling point in the range of 180 ° C. to 350 ° C., more preferably in the range of 200 ° C. to 310 ° C., and still more preferably 220 in boiling point. A conductive paste can be prepared by adding a high-boiling hydrocarbon solvent in the range of from ℃ to 310 ℃.

  As an example of a high-boiling hydrocarbon solvent that can be used, JX Nippon Mining & Co., Ltd. is an alkane having a carbon number in the range of 12 to 16 such as tetradecane (boiling point: 235.6 ° C.) or a mixed solvent of naphthene / paraffin hydrocarbon. A stone energy AF solvent (trade name) and the like can be mentioned. A mixture of a plurality of high boiling hydrocarbon solvents can also be used.

  Conductive pastes can be used for printing inks. Examples of the printing method include a screen printing method. Other known components (components other than silver fine particles and a solvent) used for the conductive paste for printing can also be added.

[Polarly dispersed silver fine particles]
Performing steps a and b, performing steps a, b, c, d, and e, or performing steps a, b, f, g, and h, thereby making affinity with non-polar solvents including hydrocarbon solvents Silver fine particles, that is, silver fine particles suitable for use in nonpolar solvents such as hydrocarbon solvents (referred to as nonpolar dispersed silver fine particles) can be obtained.

  In order to obtain silver fine particles having affinity with the polar solvent, that is, silver fine particles suitable for use in the polar solvent (referred to as polar dispersed silver fine particles), a compound having affinity with the polar solvent is used after steps a and b. Silver fine particles can be chemically modified.

Specifically, in the case of Form 1, that is, when silver fine particles dispersed in a hydrocarbon solvent are formed in step b, polar dispersed silver fine particles are obtained by performing the following steps in this order after step b. Can be prepared.
c) The hydrocarbon solvent used in steps a and b is removed by evaporation under reduced pressure from the reaction solution obtained from step b (for example, an average particle size of 5 nm to 40 nm, particularly an average particle size of 5 nm to 30 nm). (The following) recovering the residue containing silver fine particles,
d) washing the residue obtained from step c with alcohol;
i) In alcohol, the amine compound covering the surface of the silver fine particles contained in the residue obtained from step d is replaced with a protective agent having affinity for a polar solvent, and the surface is covered by the protective agent. Step j) Forming coated silver fine particles Step of performing alcohol removal and washing with a hydrocarbon solvent on the mixture containing silver fine particles and alcohol obtained from step i,
k) A step of dispersing the residue washed with the hydrocarbon solvent obtained in step j in a polar solvent to obtain a dispersion containing silver fine particles.

  The alcohol in the preparation of the polar dispersion type silver fine particles (steps d, i, j) is, for example, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol. Is mentioned. Of these, methanol is preferably used from the viewpoint of high polarity and ease of removal in a subsequent process. In the following description of step i and step j, methanol is described as an example. The alcohol used in step d may be different from the alcohol used in steps i and j, but it is easier and more preferable to use the same alcohol.

  Steps c and d are as described above.

  Step i includes a residue obtained from Step d, methanol of 150 parts by mass or more and 600 parts by mass or less per 100 parts by mass of powdered silver oxide (I) used as a raw material, and a protective agent having affinity for a polar solvent. Can be mixed and stirred as necessary. In step i, the replacement of the protective agent can be efficiently advanced by heating at 40 ° C. or higher and 60 ° C. or lower as necessary. At this time, depending on the kind of the protective agent to be introduced, silver fine particles dispersed in methanol may be obtained, or silver fine particles precipitated in methanol may be obtained.

  As a protective agent having an affinity for a polar solvent, an organic acid or a polyamine having two or more amino groups is preferable. From the viewpoint of conduction characteristics when used as a conductive paste, an organic acid having a molecular weight of 300 or less and a polyamine having a molecular weight of 450 or less are preferred in order to facilitate the desorption and decomposition of the protective agent from silver during the baking process.

  The organic acid is typically a compound having a carboxyl group. In addition to the carboxyl group, an organic acid having one or more functional groups selected from the group consisting of a hydroxyl group, a ketone group, and an ether group having affinity for a polar solvent is preferable. Specific examples include ricinoleic acid, which is an organic acid having one hydroxyl group, and monoacid maleate, monoethyl maleate, monooctyl maleate, which are organic acids having one ketone group and are maleic acid derivatives. .

  The polyamine having two or more amino groups has at least one functional group selected from the group consisting of a hydroxyl group, a ketone group, and an ether group having affinity for a polar solvent as a functional group other than the amino group. In particular, a polyether amine having a plurality of primary amino groups at the end of the polyether skeleton is more preferable. Specifically, Jeffamine EDR-148, Jeffamine T-403 (both trade names made by Huntsman Corporation), ethylene glycol bis (3-aminopropyl) ether, diethylene glycol bis (3-aminopropyl) ether, etc. Can be mentioned.

  One of these protective agents may be selected and used, or two or more protective agents may be used in combination. When two or more kinds of protective agents are used, they can be added simultaneously in methanol or can be introduced stepwise. When two or more kinds of protective agents are introduced stepwise, the silver fine particle washing step can be performed each time (except for the last introduction) (washing in step i). The cleaning in step i refers to cleaning performed after the introduction of the first stage and before the introduction of the second stage when, for example, two kinds of protective agents are introduced in stages. Alcohol can be used for the solvent used for washing in step i, and methanol is preferably used. Here again, methanol is described as an example. When the protective agent is introduced stepwise, 150 parts by mass or more per 100 parts by mass of powdered silver oxide (I) used as a raw material as described above, including the mass of methanol used for washing in process i as a whole. It is preferable to use 600 parts by mass or less of methanol.

  In step i, it is preferable to act an excessive amount of these protective agents on the amine compound covering the silver fine particles, and it is more preferable to act at a mass ratio in the range of 1.5 to 5 times. By applying an excessive amount of the protective agent, the protective agent can be efficiently replaced, and the amine compound having affinity with the hydrocarbon solvent can be replaced with the protective agent having affinity with the polar solvent. (Almost 100% of the amine compound coated with the silver fine particles can be replaced with a protective agent).

  The mass of the amine compound covering the silver fine particles can be measured by thermal analysis. For example, the solvent is completely removed from the silver fine particles obtained in step d by using an arbitrary method to obtain a dry powder of silver fine particles. A thermal analysis is performed on the dry powder, and the decreasing mass can be considered as the mass of the amine compound coated on the silver fine particles. Similarly, the mass of the protective agent can be measured by thermal analysis after removing the solvent as necessary.

  From step i, a mixture containing silver fine particles whose surface is coated with a protective agent (protective agent-coated silver fine particles) and methanol is obtained. Depending on the dispersion state of the silver fine particles, the silver fine particles are contained in methanol. In some cases, it may be in the state of a dispersion dispersed in the solution, or in a state where silver fine particles are precipitated in methanol. By removing methanol from this mixture by an appropriate method and washing with a hydrocarbon solvent, a residue containing the protective agent-coated silver fine particles can be obtained (step j). After removal of methanol, washing with a hydrocarbon solvent can be performed, or methanol removal and washing with a hydrocarbon solvent can be simultaneously performed as shown in Examples.

  The washing solvent used in step j may be the same as or different from the hydrocarbon solvent used in steps a and b. However, it is generally preferable to perform washing with a hydrocarbon solvent that is a nonpolar solvent rather than a polar solvent in which the protective agent-coated silver fine particles can be dispersed. The washing hydrocarbon solvent used here is more preferably a low-boiling hydrocarbon solvent such as hexane or isohexane in consideration of ease of removal in a subsequent step. However, in consideration of the dispersibility of the protective agent-coated silver fine particles in each solvent, if the protective agent-coated silver fine particles are not dispersed, it is possible to perform washing using a polar solvent. Although selected in consideration of the type of protective agent to be introduced and the dispersibility of the silver fine particles in the solvent, an example of a polar solvent that can be used is acetone. In order to carry out washing efficiently, a washing solvent (at least one of a hydrocarbon solvent and a polar solvent) of 500 parts by mass or more and 1000 parts by mass or less per 100 parts by mass of powdered silver oxide (I) used as a raw material is used twice. It is preferable to wash by dividing into the number of times not less than 4 times.

  The polar solvent used in the step k is not particularly limited as long as it is a polar solvent capable of dispersing the obtained protective agent-coated silver fine particles. It can be selected in consideration of the dispersibility of the silver fine particles in each polar solvent. However, a polar solvent having a boiling point of 120 ° C. or lower that can be easily replaced with a high boiling point polar solvent is more preferable, assuming use as a conductive paste. An example of such a polar solvent is alcohol. Specifically, alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, and 2-methyl-1-propanol are more preferable. It is preferable to disperse in 100 to 300 parts by mass of a polar solvent per 100 parts by mass of powdered silver oxide (I) used as a raw material.

  The substitution of the protective agent can be confirmed by measuring the physical properties of the silver fine particles before and after substitution. In addition to the change of the dispersible solvent from a nonpolar solvent such as a hydrocarbon solvent to a polar solvent, substitution of the protective agent can be confirmed by, for example, a thermal analysis result of a dry powder of silver fine particles. The protective agent was replaced by confirming changes in the pyrolysis pattern and the amount of dispersant to be coated before and after replacement (nonpolar silver fine particles) and after (polar dispersed silver fine particles). Can be judged. Moreover, substitution of a protective agent can also be determined in organic analysis by GC-MS or the like.

  The dispersion solvent of the polar dispersion type silver fine particle dispersion (dispersion obtained from step k) in Form 1 is a high boiling polar solvent having a boiling point in the range of 180 ° C. to 350 ° C., preferably in the range of the boiling point of 200 ° C. to 310 ° C. The conductive paste can be prepared by substituting with a high-boiling polar solvent preferably having a boiling point in the range of 220 ° C to 310 ° C.

  Examples of high-boiling polar solvents that can be used include alcohol solvents, among which glycol ethers having hydroxyl groups and ether groups, glycol ether acetates that are derivatives of glycol ethers, diols having two hydroxyl groups, and the like. . Specific examples include butyl carbitol, hexyl carbitol, ethyl carbitol acetate, butyl carbitol acetate, 2-ethyl-1,3-hexanediol, 2,4-diethyl-1,5-pentanediol, and the like. . A mixture of a plurality of high boiling polar solvents can also be used.

  Conductive pastes can be used for printing inks. Examples of the printing method include an inkjet printing method and a screen printing method. Further, as described later, a reverse printing method can be exemplified. Other known components (components other than silver fine particles and a solvent) used for the conductive paste can also be added.

In the case of Form 2, that is, when silver fine particles precipitated in a hydrocarbon solvent in step b are formed, polar dispersed silver fine particles can be prepared by performing the following steps in this order after step b. it can.
f) The hydrocarbon solvent used in steps a and b is removed from the reaction solution obtained in step b, and silver fine particles (for example, having an average particle size of 50 nm to 10 μm, particularly an average particle size of 50 nm to 8 μm) are obtained. A step of recovering a residue containing,
g) washing the residue obtained from step f with alcohol, and
l) In an alcohol, the amine compound covering the surface of the silver fine particles contained in the residue obtained from step g is replaced with a protective agent having affinity for a polar solvent, and the surface is covered by the protective agent. Forming a coated silver fine particle;
m) removing alcohol from the mixture containing silver fine particles and alcohol obtained from step l, and washing the resulting residue with a hydrocarbon solvent;
n) A step of collecting the residue obtained from step m to obtain silver fine particles.

  The alcohol in the preparation of the polar dispersion type silver fine particles (steps g, l, m) is, for example, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol. Is mentioned. Among these, methanol can be suitably used from the viewpoint of particularly high polarity and ease of removal in a subsequent process. In the following description of steps l and m, methanol is described as an example. The alcohol used in step g may be different from the alcohol used in steps l and m, but it is easier and more preferable to use the same alcohol.

  Steps f and g are as described above.

  In step l, a residue obtained from step g, 150 parts by mass or more and 500 parts by mass or less of methanol per 100 parts by mass of powdered silver oxide (I) used as a raw material, and a protective agent having affinity for a polar solvent are used. It can be carried out by mixing and, if necessary, stirring. In step l, the replacement of the protective agent can be efficiently advanced by heating at 40 ° C. or higher and 60 ° C. or lower as necessary. In this case, since the silver fine particles are relatively large, it is easy to separate the methanol from the reaction liquid obtained in step l to obtain silver fine particles that are not contained in the liquid.

  The protective agent used in step l is the same as the protective agent used in step i. As in step i, one type of protective agent may be selected and used, or two or more types of protective agents may be used in combination. When two or more kinds of protective agents are used, they can be added simultaneously in methanol or can be introduced stepwise. When two or more kinds of protective agents are introduced stepwise, a silver fine particle washing step can be performed each time (except for the last introduction) (washing in step l). For example, when the two kinds of protective agents are introduced stepwise, the washing in the step 1 refers to washing performed after the first stage introduction and before the second stage introduction. Alcohol can be used for the solvent used for washing in Step 1, and methanol is preferably used. Here again, methanol is described as an example. When the protective agent is introduced stepwise, 150 parts by mass or more per 100 parts by mass of powdered silver oxide (I) used as a raw material as described above as a whole, including the mass of methanol used for washing in process l It is preferable to use 500 parts by mass or less of methanol.

  In step m, methanol is removed from the mixture obtained in step l (a mixture containing protective agent-coated silver fine particles and methanol) by an appropriate method such as decantation. The resulting residue is washed with a hydrocarbon solvent. The washing with the hydrocarbon solvent can be performed using the same hydrocarbon solvent as used in the step j. Further, similarly to the step j, it is possible to perform washing using a polar solvent. Examples of polar solvents that can be used include methanol, acetone and the like. In order to perform cleaning efficiently, a cleaning solvent (at least one of a hydrocarbon solvent and a polar solvent) of 200 parts by mass or more and 800 parts by mass or less per 100 parts by mass of powdered silver oxide (I) used as a raw material is used twice. It is preferable to wash at the following times.

  In the step n, the residue obtained from the step m can be collected to obtain silver fine particles (polar dispersed silver fine particles).

  The polar dispersed silver fine particles (obtained in step n) in Form 2 are preferably high boiling polar solvents having a boiling point in the range of 180 ° C. to 350 ° C., more preferably in the range of 200 ° C. to 310 ° C., and still more preferably 220 in boiling point. A conductive paste can be prepared by adding a high-boiling polar solvent in the range of from ℃ to 310 ℃.

  Examples of high-boiling polar solvents that can be used include alcohol solvents, among which glycol ethers having hydroxyl groups and ether groups, glycol ether acetates that are derivatives of glycol ethers, diols having two hydroxyl groups, and the like. . Specific examples include butyl carbitol, hexyl carbitol, ethyl carbitol acetate, butyl carbitol acetate, 2-ethyl-1,3-hexanediol, 2,4-diethyl-1,5-pentanediol, and the like. . A mixture of a plurality of high boiling polar solvents can also be used.

  Conductive pastes can be used for printing inks. Examples of the printing method include a screen printing method. Other known components (components other than silver fine particles and a solvent) used for the conductive paste can also be added.

[Conductive paste for reverse printing]
In recent years, a reverse printing method, more specifically a letterpress reverse printing method, has been introduced as an effective printing method for forming a precise pattern below the minimum line / space width (L / S) = 50 μm / 50 μm. In the reversal printing method, ink is applied onto a blanket to form an ink application surface, and a portion of the ink applied surface that is pressed against the relief plate is removed (initial transfer) from the blanket, and then the blanket is removed. This is a printing method in which the ink remaining on the substrate is transferred (final transfer) to a printing medium.

  A silicon blanket made of silicon is widely used as a blanket used in the reverse printing method. Since the silicon blanket is excellent in ink releasability, it is possible to improve the transferability of the pattern to the substrate.

  However, since normal ink has low wettability, the applied ink is easily repelled on the surface of the silicon blanket. The ink repelling tends to cause a problem of pinholes and unevenness in the coating layer. When pinholes and unevenness occur, the releasability of the ink from the blanket decreases, and the ink pattern cannot be transferred from the silicon blanket to the substrate satisfactorily.

  Patent Document 5 describes a conductive paste for reversal printing to which an adjusting agent is added in order to obtain an ink having appropriate wettability and releasability with respect to a silicon blanket. Here, a fluorine-based surfactant or silicon oil is used as the adjusting agent. However, since the use of these modifiers may cause a decrease in the conductivity of the metal film after firing and a deterioration in the quality of the derived product, a conductive paste not containing them is desired.

  On the other hand, the conductive paste for reversal printing of the present invention has appropriate wettability and releasability with respect to the silicon blanket without including a regulator and other regulators.

  The conductive paste for reverse printing of the present invention contains a solvent. As this solvent, it is preferable to use a polar solvent. While the ink is being applied to the silicon blanket, the silicon blanket absorbs the solvent in the ink and swells. At this time, if the degree of swelling exceeds a certain level, there arises a problem that the quality of the precision pattern deteriorates. Generally, a silicon blanket has a small degree of swelling with respect to an alcohol solvent having a high polarity, and a degree of swelling with respect to a hydrocarbon solvent with a low polarity.

  Further, the solvent constituting the conductive paste for reverse printing preferably contains a low-boiling solvent (quick-drying solvent) having a quick drying property. This quick-drying solvent is used when the ink coating film is formed on the blanket so that the ink composition has good fluidity, and then in the atmosphere in a short time until the initial transfer on the relief printing plate. The ink viscosity is increased by being volatilized or absorbed by the blanket, so that the ink has a viscosity suitable for transfer.

  In addition, it is preferable to select a solvent having a low surface tension as the solvent in order to perform uniform application without repelling ink on the silicon blanket.

  In consideration of the above requirements, it is preferable to use a solvent having a boiling point of 50 ° C. to 120 ° C. (low-boiling solvent), particularly a polar solvent, as a quick-drying solvent, specifically, methanol, ethanol, 1-propanol, Examples include alcohols such as 2-propanol, 1-butanol, isobutyl alcohol, and 2-butanol.

  Moreover, these low boiling point solvents preferably include a fluorine-based solvent having a surface tension (25 ° C.) of 12 mN / m to 18 mN / m, and more preferably hydrofluoroethers. Hydrofluoroether is defined as a group of compounds each having one or more fluorinated alkyl groups and ether groups in the molecule. Examples of such solvents include NOVEC series manufactured by Sumitomo 3M Co., Ltd., NOVEC 7100 (trade name, boiling point: 61 ° C., surface tension 13.6 mN / m), NOVEC 7200 (trade name, boiling point: 76 ° C., surface tension 13.6 mN). / N), NOVEC7300 (trade name, boiling point: 98 ° C., surface tension of 15 mN / m) and the like can be used. As a low boiling point solvent, in addition to an alcohol solvent, a fluorine solvent having a surface tension (25 ° C.) of 12 mN / m to 18 mN / m can be used in combination to reduce the surface tension of the conductive paste for reverse printing. This paste can be uniformly applied to the blanket.

  When the surface tension of the fluorinated solvent is 12 mN / m or more, the wettability of the ink with respect to the surface of the silicon blanket does not exceed the preferred range, depending on the physical properties of the other solvent to be combined. Therefore, the ink pattern can be satisfactorily transferred from the silicon blanket to the substrate. On the other hand, when the surface tension of the fluorinated solvent is 18 mN / m or less, the effect of improving the wettability of the ink with respect to the surface of the silicon blanket is excellent. For this reason, when ink is applied to the surface of the silicon blanket, repelling or the like hardly occurs. As a result, ink layer unevenness and pinholes due to repellence and the like are less likely to occur.

  The content of the fluorinated solvent is preferably 10 to 50% by mass, more preferably 20 to 45% by mass in the total solvent. When the content ratio of the fluorinated solvent is 10% by mass or more, the surface tension of the ink does not decrease when the solvent contains the fluorinated solvent, thereby improving the wettability of the ink with respect to the surface of the silicon blanket. Can be improved. For this reason, when ink is applied to the surface of the silicon blanket to form a layer, repelling or the like hardly occurs. In addition, repellence or the like is not caused, and unevenness or pinholes are hardly generated in the ink layer. On the other hand, when the content ratio of the fluorinated solvent is 50% by mass or less, the dispersibility of the ink does not decrease, and the aggregate of silver fine particles is difficult to precipitate.

  As the solvent, it is preferable to use a slow drying solvent (high boiling point solvent) in addition to a fast drying solvent (low boiling point solvent). By using this slow-drying solvent in combination with a fast-drying solvent, rapid volatilization of the solvent component of the ink can be suppressed, and the time during which the ink can be transferred from the blanket can be maintained. As the slow drying solvent, a solvent having a boiling point of 150 ° C. or higher, particularly a solvent having a boiling point in the range of 150 ° C. to 300 ° C. (high boiling solvent), particularly a polar solvent is preferable. Specific examples include ethyl carbitol, butyl carbitol, 2-ethyl-1,3-hexanediol, 2,4-diethyl-1,5-pentanediol, propylene carbonate, and the like. The content of the high boiling point solvent is preferably 0.5 to 10% by mass, more preferably 1 to 5% by mass in the total solvent.

  The polar dispersion type silver fine particles in the case of Form 1 are preferable as the silver fine particles used for the conductive paste for reverse printing. More specifically, the surface is obtained by a protective agent having an affinity for a polar solvent, particularly a low-boiling alcohol solvent, obtained by a method for preparing silver fine particles comprising steps a, b, c, d, i, j and k. A conductive paste for reversal printing containing silver fine particles coated with (for example, an average particle size of 5 nm to 40 nm, particularly an average particle size of 5 nm to 30 nm) can be produced.

  The inventors of the present invention that the conductive paste produced using the solvent satisfying the above range and the polar dispersion type silver fine particles in the case of Form 1 as silver fine particles contains a regulator and other adjusting agents. In addition, the present inventors have found that it can be suitably used as a conductive paste for reversal printing, which has appropriate wettability and releasability with respect to a silicon blanket. Among the silver fine particles included in Form 1, silver fine particles coated with an organic acid are preferable, and silver fine particles coated with a protective agent composed of ricinoleic acid and a maleic acid derivative are particularly preferable. A conductive paste prepared using silver fine particles coated only with an organic acid tends to improve transferability from a blanket to a substrate as compared with a conductive paste using silver fine particles coated with a polyamine derivative.

  The conductive paste for reverse printing of the present invention does not contain a binder resin component. This is because after the paste (ink) is dried, the silver fine particle content is increased and the conductivity is increased. In general reverse printing ink, natural rubber, polyolefin, polyether, polyester, acrylic resin, phenol resin, melamine resin, benzoguanamine resin, epoxy resin, urethane resin are used to adjust the printability of paste in the reverse printing method. Binder resin components such as are blended. The conductive paste for reversal printing of the present invention is characterized by having good printability without containing a binder resin component.

  Dispersion liquid in which silver fine particles (for example, average particle diameter of 5 nm to 40 nm, particularly average particle diameter of 5 nm to 30 nm) having a surface coated with a protective agent having affinity for a polar solvent are dispersed in alcohol from step k. Is obtained. For example, by mixing this dispersion and a solvent (particularly, the low-boiling solvent and the high-boiling solvent) and stirring appropriately, a conductive paste for reverse printing, that is, a conductive ink for reverse printing, can be obtained. The other component used for the conductive ink for reverse printing can also be mix | blended.

  EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited by this.

[Example 1]
・ Process a
10.85 g of powdered silver oxide (special grade silver oxide manufactured by Toyo Kagaku Kogyo Co., Ltd., particle size distribution of 30 μm or less, average particle size of 6 μm) and 45 g of methylcyclohexane (nonpolar hydrocarbon solvent having a boiling point of 101 ° C.) were added to a 300 ml beaker and stirred. Thereafter, 4.56 g of formic acid was added and stirred over 30 seconds.

・ Process b
When the exotherm due to the formation of silver formate subsides and the liquid temperature reaches 30 ° C., 0.87 g of oleylamine (primary amine, molecular weight 267.49), 12.8 g of dibutylamine (secondary amine, molecular weight 129.24), 8 g of methylcyclohexane was simultaneously added to carry out a decomposing reduction reaction. The liquid temperature rose to 58 ° C. Stirring was stopped when the liquid temperature became 40 ° C. or lower.

  In this example, the primary amine (oleylamine 0.87 g) used in step b is 3.5 mol% based on the silver cation contained in the raw powdered silver oxide (I) and used in step b. The total amount of primary amine and secondary amine (12.8 g of dibutylamine) was 110 mol% (1.1 mol with respect to 1 mol of silver cation).

-Check of dispersion state of silver fine particles in liquid obtained from step b Here, the liquid obtained from step b (mixture of silver fine particles and hydrocarbon solvent) is allowed to stand for 10 minutes, and the dispersion state of silver fine particles Was confirmed visually. In this example, precipitation was not confirmed, and from step b, a dispersion liquid in which silver fine particles were dispersed in methylcyclohexane was obtained.

・ Process c
The resulting dark blue dispersion was transferred to a 300 ml eggplant type flask, and methylcyclohexane as a reaction solvent was distilled off under conditions of 40 ° C. and 50 hPa using an evaporator (trade name: N-1100S, manufactured by Tokyo Science Instruments). . A slurry-like residue containing silver fine particles was obtained.

・ Process d
After adding 40 g of methanol to the residue after demethylcyclohexane, the mixture was stirred for 3 minutes. Aggregation of silver fine particles occurred when methanol was added, and silver fine particles formed by coating primary amine and secondary amine were precipitated without being dispersed in methanol. A considerable amount of components such as surplus formic acid and amine, or salts containing them are dissolved in methanol. Thereafter, the supernatant layer was removed by decantation (first washing). Next, 25 g of methanol was added in the same manner, followed by stirring for 3 minutes, and the supernatant layer was removed by decantation (second washing). Similarly, 10 g of methanol was added and stirred for 3 minutes, and the supernatant layer was removed by decantation (the third washing), and the washing step was completed.

・ Process e
After 15 g of methylcyclohexane was added to the recovered precipitate layer (residue after washing with methanol obtained from step d), stirring was performed to disperse the silver fine particles in methylcyclohexane. In this methylcyclohexane dispersion, some methanol used in the washing step is mixed. Methanol was selectively removed using an evaporator at 40 ° C. and 120 hPa. The dispersion was filtered through a 0.5 μm membrane filter (manufactured by Advantech) to remove a small amount of aggregates, and a methylcyclohexane dispersion of silver fine particles coated with an amine compound was obtained.

-Physical property evaluation The following evaluation was performed about the silver fine particle contained in the dispersion liquid obtained from the process e.
The average particle diameter of the silver fine particles was measured using a nanotrack particle size analyzer (manufactured by Nikkiso). From the measurement results, it was found that the average particle diameter of the silver fine particles was 18.2 nm.

  When the mass of the amine compound covering the silver fine particles was measured by thermal analysis, it was found that 9 parts by mass of the amine compound covered the silver fine particle surface with respect to 100 parts by mass of silver. It has been found by separate examination that the mass of this amine compound is the same even when measured at the stage where step d is completed.

[Examples 2-45, Comparative Examples 1-12]
-Examples 2-12 and Comparative Examples 1-2
In Comparative Example 1, Examples 2 to 12, and Comparative Example 2, as shown in Table 1, the amount of primary amine (oleylamine) used in step b is included in the raw powdered silver oxide (I). 110, 50, 40, 30, 4.2, 3.0, 2.3, 1.5, 1.4, 0.8, respectively, based on the silver cation (the amount of silver cation is 100 mol%). It was set to 7, 0.5, 0.2, and 0 mol%. In Table 1, Example 1 is abbreviated as “Real 1”, and Comparative Example 1 is abbreviated as “Ratio 1” (the same applies to Tables 3 and 5). At this time, the total amount of primary amine and secondary amine (dibutylamine) used in step b was 110 mol% as in Example 1. Therefore, the amount of secondary amine used in step b was also changed according to the amount of primary amine. In Comparative Example 1, no secondary amine was used in Step b, and in Comparative Example 2, no primary amine was used in Step b.

Examples 13 to 24 and Comparative Example 3
In Comparative Example 3 and Examples 13 to 24, 3- (lauryloxy) propylamine (molecular weight 243.43) was used as the primary amine used in Step b instead of oleylamine. As shown in Table 1, the amount of primary amine was changed from 110 mol% to 0.3 mol% based on the silver cation contained in the raw powdered silver oxide (I). At this time, in order to make the total amount of the primary amine and secondary amine used in Step b 110 mol% as in Example 1, the amount of secondary amine was also changed.

Examples 25-33 and Comparative Example 4
In Comparative Example 4 and Examples 25 to 33, 3- (2-ethylhexyloxy) propylamine (molecular weight 187.32) was used as the primary amine used in Step b instead of oleylamine. As shown in Table 1, the amount of primary amine was changed from 110 mol% to 1.4 mol% based on the silver cation contained in the raw powdered silver oxide (I). At this time, in order to make the total amount of the primary amine and secondary amine used in Step b 110 mol% as in Example 1, the amount of secondary amine was also changed.

Examples 34 to 41 and Comparative Example 5
In Comparative Example 5 and Examples 34 to 41, 2-ethylhexylamine (molecular weight: 129.24) was used in place of oleylamine as the primary amine used in Step b. As shown in Table 1, the amount of primary amine was changed from 110 mol% to 1.5 mol% based on the silver cation contained in the raw powdered silver oxide (I). At this time, in order to make the total amount of the primary amine and secondary amine used in Step b 110 mol% as in Example 1, the amount of secondary amine was also changed.

Examples 42 to 43 and Comparative Examples 6 to 10
In these examples, diisopropylamine was used as the secondary amine in place of dibutylamine. Moreover, the primary amine of the kind shown in Table 3 is blended in the amount shown in Table 3 (110, 40, 3.0, or 0 mol% based on the silver cation contained in the raw powdered silver oxide (I)) ). At this time, in order to make the total amount of the primary amine and secondary amine used in Step b 110 mol% as in Example 1, the amount of secondary amine was also changed.

Examples 44 to 45 and Comparative Examples 11 to 12
In these examples, the total amount of primary amine and secondary amine (dibutylamine) used in step b was 150 mol% based on the silver cation contained in the raw material powdered silver oxide (I). And the primary amine of the kind shown in Table 5 is blended in the amount shown in Table 5 (110, 40, 3.0 or 0 mol% based on the silver cation contained in the raw material powdered silver oxide (I)) ). At this time, the amount of the secondary amine was also changed so that the total amount of the primary amine and secondary amine used in step b was 150 mol%.

  In Examples 2-45 and Comparative Examples 1-12, the operations of steps a and b were performed in the same manner as in Example 1 except for the above. Then, the dispersion state of the silver fine particles in the liquid obtained from step b (a mixture containing silver fine particles and a hydrocarbon solvent) is checked as described above. When obtained (when represented as (S) in Tables 2, 4, and 6), steps c to e were performed in the same manner as in Example 1. In the liquid obtained from the silver fine particle step b, when the precipitated silver fine particles and the dispersed silver fine particles were mixed (in the case of (S + A) in Tables 2, 4, and 6), the step b was finished. At the stage, the experiment was terminated. In this case, the particle diameter of the silver fine particles is not measured (because measurement is not easy). In the liquid obtained from step b, when silver fine particles are precipitated (indicated as (A) in Tables 2, 4, and 6), after step b, instead of steps c to e, the following Steps f to h were performed as described in detail below.

・ Process f
The supernatant was removed from the liquid obtained from step b by decantation, and the silver fine particle residue was recovered.

・ Process g
To the residue obtained from step f, 40 g of methanol was added and stirred for 3 minutes, and the supernatant was removed by decantation (first wash). Subsequently, a second washing was performed using 30 g of methanol.
・ Process h
The residue after washing was collected to obtain silver fine particles.

  In addition, although the liquid temperature rose by the degradative reduction reaction in the step b, the maximum temperature at that time was almost the same level in each example. For example, in Example 1, the liquid temperature rose to 58 ° C., but Example 30 (using 3- (2-ethylhexyloxy) propylamine as the primary amine and 1 as a reference based on the silver cation in the powdered silver oxide) In the case where 3.0 mol% of the primary amine was used and the total amount of the primary amine and the secondary amine was 110 mol%, the liquid temperature rose to 60 ° C.

-Physical property evaluation In each example, the same physical property evaluation as Example 1 was performed about the silver fine particle contained in the dispersion liquid obtained from the process e, or the silver fine particle obtained from the process h. That is, for these silver fine particles, the average particle size was measured, and the mass of the amine compound covering the silver fine particles was measured by thermal analysis. However, the mass of the amine compound that coats the silver fine particles is that of Example 1 and Example 30 (using 3- (2-ethylhexyloxy) propylamine as the primary amine, based on the silver cation in the powdered silver oxide) Measurement was performed only for an example in which 3.0 mol% of primary amine was used and the total amount of primary amine and secondary amine was 110 mol%.

  Tables 2, 4, and 6 show the average particle diameters measured in each example. In Example 30, the mass of the amine compound covering the silver fine particles was 2.6 parts by mass with respect to 100 parts by mass of silver.

-Details of dispersion state of silver fine particles obtained in step b Tables 2, 4, and 6 show the dispersion state of silver fine particles in the liquid obtained from step b in each example. In the table, when the liquid (mixture of silver fine particles and hydrocarbon solvent) obtained from step b was allowed to stand for 10 minutes, the case where no precipitation was visually confirmed was represented by “(S)”. The case where precipitation is recognized and the liquid is clarified is represented by “(A)”, and the intermediate state between the two is represented by “S + A”.

  In Example 1, when the mass of silver contained in the methylcyclohexane dispersion finally obtained (in step e) was measured, it was 9.2 g. In Example 1, since powdery silver oxide (I) equivalent to 10 g in terms of silver is used as a raw material, in Example 1, the silver fine particles covering the amine compound are 92% by mass in the state of dispersion. It was found that the yield was obtained. Therefore, from step b, a dispersion liquid in which 92% by mass of silver based on the raw material silver was dispersed in the liquid as silver fine particles was obtained.

  In Example 30, the mass of silver finally obtained (in step h) was measured, and was 9.3 g. In Example 30, since powdery silver oxide (I) equivalent to 10 g in terms of silver is used as a raw material, in Example 30, 93% by mass of the silver fine particles covering the amine compound are precipitated. It was found that it was obtained at a rate. Therefore, from step b, a liquid containing 93% by mass of silver as a precipitate of silver fine particles based on the raw material silver was obtained.

  For the examples other than Examples 1 and 30 (except for the examples indicated as “S + A” in the table), similarly, based on the mass of silver finally obtained, the dispersion state or precipitation state obtained in step b In the examples where “S” is shown in the table, the ratio of the silver fine particles to the raw material silver was investigated, and in each case, a dispersion liquid in which 90% by mass of silver based on the raw silver was dispersed in the liquid as a fine particle was obtained. It was confirmed that In each of the examples indicated as “A” in the table, it was confirmed that a liquid containing 90% by mass of silver as a precipitate of silver fine particles was obtained from step b based on the raw material silver.

Example 46
In this example, silver fine particles are produced using the same method as in Example 1, and silver fine particles dispersed in a polar solvent are produced in the subsequent process.

  First, as in Example 1, steps a, b, c and d were performed.

・ Process i
Next, ricinoleic acid and Jeffamine EDR-148 (trade name, polyamine manufactured by Huntsman Corporation) are introduced as a protective agent having affinity for polar solvents. In Example 1, it is known that 9 parts by mass of the amine compound coats the silver fine particles with respect to 100 parts by mass of silver at the stage of completing the step d. In the protective agent substitution step, the protective agent is efficiently substituted by causing the mass of the protective agent to be introduced to act excessively on the mass of the amine compound to be substituted. Also in this example, after assuming that 9 parts by mass of the amine compound covers silver fine particles with respect to 100 parts by mass of silver in steps a to d, 18 parts by mass (100 parts by mass of silver) A protective agent corresponding to the total amount of ricinoleic acid and Jeffamine EDR-148 described later is allowed to act.

  To the residue washed with methanol obtained from step d, 25 g of methanol and 0.6 g of ricinoleic acid (an organic acid used as a protective agent having an affinity for polar solvents) were added and stirred at 40 ° C. for 15 minutes. It was. At this stage, the silver fine particles are not completely dispersed in methanol, but are precipitated in methanol. After removing methanol by decantation, washing was performed by adding 15 g of methanol and stirring for 3 minutes, and methanol was removed by decantation (washing in step i).

  To the obtained residue, 5 g of methanol and 1.2 g of Jeffamine EDR-148 (polyamine used as a protective agent having affinity for a polar solvent) were added and stirred at 40 ° C. for 15 minutes. Through this step, silver fine particles coated with ricinoleic acid and Jeffamine EDR-148 are prepared. At this stage, the silver fine particles were not completely dispersed in methanol, and a mixture of silver fine particles and methanol was obtained.

・ Process j
20 g of hexane was added to the mixture of the silver fine particles obtained in step i and methanol and stirred for 3 minutes. Since these fine particles have an affinity for a polar solvent, they are aggregated and precipitated by adding hexane, which is a nonpolar solvent. Fine particles were washed by removing the mixed layer of methanol and hexane by decantation (first washing). Here, methanol removal and cleaning with hexane are performed simultaneously.

  After removing the mixed layer of methanol and hexane by decantation, 20 g of hexane was added and stirred for 3 minutes, and hexane was removed by decantation to wash the fine particles (second wash). 20 g of hexane was added to the obtained residue, and the mixture was stirred for 3 minutes, and hexane was removed by decantation (third washing) to complete the washing.

・ Process k
30 g of isopropanol (IPA), that is, 2-propanol, is added to the obtained silver fine particles (residue obtained from step j) to obtain an IPA dispersion (a dispersion in which silver fine particles coated with a protective agent are dispersed in IPA). It was. In this IPA dispersion, some methanol and hexane used in the washing step are mixed. These solvents were selectively removed using a difference in vapor pressure at 40 ° C. and 120 hPa using an evaporator. The IPA dispersion was filtered through a 0.5 μm glass filter (manufactured by Advantech) to remove agglomerates contained in a small amount in the dispersion. It was 28.2 mass% when the silver concentration (mass ratio of the silver fine particle coat | covered with the protective agent with respect to the whole IPA dispersion liquid) contained in IPA dispersion liquid was measured.

Production of conductive ink for reverse printing 53% by mass of the obtained IPA dispersion (solid content 28.2% by mass), 14% by mass of IPA, and fluorine-based solvent Novec 7200 (trade name, manufactured by Sumitomo 3M) A conductive ink for reversal printing was produced by blending 5% by mass and 0.5% by mass of 2,4-diethyl-1,5-pentanediol. When the surface tension of this ink was measured at 25 ° C. using a surface tension meter (manufactured by Kyowa Interface Science), it was 18.1 mN / m.

  A conductive pattern having a line width of about 40 μm was produced by the reversal printing method using a relief plate made of glass according to the following procedure. First, ink was uniformly applied to a silicon blanket, and then the glass relief was pressed against the ink application surface on the blanket to transfer and remove the ink (initial transfer). Further, the ink pattern formed on the blanket was transferred onto the glass by pressing the glass (final transfer). There was almost no ink remaining on the blanket at the time of initial transfer and at the time of final transfer.

Further, an ink thin film was formed on a slide glass (Matsunami Glass Industrial Co., Ltd.) having a thickness of 1 mm with a spin coater (1H-DX2, manufactured by Mikasa) using the conductive ink for reverse printing and baked at 180 ° C. for 30 minutes. When the specific resistance was measured, it was 6.2 × 10 ⁻⁶ Ω · cm.

Example 47
In this example, silver fine particles are produced using the same method as in Example 1, and silver fine particles dispersed in a polar solvent are produced in the subsequent process.

  First, as in Example 1, steps a, b, c and d were performed.

・ Process i
Next, ricinoleic acid and monoethyl maleate are introduced as a protective agent having affinity for the polar solvent. In Example 1, it is known that 9 parts by mass of the amine compound coats the silver fine particles with respect to 100 parts by mass of silver at the stage of completing the step d. In the protective agent substitution step, the protective agent is efficiently substituted by causing the mass of the protective agent to be introduced to act excessively on the mass of the amine compound to be substituted. Also in this example, after assuming that 9 parts by mass of an amine compound is covering silver fine particles with respect to 100 parts by mass of silver in steps a to d, 16 parts by mass (100 parts by mass of 100 parts by mass of silver ( A protective agent corresponding to the total amount of ricinoleic acid and monoethyl maleate described below is allowed to act.

  To the residue washed with methanol obtained from step d, 25 g of methanol and 0.6 g of ricinoleic acid (an organic acid used as a protective agent having an affinity for polar solvents) were added and stirred at 40 ° C. for 15 minutes. It was. At this stage, the silver fine particles are not completely dispersed in methanol, but are precipitated in methanol. After removing methanol by decantation, washing was performed by adding 15 g of methanol and stirring for 3 minutes, and methanol was removed by decantation (washing in step i).

  To the obtained residue, 5 g of methanol and 1 g of monoethyl maleate (an organic acid used as a protective agent having an affinity for a polar solvent) were added and stirred at 40 ° C. for 15 minutes. Through this step, silver fine particles coated with ricinoleic acid and monoethyl maleate are prepared. At this stage, the silver fine particles are not completely dispersed in methanol, and a mixture of silver fine particles and methanol is obtained.

・ Process j
20 g of hexane was added to the mixture of the obtained silver fine particles and methanol and stirred for 3 minutes. Since these fine particles have an affinity for a polar solvent, they are aggregated and precipitated by adding hexane as a hydrocarbon solvent. Fine particles were washed by removing the mixed layer of methanol and hexane by decantation (first washing). Here, methanol removal and cleaning with hexane are performed simultaneously.

  After removing the mixed layer of methanol and hexane by decantation, 20 g of hexane was added and stirred for 3 minutes, and hexane was removed by decantation to wash the fine particles (second wash). To the obtained residue, 20 g of acetone was added and stirred for 3 minutes, and acetone was removed by decantation (third washing). To the obtained residue, 10 g of hexane was added and stirred for 3 minutes, and hexane was removed by decantation (4th washing) to complete the washing. In addition, after introducing a protective agent having an affinity for a polar solvent, it is preferable to wash the fine particles using a hydrocarbon solvent such as hexane, but considering the dispersibility of the fine particles in each solvent. It is also possible to perform washing using a polar solvent such as acetone as described above.

・ Process k
25 g of isopropanol (IPA) was added to the obtained silver fine particles (residue obtained from step j) to obtain an IPA dispersion (a dispersion in which silver fine particles coated with a protective agent were dispersed in IPA). In this IPA dispersion, some methanol, hexane and acetone used in the washing step are mixed. These solvents were selectively removed using a difference in vapor pressure at 40 ° C. and 120 hPa using an evaporator. The IPA dispersion was filtered through a 0.5 μm glass filter (manufactured by Advantech) to remove agglomerates contained in a small amount in the dispersion. It was 27.4 mass% when the silver concentration (mass ratio of the silver fine particle coat | covered with the protective agent with respect to the whole IPA dispersion liquid) contained in IPA dispersion liquid was measured.

-Production of conductive ink for reverse printing 55% by mass of the obtained IPA dispersion (solid content 27.4% by mass), 12% by mass of IPA, and fluorine-based solvent Novec 7200 (trade name, manufactured by Sumitomo 3M) A conductive ink for reversal printing was produced by blending 5% by mass and 0.5% by mass of 2,4-diethyl-1,5-pentanediol. When the surface tension of this ink was measured at 25 ° C. using a surface tension meter (manufactured by Kyowa Interface Science), it was 17.9 mN / m.

  A conductive pattern having a line width of about 40 μm was produced by the reversal printing method using a relief plate made of glass according to the following procedure. First, ink was uniformly applied to a silicon blanket, and then the glass relief was pressed against the ink application surface on the blanket to transfer and remove the ink (initial transfer). Further, the ink pattern formed on the blanket was transferred onto the glass by pressing the glass (final transfer). There was no ink remaining on the blanket during the initial transfer and the final transfer, and good transferability was exhibited as compared with Example 46.

Further, an ink thin film was formed on a slide glass (Matsunami Glass Industrial Co., Ltd.) having a thickness of 1 mm with a spin coater (1H-DX2, manufactured by Mikasa) using the conductive ink for reverse printing and baked at 180 ° C. for 30 minutes. It was 6 * 10 <^{-6> (} omega ^| ohm) * cm when the back specific resistance was measured.

Example 48
In this example, silver fine particles are produced by using the same method as in Example 30 and polar dispersed silver fine particles are produced by the subsequent process.

  First, as in Example 30, steps a, b, f, and g were performed.

・ Process l
As a protective agent having affinity for a polar solvent, ricinoleic acid and Jeffamine EDR-148 (trade name, polyamine manufactured by Huntsman Corporation) are introduced. In Example 30, at the stage of completing step g, it is known that 2.6 parts by mass of an amine compound covers silver fine particles with respect to 100 parts by mass of silver. In the protective agent substitution step, the protective agent is efficiently substituted by causing the mass of the protective agent to be introduced to act excessively on the mass of the amine compound to be substituted. Also in this example, after assuming that 2.6 parts by mass of an amine compound is covering silver fine particles with respect to 100 parts by mass of silver in steps a, b, f and g, 100 parts by mass of silver The protective agent corresponding to 6 parts by mass (the total amount of ricinoleic acid and Jeffamine EDR-148 described later) is allowed to act on the mixture.

  To the residue washed with methanol obtained from step g, 25 g of methanol, 0.2 g of ricinoleic acid (an organic acid used as a protective agent having an affinity for polar solvents), and Jeffamine EDR-148 (for polar solvents) And 0.4 g of a polyamine used as a protective agent having an affinity) and stirred at 40 ° C. for 15 minutes. At this stage, silver fine particles were precipitated in methanol.

・ Process m
Methanol was removed from the liquid obtained from step 1 by decantation. 30 g of isohexane (trade name: Kyowasol C-600M, manufactured by KH Neochem) was added to the obtained residue, and the mixture was stirred for 3 minutes, and isohexane was removed by decantation (washing operation).

・ Process n
The residue after washing was collected to obtain silver fine particles.

Claims (14)

A method for preparing silver fine particles, comprising preparing silver fine particles using powdered silver oxide (I) as a raw material,
a) converting powdered silver oxide (I) into silver formate (I) by allowing formic acid to act on powdered silver oxide (I) in a hydrocarbon solvent; and b) in the hydrocarbon solvent, A step of reducing silver cations contained in the silver formate (I) to silver atoms by a reduction reaction with an amine compound to form silver fine particles whose surface is coated with the amine compound;
A method for preparing silver fine particles, wherein both a primary amine and a secondary amine are used as the amine compound. In the step b, when forming silver fine particles having a relatively small particle diameter, the proportion of primary amine in the amine compound is relatively increased, and when forming silver fine particles having a relatively large particle diameter, 2. The method for preparing silver fine particles according to claim 1, wherein the particle diameter of the silver fine particles is controlled by relatively reducing the proportion of primary amine in the amine compound. The hydrocarbon solvent is a straight chain alkane having 6 to 9 carbon atoms or a cycloalkane having 6 to 9 carbon atoms, and has a boiling point of 65 ° C. or more and 155 ° C. or less,
3. The method for preparing silver fine particles according to claim 1, wherein in step a, the hydrocarbon solvent is used in an amount of 350 to 600 parts by mass per 100 parts by mass of the powdered silver oxide (I). The molecular weight of the primary amine is 120 or more and 300 or less,
The method for preparing silver fine particles according to any one of claims 1 to 3, wherein the primary amine has an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent. The molecular weight of the secondary amine is 100 or more and 190 or less,
The method for preparing silver fine particles according to any one of claims 1 to 4, wherein the secondary amine has an aliphatic hydrocarbon chain having affinity for the hydrocarbon solvent. The step b uses a total of 110 mol% to 150 mol% of primary amine and secondary amine based on the silver cation contained in the powdered silver oxide (I) as a raw material. The preparation method of the silver fine particles as described in any one of -5. In step b, primary amines having a relatively large molecular weight are used when forming silver fine particles dispersed in a hydrocarbon solvent, and relative molecular weights are used when forming silver fine particles precipitated in a hydrocarbon solvent. The method for preparing silver fine particles according to claim 1, wherein the dispersion state of the silver fine particles is controlled by using a primary amine having a small particle size. Forming silver fine particles dispersed in a hydrocarbon solvent in step b;
After step b
c) a step of recovering a residue containing silver fine particles by removing the hydrocarbon solvent used in steps a and b by evaporating under reduced pressure from the reaction solution obtained from step b;
d) washing the residue obtained from step c with alcohol; and
e) Dispersing the alcohol-washed residue obtained from step d in a hydrocarbon solvent that is the same as or different from the hydrocarbon solvent used in steps a and b to obtain a dispersion containing silver fine particles. The method for preparing silver fine particles according to any one of claims 1 to 7. Forming silver fine particles precipitated in a hydrocarbon solvent in step b;
After step b,
f) removing the hydrocarbon solvent used in steps a and b from the reaction solution obtained from step b, and collecting a residue containing silver fine particles;
g) washing the residue obtained from step f with alcohol, and
The method for preparing silver fine particles according to any one of claims 1 to 7, further comprising a step of h) collecting the residue obtained from step g to obtain silver fine particles. Forming silver fine particles dispersed in a hydrocarbon solvent in step b;
After step b,
c) a step of recovering a residue containing silver fine particles by removing the hydrocarbon solvent used in steps a and b by evaporating under reduced pressure from the reaction solution obtained from step b;
d) washing the residue obtained from step c with alcohol; and
i) In alcohol, the amine compound covering the surface of the silver fine particles contained in the residue obtained from step d is replaced with a protective agent having affinity for a polar solvent, and the surface is covered by the protective agent. Forming a coated silver fine particle;
j) Step of performing alcohol removal and washing with a hydrocarbon solvent on the mixture containing silver fine particles and alcohol obtained from step i.
k) The residue washed with the hydrocarbon solvent obtained from step j is dispersed in a polar solvent to obtain a dispersion containing silver fine particles. A method for preparing the silver fine particles as described. Forming silver fine particles precipitated in a hydrocarbon solvent in step b;
After step b,
f) removing the hydrocarbon solvent used in steps a and b from the reaction solution obtained from step b, and collecting a residue containing silver fine particles;
g) washing the residue obtained from step f with alcohol, and
l) In an alcohol, the amine compound covering the surface of the silver fine particles contained in the residue obtained from step g is replaced with a protective agent having affinity for a polar solvent, and the surface is covered by the protective agent. Forming a coated silver fine particle;
m) removing alcohol from the mixture containing silver fine particles and alcohol obtained from step l, washing the resulting residue with a hydrocarbon solvent, and n) collecting the residue obtained from step m, The method for preparing silver fine particles according to any one of claims 1 to 7, further comprising a step of obtaining silver fine particles. The method for preparing silver fine particles according to claim 10 , wherein the protective agent is any one of organic acids and polyamines having two or more amino groups, or a combination thereof. The method for preparing silver fine particles according to claim 11 , wherein the protective agent is any one of organic acids and polyamines having two or more amino groups, or a combination thereof. The method for preparing silver fine particles according to any one of claims 8 to 11, wherein the alcohol is all methanol.