JP7688979B2 - Coating liquid for forming conductive film - Google Patents

Description

The present invention relates to a coating liquid for forming a conductive film that contains conductive particles and a binder component. In particular, the present invention relates to a coating liquid that has excellent storage stability.

Conductive coatings have been formed on substrates using coating liquids containing conductive particles. For example, transparent conductive films are used in display devices, touch panels, solar cells, etc. As a coating liquid for forming conductive films, a composition containing chain-shaped conductive particles and an alkoxysilane oligomer is known (for example, Patent Document 1). In Patent Document 1, an alkoxysilane oligomer is used as a binder component. Since the alkoxysilane oligomer easily bonds with the chain-shaped conductive particles, the strength of the resulting film can be improved.

JP 2019-157026 A

As in Patent Document 1, in a coating liquid containing conductive particles and a binder component that bonds to the conductive particles, the surfaces of the conductive particles become covered with the binder component over time. Because the binder component is insulating, a film containing conductive particles covered with the binder component is no longer able to form a conductive path through a large number of conductive particles, resulting in a problem that the conductivity is lower than that of a film formed with the initial coating liquid.

Therefore, the object of the present invention is to provide a coating liquid for forming a conductive film that has excellent storage stability.

The coating liquid for forming a conductive film of the present invention contains chain-shaped conductive particles, a high-boiling point solvent, a low-boiling point solvent, and an alkoxysilane oligomer capable of bonding to the chain-shaped conductive particles. The chain-shaped conductive particles are contained in an amount of 35 to 75 mass % based on the total amount of the chain-shaped conductive particles and the alkoxysilane oligomer. In addition, in a particle size distribution measured by a dynamic light scattering type particle size distribution analyzer, the average particle size is 100 nm or more, and the difference (D 84 -D 16 ) between the particle size D ₁₆ at 16% cumulative from the smaller particle size side and the particle size D ₈₄ at 84% cumulative from the _smaller particle size side is 200 nm or more on a _volume basis.

The mass ratio of the high-boiling point solvent to the low-boiling point solvent was set to a range of 1:9 to 1:3. Furthermore, the water content of the coating solution was set to less than 5% by mass.

The coating solution contains 0.0005 to 0.008 parts by mass of acid relative to the mass of the chain-shaped conductive particles. Furthermore, the weight-average molecular weight of the alkoxysilane oligomer is set to 3,500 to 5,000.

The coating liquid of the present invention has chain-shaped conductive particles and a binder component dispersed in a high-boiling point solvent and a low-boiling point solvent. Here, the binder component is an alkoxysilane oligomer capable of bonding to the surface of the chain-shaped conductive particles. This coating liquid contains 35 to 75 mass% of the chain-shaped conductive particles relative to the total mass of the chain-shaped conductive particles and the binder component. When the coating liquid is measured with a dynamic light scattering particle size distribution meter to obtain a particle size distribution, the difference (D 84 -D 16 ) between the particle diameter D ₁₆ at 16% cumulative from the small particle diameter side and the particle diameter D ₈₄ at 84% cumulative from the small particle diameter side on a volume basis is 200 nm or more, and the average particle diameter is 100 nm or more. Hereinafter, the above particle diameter difference (D _{84 -D 16} ) _is referred to as the width of the particle diameter distribution.

Here, a high boiling point solvent is a solvent with a boiling point of 100°C or higher at normal pressure, and a low boiling point solvent is a solvent with a boiling point of less than 100°C at normal pressure. The mass ratio of the high boiling point solvent to the low boiling point solvent is preferably 1:9 to 13:7, and most preferably 1:9 to 1:3. When expressed as the mass ratio of the high boiling point solvent to the total amount of the high boiling point solvent and the low boiling point solvent, a ratio of 0.10 to 0.65 is suitable, and 0.10 to 0.25 is most preferable. Within this range, the drying speed during application is appropriate, so that defects in the coating film such as streaks and unevenness, and solvent residue in the film, etc. are unlikely to occur. Therefore, a smooth and uniform conductive film can be formed.

Chain-shaped conductive particles are particles in which three or more primary particles (conductive inorganic particles) are linked in a chain shape. There may be a portion where the particles are branched and linked. In other words, there is a structure in which three or more primary particles are linked at the main chain portion, and branched portions may be present. In this case, the average particle size of the primary particles is preferably 2 to 50 nm. If the average particle size is too small, the crystal structure is underdeveloped and the primary particles themselves may not have sufficient conductivity. Conversely, if the average particle size is too large, it is difficult to develop a chain structure, and even if the chain structure is formed, it is difficult to effectively form a conductive path, and there is a risk that the film may not have sufficient conductivity.

Alkoxysilane oligomers are hydrolysis products (partial hydrolysates, hydrolysis polymers, etc.) obtained by hydrolyzing at least a portion of alkoxysilane, and preferably have a weight average molecular weight of 3,000 to 6,500, more preferably 3,500 to 5,000. Here, the weight average molecular weight is a value calculated in terms of polystyrene by GPC.

The coating liquid contains 35 to 75% by mass of chain-shaped conductive particles relative to the total amount of the chain-shaped conductive particles and binder components. 45 to 70% by mass is particularly preferable. If the concentration of chain-shaped conductive particles in the coating liquid is low, the formation of conductive paths may be insufficient, and the conductivity of the conductive film may be insufficient or non-uniform. Also, if the concentration of chain-shaped conductive particles is too high, the transmittance and hardness of the resulting conductive film may decrease.

In the coating liquid according to the present invention, the chain-shaped conductive particles not only exist individually, but also form aggregates that do not precipitate. In addition, primary particles that remain in an unlinked form may also exist in this coating liquid. Such a coating liquid is measured by a dynamic light scattering particle size distribution meter to have an average particle size of 100 nm or more and a particle size distribution width of 200 nm or more. In a coating liquid containing such non-precipitating aggregates, the surface area of the chain-shaped conductive particles that can react with the binder component (having hydroxyl groups, etc.) is relatively small, so the surface of the chain-shaped conductive particles is not easily covered with the insulating binder component. In addition, a conductive path is easily formed sufficiently in the conductive film formed by applying the coating liquid. On the other hand, in a coating liquid that does not have the above-mentioned particle size distribution, the chain-shaped conductive particles are highly dispersible, so that conductive paths between the chain-shaped conductive particles are not easily formed, and the surface area of the chain-shaped conductive particles is large, so the reaction between the particles and the binder component proceeds quickly, and the surface of the particles is easily covered with the insulating component. Therefore, it does not become a coating liquid for forming a conductive film with excellent storage stability.

In addition, the coating solution preferably has a combined solids concentration (total solids concentration) of the chain-shaped conductive particles and the alkoxysilane oligomer in the range of 0.1 to 15% by mass, and more preferably in the range of 0.5 to 10% by mass. When the total solids concentration is in the above range, the film thickness obtained under normal coating conditions can be controlled to an appropriate thickness. In addition, the storage stability of the coating solution is sufficient, so that the characteristics of the obtained conductive film, such as film thickness, conductivity, hardness, etc., can be stably obtained.

The viscosity of the coating solution at 20°C is preferably in the range of 0.5 to 20 mPa·s, and more preferably in the range of 1 to 10 mPa·s. If the viscosity at 20°C is in the above range, the film thickness obtained by a general coating method can be controlled to an appropriate thickness.

Furthermore, because this coating liquid contains aggregates of chain-shaped conductive particles, the viscosity is likely to increase when concentrated from a low concentration state. In a coating liquid that shows an increase in viscosity, the chain-shaped conductive particles are less likely to break apart when forming a conductive film, making it easier to form conductive paths. Specifically, when concentrated to a total solids content of 30%, the viscosity at 20°C is preferably in the range of 10 to 300 mPa·s, and more preferably in the range of 20 to 100 mPa·s.

The water content of the coating solution is preferably less than 5% by mass. If the coating solution contains 5% or more by mass of water, the hydrolysis reaction of the alkoxysilane oligomer is accelerated, and furthermore, the reaction between the surface of the chain-shaped conductive particles and the alkoxysilane oligomer is accelerated, and there is a risk that the surface of the chain-shaped conductive particles will be covered with an insulating component. A conductive film containing chain-shaped conductive particles in such a state may not have sufficient conductive paths formed within the film, and good conductivity may not be obtained.

Below, we will explain in detail each component that makes up the coating solution.

<Alkoxysilane Oligomer>
The weight average molecular weight of the alkoxysilane oligomer is preferably 3500 to 5000. The alkoxysilane oligomer is a hydrolysis polymer of an alkoxysilane represented by the following formula (1), where n is an integer of 0 to 2. When n is 3, only bonds between two molecules can be formed, so that an oligomer having a weight average molecular weight of 3500 or more cannot be formed. Here, the smaller n is the more preferable, and it is most preferable to use an alkoxysilane with n being 0. If such an alkoxysilane is used, the alkoxysilanes form a dense three-dimensional network structure, and the hardness of the entire film can be increased.

R ¹ _n -Si(OR ² ) _4-n ...(1)
Here, ^R1 and ^R2 are hydrogen atoms, halogen atoms, or unsubstituted or substituted hydrocarbon groups having 1 to 10 carbon atoms, and may be the same or different. Generally, n is an integer of 0 to 3.

The alkoxysilane oligomer present in the coating solution is preferably 1.2 to 3.3 mass %, more preferably 1.5 to 2.8 mass %, in terms of solid content. If the concentration of the alkoxysilane oligomer is low, the binder component of the film will be reduced, and the adhesion between the chain-shaped conductive particles in the film will be insufficient, which may result in insufficient hardness. If the concentration is too high, the reaction with the surface of the chain-shaped conductive particles in the coating solution will proceed excessively, which may reduce storage stability.

Specific examples of alkoxysilanes are shown in Table 1.

<Chain-shaped conductive particles>
As described above, the chain-like conductive particles are particles in which three or more primary particles are linked in a chain shape. The primary particles are conductive inorganic particles in a monodispersed state. The average particle diameter of the primary particles is 2 to 50 nm. From an image taken with a transmission electron microscope (TEM), the particle diameters of 100 random primary particles are measured, and the average value is taken as the average particle diameter of the primary particles. Furthermore, 50 random particles are selected from this image, and the number of connections of each particle is measured. The average number of connections of the 50 particles is taken as the average number of connections. The average number of connections is preferably 3 or more, and particularly preferably 5 or more. If the average number of connections of the primary particles is small, there is a risk that the effect of improving conductivity will not be sufficiently obtained.

Here, the primary particles may be conductive particles. For example, metal oxide particles can be used. Depending on the application of the film, transparency may be required. Examples of such metal oxide particles include indium oxide and tin oxide. In addition, particles containing one or more types of metal oxide as the main component and doped with tin or antimony, such as antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO), can also be used. In terms of excellent transparency, conductivity, and chemical properties, particles containing at least one of ATO and ITO are preferred, and ATO particles are particularly preferred.

The chain-shaped conductive particles are preferably surface-treated with the alkoxysilane shown in formula (1) above. By performing the surface treatment, it is possible to prevent the chain-shaped conductive particles from dispersing again into the original primary particles, as well as to improve dispersibility in the coating liquid and suppress the occurrence of aggregation such as settling.

The alkoxysilanes used here include those shown in Table 1 as well as trimethylchlorosilane where n=3, and these can be used alone or in combination of two or more.

The amount of such alkoxysilane used varies depending on the type of alkoxysilane, the particle size of the chain-shaped conductive particles, etc., but it is preferable that the ratio of the chain-shaped conductive particles to the alkoxysilane (hydrolyzable organic silicon compound/chain-shaped conductive particles, weight ratio) is in the range of 0.01 to 0.5, and more preferably 0.02 to 0.3. If the ratio of the chain-shaped conductive particles to the alkoxysilane is below the lower limit, the particles connected in a chain may return to their original primary particles in the coating liquid, and even if the connections are maintained, the dispersibility in the paint may be insufficient. For this reason, the coating formed using this paint may have high haze and insufficient antistatic performance. If the ratio of the chain-shaped conductive particles to the alkoxysilane exceeds the upper limit, the surface of the chain-shaped conductive particles may be thickly coated with hydrolyzates of the alkoxysilane, and the conductivity may decrease.

<Solvent>
The solvent used here may be one that can be removed after application by a drying process, etc. Examples of high boiling point solvents include 1-methoxy-2-propanol, diacetone alcohol, ethylene glycol, and diethylene glycol, while examples of low boiling point solvents include methanol, ethanol, isopropyl alcohol, and acetone.

The high boiling point solvent and the low boiling point solvent are preferably present as a mixture, and the mass ratio of the high boiling point solvent to the total amount of the high boiling point solvent and the low boiling point solvent is preferably 0.10 to 0.65. Using such a mixed solvent ensures an appropriate drying speed when applying the coating liquid. This mass ratio is more preferably 0.10 to 0.40, and most preferably 0.10 to 0.25.

<Acid>
Moreover, the coating liquid preferably contains an acid. Examples of the acid include hydrochloric acid, nitric acid, acetic acid, and phosphoric acid. Such an acid promotes the aggregation of the chain-shaped conductive particles and promotes the formation of a conductive path in the coating liquid. The amount of the acid is preferably 0.0005 to 0.008 parts by mass, more preferably 0.001 to 0.004 parts by mass, relative to the mass of the chain-shaped conductive particles. If the amount of the acid is large, the chain-shaped conductive particles may be excessively aggregated, and precipitation of the chain-shaped conductive particles may occur. The conductive film formed from such a coating liquid may have high haze and is not suitable as a transparent conductive film. If the amount of the acid is small, the aggregation of the chain-shaped conductive particles may not occur sufficiently, and the bond between the alkoxysilane oligomer and the chain-shaped conductive particles may be promoted, and the surface of the chain-shaped conductive particles may be covered with an insulating component. The conductive film formed using such a coating liquid may not have a sufficient conductive path formed in the film, and may show a lower conductivity than the conductive film formed from the initial coating liquid.

The manufacturing method for the coating liquid is explained in detail below.

First, a dispersion liquid is prepared in which chain-shaped conductive particles made of a conductive inorganic oxide are dispersed. Such a dispersion liquid can be prepared by the method disclosed in JP-A-2006-339113, for example. The concentration of the chain-shaped conductive particles is preferably 20% by mass or more.

Next, a first solvent containing at least one of a high boiling point solvent and a low boiling point solvent, and the aforementioned alkoxysilane oligomer are mixed into this dispersion of chain-shaped conductive particles. At this time, a total of 8.5 to 15 parts by mass of the chain-shaped conductive particles and the alkoxysilane oligomer are mixed with respect to 100 parts by mass of the dispersion. Note that the first solvent may be added after mixing the dispersion of chain-shaped conductive particles and the alkoxysilane oligomer. The mixture thus obtained is stirred for one hour at a temperature between room temperature and below the boiling point of the first solvent.

Next, a second solvent containing at least one of a high boiling point solvent and a low boiling point solvent is added to this mixture to obtain a coating liquid. The first solvent and the second solvent may be the same or different, but the obtained coating liquid contains both a high boiling point solvent and a low boiling point solvent. At this time, the chain-like conductive particles and the alkoxysilane oligomer are mixed in a total amount of 3.5 to 10 parts by mass per 100 parts by mass of the coating liquid.

The conductive film obtained by curing the coating liquid is explained below. The liquid film is heated to remove the solvent, and the dehydration condensation reaction between the chain-like inorganic conductive particles and the alkoxysilane oligomer proceeds. The conductive film has excellent electrical properties, including excellent conductivity, and is suitable for parts that require optical and electrical properties.

The substrate is not particularly limited, and any substrate that can form a uniform liquid film and can withstand the curing temperature, such as glass or plastic, can be used. The coating method is also not particularly limited, and bar coater method, dipping method, spray method, spinner method, roll coating method, gravure coating method, slit coating method, pressure coating method, etc. can be used. The average thickness of the film can be appropriately selected depending on the application. When forming a film by coating, the average thickness is preferably 80 to 300 nm.

Below, we will explain in detail examples of coating solutions that contain acid.

[Example 1]
First, 23.0 g of alkoxysilane oligomer (solid concentration 9.8% by mass) and 13.4 g of chain-shaped conductive particles (solid concentration 20.5% by mass) were mixed and stirred for 60 minutes. The total solid concentration of the alkoxysilane oligomer and chain-shaped conductive particles in this solution was 13.7% by mass. The alkoxysilane oligomer used here was obtained by hydrolyzing tetramethoxysilane and had a molecular weight of 3810. It also contained 0.002 parts by mass of hydrochloric acid relative to the solid mass. The chain-shaped conductive particles are particles in which ATO particles surface-treated with tetraethoxysilane are linked in a chain shape as primary particles.

43.6 g of isopropyl alcohol was added to this mixed solution while stirring, and the mixture was stirred for 5 minutes. Next, 20.0 g of 1-methoxy-2-propanol was added, and the mixture was stirred for 15 minutes and then filtered to prepare a coating solution with a total solids concentration of 5% by mass. The preparation conditions for the coating solution are shown in Table 2, and the physical properties of the obtained coating solution are shown in Table 3.

This coating solution was applied onto a glass substrate using a bar coater method, dried at 80°C for 1 minute, and then heated in a dryer at 130°C for 30 minutes. The surface resistance of the resulting film-coated substrate was measured. The results are shown in Table 2.

This coating solution was left to stand for 24 hours in a bath with the water temperature adjusted to 40°C. This coating solution after heating was applied to a glass substrate by spin coating, dried at room temperature for 1 minute, and then heated in a dryer at 130°C for 30 minutes. The surface resistance of the obtained film-coated substrate was measured. The results are shown in Table 4.

In the examples and comparative examples described below, the physical properties of the coating liquid and the film-coated substrate were measured and evaluated as follows.

(Average particle size, particle size distribution range)
The coating solution was measured using a dynamic light scattering particle size distribution analyzer (MICROTRAC Corp., NANOTRAC Wave 2-UT151) to determine the average particle size and particle size distribution width (particle size difference: D ₈₄ -D ₁₆ ) of the coating solution. Table 3 shows half the value of this particle size distribution width.

(viscosity)
The viscosity was measured at 20° C. using an E-type viscometer (TV-25 model, manufactured by Toki Sangyo Co., Ltd.) The viscosity was measured for a coating solution having a total solids concentration of 5% by mass and a coating solution obtained by concentrating this to a total solids concentration of 30%.

(Surface resistance)
The surface resistance was measured using a surface resistance measuring device (Hirester UX MCP-HT800 manufactured by Mitsubishi Chemical Analytech Co., Ltd.).

(Storage stability)
The storage stability was evaluated based on the ratio calculated by (surface resistance value of the substrate with a film formed by the coating liquid after heat treatment)/(surface resistance value of the substrate with a film formed by the initial coating liquid).

[Example 2]
In this example, the amount of alkoxysilane oligomer added was 20.4 g, the amount of chain-like conductive particles added was 14.6 g, and the amount of isopropyl alcohol added was 45.0 g. Except for this, the coating liquid and the film-coated substrate were prepared and evaluated in the same manner as in Example 1.

[Example 3]
In this example, the amount of alkoxysilane oligomer added was 28.1 g, the amount of chain-like inorganic conductive particles added was 11.1 g, and the amount of isopropyl alcohol added was 40.8 g. Except for this, the coating solution and the film-coated substrate were prepared and evaluated in the same manner as in Example 1.

[Comparative Example 1]
13.4 g of the same chain conductive particles (solid content concentration 20.5% by mass) as in Example 1 was added to 43.6 g of isopropyl alcohol and stirred for 5 minutes. Then, 20.0 g of 1-methoxy-2-propanol was mixed and stirred for 15 minutes. 23.0 g of the same alkoxysilane oligomer as in Example 1 was mixed and stirred for 60 minutes. At this time, the total solid content concentration of the alkoxysilane oligomer and the chain conductive particles was 5% by mass. Then, the mixture was filtered with a 1 μm nylon filter to prepare a coating solution with a total solid content concentration of 5% by mass. The concentrations of each component in the obtained coating solution are shown in Table 1. Other than this, the preparation and evaluation were performed in the same manner as in Example 1.

[Comparative Example 2]
The amount of isopropyl alcohol added was 45.0 g, the amount of chain-like conductive particles added was 14.6 g, and the amount of alkoxysilane oligomer added was 20.4 g. Other than this, a coating solution and a film-coated substrate were prepared and evaluated in the same manner as in Comparative Example 1.

[Comparative Example 3]
The amount of isopropyl alcohol added was 40.8 g, the amount of the chain-like conductive particles added was 11.1 g, and the amount of the alkoxysilane oligomer added was 28.1 g, the same as in Example 1. Except for this, a coating solution and a film-coated substrate were prepared and evaluated in the same manner as in Comparative Example 1.

Claims (7)

A coating liquid for forming a conductive film, comprising: chain-shaped conductive particles; a high-boiling point solvent; a low-boiling point solvent; and an alkoxysilane oligomer capable of bonding to the chain-shaped conductive particles,
the coating liquid contains the chain-like conductive particles in an amount of 35 to 75 mass % based on the total amount of the chain-like conductive particles and the alkoxysilane oligomer;
In the particle size distribution of the coating liquid measured with a dynamic light scattering particle size distribution meter, the coating liquid has an average particle size of 100 nm or more, and the difference (D ₈₄ -D ₁₆ ) between the particle size D ₁₆ at which the cumulative volume from the smaller particle size side is 16% and the particle size D ₈₄ at which the cumulative volume is 84% is 200 nm or more. The coating solution according to claim 1, characterized in that the mass ratio of the high boiling point solvent to the low boiling point solvent is in the range of 1:9 to 1:3. The coating solution according to claim 1 or 2, characterized in that the coating solution contains less than 5% by mass of water. The coating liquid according to any one of claims 1 to 3, characterized in that the coating liquid contains an acid , and a ratio of a mass of the acid to a mass of the chain- like conductive particles is 0.0005 to 0.008. The coating solution according to any one of claims 1 to 4, wherein the weight average molecular weight of the alkoxysilane oligomer is 3500 to 5000. The coating liquid according to any one of claims 1 to 5, wherein the chain-like conductive particles are formed by connecting three or more primary particles, and the average particle size of the primary particles is 2 to 50 nm. The coating solution according to claim 6, wherein the average number of connections of the chain-like conductive particles is 3 to 20.