Title of Invention: METHOD FOR PRODUCING CONDUCTIVE

INK

Technical Field

[1] The present invention relates to a method for producing a conductive ink containing silver particles. And, the present invention relates to a conductive film with a wire formed using the conductive ink produced by said method. Background Art

[2] Information communication devices such as liquid crystal displays move toward miniaturization and high performance, and attempts have been steadily made to incorporate these devices on flexible material supports. A desired wire has been generally formed by preparing a film by vapor deposition such as chemical vapor deposition (CVD), sputtering and so on, and etching out an unnecessary portion by photolithography and so on, when said devices are produced.

[3] However, the conventional method for forming a wire has disadvantages caused by repetition of film formation and etching, for example a low usage efficiency of raw materials, generation of a large amount of waste, a long manufacturing time and considerable costs of equipment. And, the conventional method for forming a wire encounters with many problems in forming a fine wire required for miniaturization of said information communication devices.

[4] Recently the related industries pay attention to an inkjet printing or roll printing technique for forming a fine wire that allows a low loss of raw materials, non-use of hazardous components such as lead or the like, and a simple process for forming a wire, printingo use the inkjet or roll printing technique in earnest to form a wire, development of a high-performance conductive paste or conductive ink is needed.

[5] A conductive paste or ink suitable for forming a wire should have a high conductivity. If a conductive ink is applied to an antenna for a radio-frequency identification (RFID) that attracts expectations for a significant increase in use thereof, a specific resistivity of the conductive ink should be 1.7x10 ⁵ Ω-cm or less. It is difficult to make mass production of the conductive ink, if an expensive conductive filler should be used to meet the specific resistivity requirement of the conductive ink. And, if the conductive ink is printed on a material for a flexible circuit board such as polyethylene terephthalate (PET) and so on, the conductive ink should be sintered at a sufficiently low temperature after it is applied on the circuit board. This is because plastics or the like have a low glass transition temperature (T _g ). The sintering temperature is determined depending on the characteristics of a conductive filler (mainly metal particles) and other components of the conductive paste. But, because smaller metal particles have higher surface energy, the sintering temperature tends to be lowered than an intrinsic melting point of a metal.

[6] Silver has a high conductivity, and thus is suitable as a conductive filler of a conductive paste for forming a fine wire.use a large amount of silver. To ensure storage stability of an ink, an additive such as a dispersion, a stabilizer and so on may be added. However, the additive increases the sintering temperature again, which was low once due to size reduction of silver particles.

[7] For low sintering temperature and high conductivity, silver particles should be uniformly dispersed in a paste or ink. Typically, the ink is produced by introducing a chelating agent or the like to silver ions to form a silver ion complex, adding a reducing agent to the silver ion complex to reduce the silver ions into silver atoms, and uniformly dispersing the reduced silver atoms in a liquid organic solvent or liquid dispersion medium. A small amount of remaining chelating agent after reduction serves as capping molecules to suppress agglomeration of the reduced silver nanoparticles. However, this process takes a long time, and has difficulty in dispersing silver nanoparticles in an organic solvent again to produce an ink. To ensure dispersion of an ink, it is preferred to disperse a silver ion complex in a suitable organic solvent for inkjet printing and then reduce the silver ion complex. However, because most of silver ion complexes do not have solubility to an organic solvent, it is difficult to dissolve or disperse a silver ion complex in an organic solvent. This is because of an insignificant interaction of the silver ion complex with the organic solvent. When silver ions bind to ligands to form a complex, a formed coordinate covalent bond between the silver ions and the ligand is a strong bond having polarity and the formed silver ion complexes form a highly systematical and stable square-octagon dimmer to result in said interaction.

[8] So conventional methods for producing a conductive ink selected a method comprising reducing silver ions to silver atoms by addition of a reducing agent prior to dispersing silver ion complexes in an organic solvent, to break up the organized structure between the complexes. In this case, it is easy to disperse a mixture of silver and ligand in an organic solvent, but the reduced silver particles agglomerate to result in poor storage stability of the ink. In addition, because the conductive silver complexes have a high decomposition temperature and the silver particles agglomerate to increase the effective particle size, there is a weak point of increased sintering temperature. The conventional methods for producing a conductive ink did not yet find any satisfactory solution for uniform dispersion, low sintering temperature and high conductivity. Disclosure of Invention

Technical Problem

[9] It is an object of the present invention to develop a conductive ink containing silver that can be processed at lower sintering temperature than a conventional technique and has good storage stability and high conductivity by using a silver ion complex soluble in an organic solvent easily. Solution to Problem

[10] The present invention provides a method for producing a conductive ink that can be sintered at temperature of 150 ⁰ C or lower, and the method disperses a silver ion complex in an organic solvent prior to reducing the silver ion complex. According to one aspect of the present invention, the method comprises reacting silver ions with branched chain carboxylic acid having 6 to 14 carbon atoms or its salt to form a silver ion complex; dispersing the silver ion complex in an organic solvent to prepare a silver ion complex-dispersed mixture; and adding a reducing agent to the silver ion complex- dispersed mixture to reduce the silver ions. At this time, the reducing agent is preferably added at an equivalence ratio of silver ion complex reducing agent= 1:0.5 to 1:1.

[11] According to another aspect of the present invention, a method for manufacturing a conductive film comprising printing the conductive silver ink produced by said method on a film to form a pattern thereon; and sintering the film having the pattern is provided. At this time, preferably the sintering process is performed at temperature of 130 to 150 ⁰ C. In the present invention, a ratio of the reducing agent and the sintering temperature are controlled so that the sintered silver ink of the conductive film produced by said method has a specific resistivity of 1.7xlO ^~5 Ω-cm or less.

Advantageous Effects of Invention

[12] The conductive silver ink of the present invention has a high conductivity, and can be sintered at temperature of 150 ⁰ C or lower, consequently has excellent processability. And, in the present invention, a process of dispersing silver in an organic solvent is performed prior to a process of reducing silver, resulting in high dispersion of the ink. Consequently the storage stability of the ink is improved. The conductive ink of the present invention can be widely used to form a wire by printing techniques, for example, to form a wire for a printed circuit board, to form a wire for a display device such as a liquid crystal display, a plasma display panel, an organic light-emitting diode and so on, to form an antenna for a RFID system, to produce an electrode for a solar cell, to form a reflective film for a solar cell and so on. Best Mode for Carrying out the Invention

[13] Hereinafter, the present invention will be described in detail. The present invention provides a method for producing a conductive ink containing silver and a method for manufacturing a conductive film using the same, and the resultant ink has high conductivity and low sintering temperature.

[14] According to an aspect of the present invention, a method for producing a conductive silver ink comprises the steps of A) reacting silver ions with branched chain carboxylic acid having 6 to 14 carbon atoms or its salt to form a silver ion complex, B) dispersing the silver ion complex in an organic solvent to prepare a silver ion complex-dispersed mixture, and C) adding a reducing agent to the silver ion complex-dispersed mixture to reduce the silver ions into silver atoms.

[15] In the method of the present invention, the branched chain carboxylic acid having 6 to 14 carbon atoms or its salt binds, as a ligand, to silver ions to form a silver ion complex or encompasses silver ions as a capping material. The branched chain carboxylic acid has a bulky effective volume due to carbon branch, and cannot achieve a close bond between complexes as a straight chain carboxylic acid ligand can. Thus, a complex of straight chain carboxylic acid ligand and silver ions forms a highly systematical and stable organization between complexes, while a complex of the branched chain carboxylic acid ligand and silver ions does not do so. Accordingly, the complex of branched chain carboxylic acid ligand and silver ions has a weaker interaction between complexes than the complex of straight chain carboxylic acid ligand and silver ions, and thus can be dissolved or dispersed in a nonpolar organic solvent more easily, unlike the complex of straight chain carboxylic acid ligand and silver ions. The present invention reduces a silver ion complex dispersed uniformly in an organic solvent, and thus can produce a conductive ink with better dispersion of silver particle than the prior art.

[16] In the present invention, the branched chain carboxylic acid having 6 to 14 carbon atoms or its salt is not limited to a specific type, and preferably is neohexanoic acid, neoheptanoic acid, isooctanoic acid, isononanoic acid, neodecanoic acid, neoundecanoic acid and so on. And, the source of silver ions is preferably silver- containing materials used typically in the art, for example, silver salts such as silver nitrate.

[17] In the present invention, the organic solvent used to disperse the silver ion complex may be properly selected by an ordinary person skilled in the art, depending on the conditions for producing an end-use conductive ink, for example, inkjet printing characteristics such as viscosity, sintering temperature, sintering time, conductivity and so on. The organic solvent follows a common definition given in the art. Actually, it is enough if the organic solvent is capable of dissolving or dispersing a branched chain carboxylic acid ligand having 6 to 14 carbon atoms or a capping material, and has neither too high nor too low viscosity and a lower boiling point than a sintering tern- perature. The non-polar organic solvent is mainly effective to dissolve or disperse a hydrocarbon part of the carboxylic acid ligand or capping material. The non-polar organic solvent is, for example, tetradecane, toluene, xylene, hexane, tetrahydrofuran or cyclohexane. The silver ion complex of the present invention can be dissolved or dispersed in the organic solvent by simple agitation or mild reaction conditions of agitation with weak heating.

[18] In the present invention, a silver ion complex-dispersed mixture, in which the complex of silver ions and the branched chain carboxylic acid having 6 to 14 carbon atoms or its salt is dispersed in the organic solvent, is prepared in this way. A reducing agent is added to the silver ion complex-dispersed mixture to obtain a conductive silver ink. The reducing agent used to reduce the silver ion complex is not limited to a specific type if it is a typical one used in the art. For example, the reducing agent is phenylhydrazine, triethylamine, hydrazine, aldehyde, ascorbic acid, ten - butylhydroxytoluene and so on.

[19] Preferably, when mixing the silver ion complex with the reducing agent, an equivalence ratio (a mole ratio when 1 mole is 1 equivalent in the reducing agent) is silver ion complex:reducing agent = 1:0.5 to 1:1. When the equivalence ratio is in this range, it results in low specific resistivity and low sintering temperature while maintaining the particle size of silver particles to a small level. And, it prevents waste of a reducing agent, resulting in economical efficiency. If the equivalence ratio of the reducing agent to the silver ion complex is less than 0.5, silver particles are not completely reduced, failing to reach a desired level of conductivity. If the equivalence ratio of the reducing agent to the silver ion complex is more than 1, the amount of a reducing agent is excessively larger relative to silver particles, which is a waste of the reducing agent, and the particle size of the reduced silver particles is large and consequently the sintering temperature increases, bringing about unfavorable results. Meanwhile, the amount of the reducing agent may be decreased to a proper level in conformity with a reducing solvent, for example, ethylene glycol.

[20] The conductive ink of the present invention may further comprise a binder, and selectively an additive. For desired end-use material characteristics, the binder and additive may be selected by an ordinary person skilled in the art with reference to the prior art, and the detailed description is omitted. For example, the binder is nitrocellulose. And, the additive includes a stabilizer, a dispersant, a reducing agent, a surfactant, a wetting agent, a thixotropic agent, a levelling agent, an anti-foamant, a coupling agent, a surface tension adjusting agent, a thickener and so on.

[21] According to another aspect, the present invention provides a conductive circuit board or conductive film having a wire formed using the conductive ink. An exemplary method for manufacturing a conductive circuit board or conductive film is described in brief. A wire can be formed by printing the conductive paste or ink on a circuit board made of metal, glass, plastic and so on, by inkjet printing, spin coating, screen printing and so on. At this time, the wire is formed on a base film that is formed on the surface of the circuit board. The base film of the circuit board may have a wiring pattern scanned thereon in advance by photolithography or screen printing. The paste is sprayed in conformity with the scanned wiring pattern to form a film including a conductive filler. The circuit board having the paste printed thereon is sintered to remove a solvent and so on, and to merge silver particles. If necessary, a multilayered circuit board may be fabricated through subsequent processes including stacking, thin- film forming, plating and so on.

[22] For low temperature sintering, the sintering process in the method of the present invention is preferably performed at temperature of 130 to 15O ⁰ C, thereby achieving compatibility between low sintering temperature and conductivity. If the sintering process is performed at temperature lower than 130 ⁰ C, as sintering of silver nanoparticles is delayed or incomplete, it is difficult to form a wire having a desired level of conductivity. If the sintering process is performed at temperature higher than 150 ⁰ C, it results in a narrow range of materials suitable for a film to be printed thereon.

[23] The conductive silver ink obtained by the method of the present invention has a promising application to an antenna for a RFID system. For this purpose, the conductive silver ink preferably should have a specific resistivity of 1.7xlO ⁵ Ω-cm or less after sintering. The ink can satisfy a specific resistivity of the above-mentioned level or lower by properly controlling a ratio of a reducing agent and sintering temperature according to the technical spirit of the present invention. For example, when an equivalence ratio of a reducing agent to a silver ion complex approaches 1:1, the specific resistivity tends to be lowered. For this reason, a more amount of the reducing agent is added or a solvent is replaced by a reducing solvent so as to control conductivity. And, compatibility between low sintering temperature and conductivity can be achieved by optimizing the sintering temperature. Control of specific resistivity through an equivalence ratio of a reducing agent and sintering temperature will be understood more clearly with reference to the following examples and Tables 1 to 3. Mode for the Invention

[24] Hereinafter, the present invention will be described in detail through examples and an exemplary production method. The description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention. [25] Conductive silver inks of examples according to the method of the present invention and conventional conductive silver inks of comparative examples were prepared, and their performance was compared. Production of an ink according to the examples and the comparative examples is described in detail below.

[26] Production of a soluble silver ion complex in a nonpolar organic solvent

[27] 0.4 mol of branched chain carboxylic acid was dissolved in methanol at a concentration of about 0.4 M. This methanol solution was mixed with about 1000 mL of an aqueous solution including 0.4 mol of sodium hydroxide, and the mixed solution was agitated. And, about 1000 mL of an aqueous solution including 0.4 mol of silver nitrate was added to the mixed solution and agitated to obtain a white silver complex precipitate. The silver complex precipitate was filtered, washed with an aqueous solution several times and then with a polar organic solvent (acetone or ethanol) several times, and dried to obtain a silver ion complex.

[28] Example 1

[29] 9.9x10 ³ mol of the silver ion complex was dissolved in 20 mL of tetradecane, and

9.9x10 ³ mol of phenylhydrazine as a reducing agent and 0.005 g of terpineol were added to obtain a dark brown silver ink (a mole ratio of silver ion complex:reducing agent = 1:1).

[30] Example 2

[31] 9.9x 10 ³ mol of the silver ion complex was dissolved in 20 mL of tetradecane, and

5x IO ⁴ mol of phenylhydrazine as a reducing agent and 0.005 g of terpineol were added to obtain a dark brown silver ink (a mole ratio of silver ion complex:reducing agent = 1:0.5).

[32] Comparative example 1

[33] 9.9x10 ³ mol of the silver ion complex was dissolved in 20 mL of tetradecane, and

9.9xlO ⁴ mol of phenylhydrazine as a reducing agent and 0.005 g of terpineol were added to obtain a dark brown silver ink (a mole ratio of silver ion complex:reducing agent = 1:0.1).

[34] Comparative example 2

[35] 9.9x10 ³ mol of the silver ion complex was dissolved in 20 mL of tetradecane, and

0.005 g of terpineol was added to obtain a dark brown silver ink (a mole ratio of silver ion complex:reducing agent = 1:0).

[36] Comparative example 3

[37] 9.9x10 ³ mol of the silver ion complex was dissolved in 20 mL of tetradecane, and

1.98xlO ^~2 mol of phenylhydrazine as a reducing agent and 0.005 g of terpineol were added to obtain a dark brown silver ink (a mole ratio of silver ion complex:reducing agent = 1:2).

[38] Comparative example 4 [39] 9.9x10 ³ mol of silver nanoparticles obtained by reacting the silver ion complex with phenylhydrazine were dissolved in 20 rnL of tetradecane, and 0.005 g of terpineol was added to obtain a dark brown silver ink.

[40] Manufacture of a conductive film

[41] Each conductive silver ink according to examples and comparative examples was filled in a polyethylene container, and printed on a polyimide film using a lithographic printer with a piezoelectric inkjet print head. The printed film was sintered at each temperature shown in Table 1 for 10 minutes to obtain a conductive film.

[42] Measurement of surface tension

[43] Surface tension was measured using KSV BPA tensiometer. The preferred surface tension range of a conductive silver ink is 20 to 40 dyn/cm.

[44] Measurement of viscosity

[45] Viscosity was measured using Malcom viscometer according to JIS standards. The preferred viscosity range of a conductive silver ink is 10 t 25 cPs.

[46] Measurement of specific resistivity

[47] Specific resistivity was measured using a 4-probe tester (Hitachi) from the inputted area and thickness. The preferred specific resistivity range of a conductive silver ink is 1.7OxIO ⁵ Ω-cm or less that is suitable for printed electronics such as a RFID antenna.

[48] The composition of the conductive silver inks according to examples and comparative examples and their surface tension, viscosity and specific resistivity are shown in Table 1.

[49] Table 1

[Table 1] [Table ]

[50] The inks of comparative examples 1 to 3 comprising no reducing agent or a small or excessive amount of a reducing agent have higher specific resistivity than a suitable level for a RFID antenna, and cannot be sintered at low temperature. The ink of comparative example 4 produced by reducing a silver ion complex and then dissolving the silver ion complex in an organic solvent has a proper specific resistivity but cannot be sintered at low temperature. The inks of examples according to the present invention satisfied all the conditions of low specific resistivity and low sintering temperature. In addition to specific resistivity and sintering temperature, the inks of examples satisfied the surface tension and viscosity ranges for printing.

[51] Meanwhile, the ink of the present invention that can be sintered at low temperature exhibits a satisfactory specific resistivity as a wiring material, but if the ink is sintered at high temperature, it is questionable whether the ink can achieve a lower specific re- sistivity. Thus, the inventors studied the relationship between sintering temperature and specific resistivity in the same ink composition. Tables 2 and 3 show the final specific resistivity of conductive inks according to example 1 and comparative example 2, respectively, while varying the sintering temperature of the ink composition.

[52] Table 2 [Table 2] [Table ]

The relationship between sintering temperature and specific resistivity (composition according to example 1)

[53] Table 3 [Table 3] [Table ]

The relationship between sintering temperature and specific resistivity (composition according to comparative example 2)

[54] In Table 2, a low sintering temperature range of the present invention is preferably 130 to 150 ⁰ C, and it is found that the sintered ink has a lowest specific resistivity or a next level thereto. In Table 2, when the sintering temperature is 15O ⁰ C, the lowest specific resistivity is exhibited. Although the sintering temperature is more than 150° C, the specific resistivity does not decrease any longer. On the contrary, in Table 3, the ink of comparative example does not reach a target conductivity of the present invention over the entire temperature range. Therefore, it is found through data of Tables 2 and 3 that if an ink composition according to the method of the present invention is sintered at 130 to 15O ⁰ C, both of advantages obtained from low sintering temperature and high conductivity can be satisfied without sacrifice of specific resistivity.

[55] As such, the preferred embodiments of the present invention were described hereinabove. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.