JP2021028416A - Coating liquid for conductive film formation and method for producing conductive film - Google Patents

Abstract

To stably remarkably reduce the generation frequency of recessed defects in a surface of a conductive film in which a metal nanowire is used for a conductive filler.SOLUTION: An aqueous coating liquid is obtained by dispersing metal nanowires into a liquid medium comprising water, water-soluble cellulose ether and propylene glycol and is attained by a coating liquid for conductive formation in which the water-soluble cellulose ether has a gelling temperature T1 is 70°C or more in a cellulose ether aqueous solution with a concentration of 2 mass%. The content of the water-soluble cellulose ether in the coating liquid can be controlled to, for example, 0.01 to 1.00 mass%. The content of the propylene glycol in the coating liquid can be controlled to, for example, 0.1 to 5.0 mass%. The water-soluble cellulose ether is, for example, HEMC (hydroxyethylmethyl cellulose).SELECTED DRAWING: None

Description

The present invention relates to a coating liquid for forming a conductive film containing silver nanowires, and a method for producing a conductive film using the same.

In the present specification, a fine metal wire having a thickness of about 200 nm or less is referred to as “nanowire (s)”. The metal nanowire is useful as a conductive material for forming a translucent conductive film. Translucent conductive films using metal nanowires as conductive materials are being put to practical use in displays for large electronic devices such as personal computers and televisions, as well as touch panels for small electronic devices such as car navigation systems, smartphones, and electronic dictionaries. The liquid applied on the base material to form a coating film is called a "coating liquid". The metal nanowires are dispersed in the coating liquid for obtaining a conductive film using metal nanowires as a conductive material. The liquid that is used is usually used. The coating liquid usually contains a thickening substance or the like in order to obtain properties suitable for coating. As a thickening substance that can be used in a coating liquid containing metal nanowires. , HPMC (Hydroxypropyl Methyl Cellulose) and HEMC (Hydroxyethyl Methyl Cellulose) are known as water-soluble cellulose ethers.

Patent Document 1 describes that water and a co-solvent are used as a solvent for an ink composition containing metal nanowires. Examples of the co-solvent include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, propylene glycol, propylene glycol methyl ether, and ethylene glycol (claim 5). These include high boiling alcohols such as propylene glycol and ethylene glycol. According to Patent Document 1, the use of a high boiling point additive such as propylene glycol (PG) is effective as a non-aqueous carrier of an organic ink (paragraphs 0052, 0053, Table 2). However, it is taught that the co-solvent of the aqueous liquid carrier has a boiling point of 150 ° C. or lower (paragraph 0030), and no specific example of the aqueous ink containing a high boiling point alcohol such as propylene glycol is shown (paragraph 0030). Paragraphs 0066 to 0073). Further, Patent Document 1 includes hydroxypropylmethyl cellulose (HPMC), hydroxypropyl cellulose (HPC), mail cellulose, ethyl cellulose, xanthan rubber, polyvinyl alcohol, polyvinylpyrrolidone (PVP) carboxylmethyl cellulose, and hydroxyethyl cellulose as thickening substances. (Paragraph 0039), HPMC is used in examples and comparative examples of water-based inks (paragraphs 0066, 0070).

Patent Document 2 lists many substances other than water for use as an aqueous solvent for silver nanowire dispersions, such as alcohol-based, polyol-based, ether-based, amide-based, nitrile-based, and aldehyde-based substances. (Paragraph 0020). There is also a description of propylene glycol in it. However, the coating liquids specifically shown are only organic ones using isopropyl alcohol (paragraph 0053). Further, it is taught that there is no limitation on the thickener of the dispersion liquid, and examples of the water-soluble cellulose ether include methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxymethyl cellulose, hydroxypropyl cellulose, and hydroxypropyl methyl cellulose (paragraph 0034). ..

Patent Documents 3 and 4 describe water-based silver nanowire inks containing HEMC (hydroxyethyl methyl cellulose). In these documents, low boiling point alcohols such as methanol, ethanol, and isopropyl alcohol are preferably used as the solvent component of the water-based ink (paragraph 0026 of Patent Document 3, paragraph 0017 of Patent Document 4), and Examples thereof. , Isopropyl alcohol is used in the comparative example. These documents do not describe the inclusion of high boiling point alcohols such as propylene glycol in water-based inks.

Special Table 2013-544917 Special Table 2016-507640 JP-A-2018-3014 JP-A-2018-141239

When a translucent conductive film is produced by applying a coating liquid containing metal nanowires to the surface of a substrate and then drying the conductive film, a point-like coating film defect in which a part of the surface of the conductive film is locally recessed occurs. There is. This type of defect is called a "concave defect". Conductivity is reduced in the portion where the concave defect is generated. When applying the conductive film to an industrial product such as a touch panel, it is necessary to collect a portion that does not contain a concave defect. The occurrence of concave defects becomes a major problem because it causes a significant decrease in product yield, especially in large display applications such as televisions.

It is common practice to adjust the properties of the coating liquid so as to maintain good coatability of the conductive film. However, in the conventional techniques such as Patent Documents 1 to 4, the above-mentioned concave defect is not taken into consideration. It can be said that the occurrence of concave defects is a new problem that has emerged in order to develop it for large display applications.

The present invention is to provide a technique for stably and remarkably reducing the frequency of occurrence of concave defects on the surface of a conductive film, which has become a problem in the manufacture of a conductive film using metal nanowires. is there.

The above object is an aqueous coating solution in which metal nanowires are dispersed in a liquid medium containing water, water-soluble cellulose ether, and propylene glycol, and the water-soluble cellulose ether is an aqueous cellulose ether solution having a concentration of 2% by mass. It is achieved by a coating liquid for forming a conductive film having a gelling temperature T _{1 of 70 ° C. or higher.} The content of the water-soluble cellulose ether in the coating liquid can be, for example, 0.01 to 1.00% by mass. The content of propylene glycol in the coating liquid can be, for example, 0.1 to 5.0% by mass. For example, HEMC (hydroxyethyl methyl cellulose) can be applied as the water-soluble cellulose ether.

A monohydric alcohol having 4 or less carbon atoms can be contained in the coating liquid as a component of the liquid medium. The content of the monohydric alcohol is preferably in the range of, for example, 60% by mass or less. Preferred specific examples of the metal nanowires include silver nanowires.

Further, in the present invention, the step of applying the above coating liquid onto the substrate,
The step of drying the coating film after coating,
A method for producing a conductive film including.
As a condition for drying the coating film, for example, a condition in which the coating film temperature satisfies the following equation (1) can be adopted.
Coating film temperature (° C) ≤ T ₁ +50 ... (1)

As an index for specifying the gelation-related properties of the water-soluble cellulose ether, in the present invention, the gelation temperature T ₁ (° C.) measured for an aqueous solution sample in which the water-soluble cellulose ether is dissolved at a concentration of 2% by mass is used. adopt. In the case of a coating liquid containing two or more kinds of water-soluble cellulose ethers, gelation is performed with a sample of a 2% by mass cellulose ether aqueous solution containing each water-soluble cellulose ether at a mixing ratio of each water-soluble cellulose ether in the coating liquid. Identify the temperature T _1. T ₁ can be obtained as follows.

_{[How to determine the gelation temperature T 1} (° C) of water-soluble cellulose ether]
Prepare an aqueous solution sample in which the water-soluble cellulose ether to be measured is dissolved at a concentration of 2% by mass. The viscosity (mPa · s) is measured by a vibration viscometer of a method in which the initial temperature of the aqueous solution sample is set to 30 ° C. and the temperature is raised at a heating rate of 1 ° C./min while the viscous resistance is obtained by rotational vibration (twisting). The measurement results are plotted on a semi-logarithmic graph in which the horizontal axis is the temperature (° C.) and the vertical axis is the logarithm of the viscosity (mPa · s) to obtain a temperature-viscosity curve. In the case of a 2% by mass aqueous solution of water-soluble cellulose ether, in the above semi-log graph, the viscosity decreases substantially linearly as the temperature rises from the initial temperature until the property changes due to gelation. Then, when gelation occurs, the behavior of viscosity with temperature change suddenly changes. That is, the slope of the temperature-viscosity curve changes suddenly. The behavior of sudden changes differs slightly depending on the type of water-soluble cellulose ether. For example, in the case of methyl cellulose, the slope generally starts to rise. In the case of HPMC (hydroxypropyl methylcellulose) and HEMC (hydroxyethylmethylcellulose), the slope generally suddenly changes from a gentle negative value to a large negative value. It is considered that this is because the hydroxy group and the surrounding water molecules are phase-separated by heating, and as a result, the molecular chains are aggregated and the viscosity is lowered. As the temperature is further increased, as a result of hydrophobic interaction, the slope often starts to increase as in the case of methyl cellulose. Therefore, the temperature-viscosity behavior changes depending on the content ratio of hydroxy group and methyl group. Here, the temperature at which the behavior of the slope of the temperature-viscosity curve first begins to change in the temperature raising process, that is, the first sudden change start temperature is defined as the gelation temperature T ₁ (° C.).

Japanese Patent Application Laid-Open No. 9-197306 describes that the gelation temperature of the water-soluble cellulose ether was determined by such a method.

According to the present invention, in the production of a conductive film using metal nanowires as a conductive filler, it is possible to stably suppress the occurrence of the above-mentioned "concave defect", and a translucent conductive film particularly applied to a large display. It is expected that the yield will be greatly improved in mass production.

According to the studies by the inventors, it was found that the above-mentioned concave defects occur in the step of drying the coating film applied on the substrate. It was speculated that the thickening substance contained in the coating liquid was locally gelled due to the temperature rise during drying of the coating film, and the gelation phenomenon was the cause of the occurrence of concave defects. In order to prevent gelation of the thickening substance, it is effective to dry the coating film at a temperature lower than the gelling temperature of the thickening substance. However, drying at a low temperature (for example, about 50 ° C. or lower) is inefficient in volatilizing an aqueous solvent substance, and is not suitable for industrial mass production.

On the other hand, if a thickening substance having a high gelation temperature is used, it is considered that the occurrence of concave defects can be suppressed without maintaining the coating film temperature at the time of drying at a low temperature. Water-soluble cellulose ether is a thickening substance useful for obtaining a conductive film using metal nanowires. Some of them have a relatively high gelation temperature, such as HEMC (hydroxyethyl methyl cellulose). However, even if a water-soluble cellulose ether having a relatively high gelation temperature is used, it is not possible to sufficiently suppress the occurrence of concave defects when the coating film temperature during drying is raised to, for example, 80 ° C. or higher. There wasn't.

Therefore, the inventors further researched. As a result, it is extremely effective to (i) use a water-soluble cellulose ether having a relatively high gelation temperature as the thickening substance to be contained in the coating liquid, and (ii) to contain propylene glycol in the coating liquid. It turned out that there was. The present invention is based on such findings. The mechanism by which the occurrence of concave defects is remarkably suppressed by containing propylene glycol has not been fully elucidated. It is presumed that the coexistence of propylene glycol exerts some action that acts to prevent gelation, such as an increase in the apparent gelation temperature of the water-soluble cellulose ether.

[Water-based coating liquid]
The coating liquid according to the present invention is composed of a liquid medium and a solid substance existing in the liquid medium. The solid substance is specifically a metal nanowire. The liquid medium contains at least water, water-soluble cellulose ether, and propylene glycol as components. The water-soluble cellulose ether is a solid substance at room temperature by itself, but it is a component of a liquid medium because it is dissolved in a solvent substance in the coating liquid of the present invention. The "water-based coating liquid" means a coating liquid in which the mass ratio of water is 30% or more with respect to the total mass of the liquid including the metal nanowires which are solid substances. Water is the main solvent for dissolving water-soluble cellulose ethers. Considering the wettability to a translucent substrate such as PET (polyethylene terephthalate), it is preferable to use a "mixed solvent" in which an organic solvent that mixes well with water, such as a monohydric alcohol having 4 or less carbon atoms, is added. There is also. The water-soluble cellulose ether used as a thickener has high solubility in water, but it may be difficult to dissolve in a "mixed solvent" of water and an organic solvent when the content ratio of water is small. As a result of the examination, if the coating liquid has a water content of 30% by mass or more, it is possible to dissolve the water-soluble cellulose ether and adjust the viscosity to be suitable for coating. The water content may be set to 50% by mass or more, or 70% by mass or more. Increasing the proportion of water improves the solubility of the water-soluble cellulose ether and increases the degree of freedom in adjusting the viscosity.

When a "mixed solvent" of water and an organic solvent is used as the liquid medium of the water-based coating liquid, it is preferable to use a monohydric alcohol having 4 or less carbon atoms as the organic solvent. Specifically, alcohols such as methanol, ethanol, 2-propanol (isopropyl alcohol), 2-methyl-1-propanol, and 1-butanol are suitable targets. Since these do not exhibit excessive hydrophobicity, they are preferable organic solvents for ensuring sufficient solubility of the water-soluble cellulose ether in a mixed solvent with water. Moreover, since the boiling point is not excessively high, it is relatively easy to evaporate and remove the coating film in the drying step even if it is contained in a large amount as a solvent. If the content of the monohydric alcohol having 4 or less carbon atoms is too high, the solubility of the water-soluble cellulose ether decreases. Since the coating liquid of the present invention has a water content of 30% by mass or more, it is possible to obtain a coating liquid in which the water-soluble cellulose ether is completely dissolved, but the degree of freedom of the water-soluble cellulose ether content is high. From the viewpoint of expanding the above, it is desirable that the content of monohydric alcohol having 4 or less carbon atoms in the coating liquid is 60% by mass or less, and even if it is limited to the range of 50% by mass or less or 30% by mass or less. Good. In order to improve the wettability of the coating liquid with a translucent base material such as PET, it is effective that the content of the monohydric alcohol having 4 or less carbon atoms in the coating liquid is 1% by mass or more. Yes, it is more effective to set it to 5% by mass or more.

A substance having a function as a binder or the like can be contained in the coating liquid as necessary, as long as the effect of the present invention is not impaired.

[Water-soluble cellulose ether]
When preparing the coating liquid, it is common to add a thickening substance in order to obtain properties suitable for coating. There are various thickening substances, and as one of the thickening substances suitable for a water-based coating liquid for forming a conductive film using metal nanowires as a conductive filler, water-soluble cellulose ether can be mentioned. The water-soluble cellulose ether imparts water solubility by substituting most of the hydrogen atoms of the OH group contained in cellulose with substituents. For example, when the hydrogen atom of the OH group _{is replaced with a methyl group (-CH 3} ), a methoxy group (-OCH ₃ ) is formed, and when _{it is replaced with a hydroxypropyl group (-CH 2} CHOHCH ₃ ), a hydroxypropoxy group (-OCH ₂ CHOHCH) is formed. ₃ ) is formed, and when _{substituted with a hydroxyethyl group (-CH 2} CH ₂ OH), a hydroxyethoxy group (-OCH ₂ CH ₂ OH) is formed. At the portion having these substituents, the hydrogen bond between the cellulose molecules is dissolved, so that there is room for water molecules to enter between the cellulose molecules, and the cellulose molecules become water-soluble. HPMC (hydroxypropylmethylcellulose) is obtained by introducing a methoxy group (-OCH ₃ ) and a hydroxypropoxy group (-OCH ₂ CHOHCH ₃ ) as its substituents, and a methoxy group and a hydroxyethoxy group (-OCH ₂ CH ₂ OH) are introduced. ) Is introduced into HEMC (hydroxyethyl methyl cellulose). In addition, MC (methyl cellulose) is introduced with only a methoxy group (-OCH ₃ ), and HEC (hydroxyethyl cellulose) is introduced with only _{a hydroxyethoxy group (-OCH 2} CH _{2 OH).} HPMC, HEMC, MC, HEC and the like are usually supplied to the market as industrial materials in the form of dry powder.

In the present invention, a water-soluble cellulose ether is contained as a thickening substance. However, it is important that the water-soluble cellulose ether to be contained has the above-mentioned gelation temperature T ₁ of 70 ° C. or higher. When a gelling temperature T ₁ of 70 ° C. or higher is applied, concave defects are generated even when the coating film is dried at a highly practical temperature (for example, 80 ° C. or higher) due to coexistence with propylene glycol described later. Can be significantly suppressed. Two or more kinds of water-soluble cellulose ethers may be used in combination. In that case, a sample of a 2% by mass aqueous solution of water-soluble cellulose ether can be prepared at a mixing ratio of each water-soluble cellulose ether, and the gelation temperature T ₁ can be specified.

The gelation temperature of the water-soluble cellulose ether varies depending on the type of substituent. According to the studies by the inventors, HEMC (hydroxyethyl methyl cellulose) can be mentioned as a preferable water-soluble cellulose ether for setting _{the gelation temperature T 1 to 70 ° C. or higher.} For example, in the case of HEMC having a methoxy group content of 23% by mass and a hydroxyethoxy group content of 12% by mass, the above-mentioned gelation temperature T _{1 with} a 2% by mass aqueous solution was 75 ° C. As the thickener, such a water-soluble cellulose ether having a high gelation temperature may be used alone, or such a water-soluble cellulose ether having a high gelation temperature may be used in combination with another water-soluble cellulose ether. _{Therefore, the gelation temperature T 1} of the water-soluble cellulose ether composed of one or more can be set to 70 ° C. or higher.

The content of the water-soluble cellulose ether present in the coating liquid can be adjusted according to the viscosity required for the coating liquid. For example, when the content of the water-soluble cellulose ether in the coating liquid is in the range of 0.01 to 1.00% by mass, it becomes easy to optimize the viscosity of the coating liquid for forming a conductive film using metal nanowires. It is more preferably in the range of 0.05 to 0.50% by mass, and may be in the range of 0.05 to 0.20% by mass.

[Propylene glycol]
In the present invention, propylene glycol is present in the coating liquid together with the above-mentioned water-soluble cellulose ether. In a coating liquid in which propylene glycol coexists with water-soluble cellulose ether and is dissolved in an aqueous liquid medium, it is possible to remarkably suppress the occurrence of concave defects. Even if propylene glycol is not contained, the effect of reducing the frequency of occurrence of concave defects can be obtained by adopting the above-mentioned water-soluble cellulose ether having a high gelation temperature as the thickening substance. However, the effect is limited, and it is insufficient as a remarkable improvement effect for stably suppressing the occurrence of concave defects. The mechanism by which the synergistic action of coexistence of water-soluble cellulose ether having a high gelation temperature and propylene glycol can obtain a remarkable improving effect, which is the object of the present invention, has not been elucidated. It is considered that some action that makes it difficult for the water-soluble cellulose ether to gel, such as an increase in the apparent gelation temperature of the ether, occurs. Propylene glycol is an alcohol having a relatively high boiling point of 188 ° C. under atmospheric pressure, but propylene glycol present in the water-based coating liquid can be evaporated together with water in the step of drying the coating film. The content of propylene glycol in the coating liquid is preferably in the range of 0.1 to 5.0% by mass, more preferably 0.3 to 3.0% by mass. The content ratio of propylene glycol to the water-soluble cellulose ether is preferably in the range of 4 to 100 by mass.

[Metal nanowires]
The metal nanowire used in the coating liquid for forming a conductive film of the present invention preferably has a shape as long as possible from the viewpoint of forming a conductive film having excellent conductivity, and has excellent visibility (characteristic with less haze). From the viewpoint of forming a conductive film, it is preferable that the shape is as thin as possible. Specifically, it is preferable that the average length is 8 μm or more and the average diameter is 50 nm or less. More preferably, the average length is 10 μm or more and the average diameter is 30 nm or less. The average aspect ratio is preferably 200 or more, more preferably 450 or more. The content of metal nanowires in the coating liquid can be, for example, in the range of 0.05 to 3.0% by mass. As the metal nanowire, known metal nanowires such as silver nanowires and copper nanowires can be used. Particularly for silver nanowires, the development of highly practical manufacturing technology is progressing, and commercially available silver nanowires can be used in the present invention. The average length, average diameter, and average aspect ratio of metal nanowires follow the definitions below.

(Average length L _M)
The trace length from one end to the other end of a single metal nanowire on an image observed by a field emission scanning electron microscope (FE-SEM) is defined as the length of the wire. The value obtained by averaging the lengths of the individual metal nanowires present on the microscope image, is defined as the average length L _M. In order to calculate the average length, the total number of wires to be measured is 100 or more.

(Average diameter D _M )
On a bright-field observation image by a transmission electron microscope (TEM), the distance between contours on both sides in the thickness direction of a single metal nanowire is defined as the diameter of the wire. Each wire can be considered to have a substantially uniform thickness over the entire length. Therefore, the thickness can be measured by selecting a portion that does not overlap with other wires. In a TEM image showing one field of view, among the metal nanowires observed in the image, the diameters of all the wires except the wires that completely overlap with the other wires and whose diameter is difficult to measure are measured. The operation is performed on a plurality of randomly selected visual fields, the diameters of a total of 100 or more different metal nanowires are obtained, the average value of the diameters of the individual metal nanowires is calculated, and the value is defined _{as the average diameter D M.}

(Average aspect ratio)
_{The average aspect ratio A M} is calculated by substituting the above average diameter D _M and average length L _M into the following equation (2). However, the D _M, L _M is expressed in units of any nm value assigned to equation (2).
_{A M = L M / D M} ... (2)

[Manufacturing method of conductive film]
The above-mentioned coating liquid according to the present invention can be applied to a conventional general coating film forming method in which a coating film is applied onto a substrate and then the coating film is dried. As the coating method, a method suitable for mass production such as a die coater method or a bar coater method may be adopted. It is known that the above-mentioned concave defects, which are surface defects of the conductive film, occur in the process of drying the coating film. The main cause is considered to be the phenomenon that the thickening substance added to the coating liquid locally gels. It is effective to raise the coating film temperature during drying in order to efficiently dry the coating film, but with the conventional coating liquid, if the coating film temperature during drying is increased, the frequency of occurrence of concave defects increases. It becomes a problem. Since gelation of the thickening substance is unlikely to occur in the coating liquid according to the present invention, it is possible to remarkably reduce the frequency of occurrence of concave defects without keeping the coating film temperature at the time of drying particularly low. As a result, productivity and quality improvement of the conductive film can be balanced at a high level. In order to suppress the concave defects of the conductive film extremely remarkably, it is effective to dry under the condition that the coating film temperature satisfies the following equation (1).
Coating film temperature (° C) ≤ T ₁ +50 ... (1)
Here, T _{1 means the above-mentioned gelation temperature T 1} (° C.) evaluated by a 2% by mass aqueous solution of water-soluble cellulose ether. If the coating film temperature is low, the productivity decreases. Considering the productivity, it is more effective to dry under the condition that the coating film temperature satisfies the following formula (1)'.
T ₁ ≤ coating film temperature (° C) ≤ T ₁ +50 ... (1)'

[Comparative Example 1]
(Metal nanowire dispersion)
Silver nanowires were synthesized based on the method disclosed in JP-A-2018-071000 to obtain a metal nanowire dispersion liquid A in which the silver nanowires were dispersed. This metal nanowire dispersion liquid A contains 0.4% by mass of silver nanowires in terms of silver concentration, and the residual component is water. Average length L _M of the silver nanowires contained in the metal nanowire dispersion A is 16 [mu] m, an average diameter D _M was 23 nm. This silver nanowire has a copolymer of vinylpyrrolidone and diallyldimethylammonium nitrate attached to the surface as an organic protective agent, and has good dispersibility in a liquid.

(Water-soluble cellulose ether aqueous solution)
As a water-soluble cellulose ether, a powder of HPMC (hydroxypropylmethyl cellulose; containing 21.5% by mass of methoxy groups and 30.0% by mass of hydroxypropoxy groups) was prepared. The HPMC powder was put into hot water that had been strongly stirred with a stirrer, and then naturally cooled to 40 ° C. while continuing strong stirring, and then cooled to 10 ° C. or lower using a chiller. The cooled liquid was filtered through a metal mesh having an opening of 100 μm to remove the gel-like insoluble component, and a water-soluble cellulose ether aqueous solution C1 having a water-soluble cellulose (HPMC) concentration of 1.24% by mass was obtained.

(Gelation temperature of water-soluble cellulose ether T ₁ )
As an aqueous solution sample for measuring the gelation temperature T ₁ , a water-soluble cellulose ether aqueous solution X1 having a water-soluble cellulose ether (HPMC) concentration of 2% by mass was used in the same process as the above-mentioned water-soluble cellulose ether aqueous solution C1. Obtained. Here, a preliminary experiment is conducted to understand the relationship between the amount of HPMC powder charged and the concentration of water-soluble cellulose ether obtained after filtration, and based on the preliminary experiment data, the concentration of water-soluble cellulose ether is 2% by mass. A water-soluble cellulose ether aqueous solution X1 adjusted to the above was obtained. Using this water-soluble cellulose ether aqueous solution X1, the _{gelation temperature T 1} was determined _{according to the above-mentioned “Method for determining the gelation temperature T 1} (° C.) of the water-soluble cellulose ether”. The viscosity was measured using a rheometer (model number MCR92) manufactured by Anton Pearl Co., Ltd. in a vibration mode.
_{The gelation temperature T 1} of the water-soluble cellulose ether (HPMC) used in this example was 45 ° C.

(Coating liquid for forming a conductive film)
To 5000 g of the metal nanowire dispersion liquid A, 583 g of pure water and 867 g of the water-soluble cellulose ether aqueous solution C1 were added, and the mixture was stirred with a stirrer for 2 hours. Then, 4300 g of a 50% aqueous solution of 2-propanol was added, and the mixture was stirred for 12 hours to obtain a coating liquid for forming an aqueous conductive film. When the metallic silver concentration of the coating liquid for forming a conductive film was measured by ICP emission spectroscopic analysis, the metal nanowire content was 0.18% by mass in terms of metallic silver. In this example, the liquid medium constituting the coating liquid consists of a mixed solvent composed of water and 2-propanol, and a water-soluble cellulose ether dissolved in the solvent. The metal nanowire content in the total mass of the coating liquid is 0.18% by mass, the water-soluble cellulose ether content is 0.10% by mass, the 2-propanol content is 20% by mass, and the remainder thereof is water. This coating liquid does not contain propylene glycol. Since the mass ratio of the organic protective agent adhering to the surface of the metal nanowire is very small with respect to the total mass of the coating liquid, it can be ignored in the composition display of the coating liquid (the same in each of the following examples). .).

(Conducting film)
As a base material, a PET film having a thickness of 100 μm (Cosmo Shine (registered trademark) A4100 manufactured by Toyobo Co., Ltd.) was prepared. Using a coating device equipped with a die coater and a drying furnace, the coating liquid was applied onto the substrate by a slot die coater under the condition of a wet coating thickness of 15 μm while moving the substrate film in one direction at a transport speed of 2 m / min. .. The formed coating film was continuously sent to a drying furnace, and the coating film was dried under the condition that the coating film temperature was 90 ° C. In this way, coating was performed so that the total area of the coating film was about 500 m ^2, and a conductive film (dry coating film) having silver nanowires as a conductive filler was obtained.

(Evaluation of concave defects)
In the above coating apparatus, the surface of the dried coating film discharged from the drying furnace was visually observed over the entire length in the film transport direction, and the number of concave defects having a maximum diameter of the contour of the recessed portion of 1 mm or more was counted. By dividing the total number of counts by the total area of the dry coating film, the ^{number of counts per 1 m 2} was calculated, and the value was defined as "concave defect occurrence frequency (pieces / m ² )". Under these test conditions, if the frequency of concave defects is ^{less than 0.01 pieces / m 2,} the water-based coating liquid used to form the coating film will be the conventional water-based coating liquid using metal nanowires as the conductive filler. In comparison, it is evaluated as having a very excellent performance of suppressing the occurrence of concave defects.
The frequency of concave defects in this example was 4.2 / m ² .

[Comparative Example 2]
The experiment was carried out under the same conditions as in Comparative Example 1 except that the coating liquid for forming the conductive film was prepared as follows.
(Coating liquid for forming a conductive film)
To 5000 g of the metal nanowire dispersion liquid A, 368 g of pure water and 867 g of the water-soluble cellulose ether aqueous solution C1 were added, and the mixture was stirred with a stirrer for 2 hours. Then, 4300 g of a 50% aqueous solution of 2-propanol and 215 g of propylene glycol were added and stirred for 12 hours to obtain an aqueous coating film for forming a conductive film. When the metallic silver concentration of the coating liquid for forming a conductive film was measured by ICP emission spectroscopic analysis, the metal nanowire content was 0.18% by mass in terms of metallic silver. In this example, the liquid medium constituting the coating liquid consists of a mixed solvent consisting of water and 2-propanol, propylene glycol, and a water-soluble cellulose ether dissolved in the liquid components thereof. The metal nanowire content in the total mass of the coating liquid is 0.18% by mass, the water-soluble cellulose ether content is 0.10% by mass, the propylene glycol content is 2.0% by mass, and the 2-propanol content is 20. It is by mass% and these residues are water.

Using this coating liquid, a conductive film was prepared by the same procedure as in Comparative Example 1, and the frequency of occurrence of concave defects was investigated. As a result, the frequency of concave defects in this example was 1.0 pieces / m ² .

[Comparative Example 3]
The experiment was carried out under the same conditions as in Comparative Example 1 except that the water-soluble cellulose ether aqueous solution was prepared, the gelation temperature T ₁ was measured, and the coating liquid for forming the conductive film was prepared as follows.
(Water-soluble cellulose ether aqueous solution)
As a water-soluble cellulose ether, a powder of HEMC (hydroxyethyl methyl cellulose; containing 23.9% by mass of methoxy groups and 9.4% by mass of hydroxyethoxy groups) was prepared. 37.5 g of the above HEMC powder was put into 2500 g of boiling water at 99 ° C. which was strongly stirred with a stirrer, and the strong stirring was continued for 24 hours, and then cooled to 10 ° C. The cooled liquid was filtered through a metal mesh having an opening of 100 μm to obtain a HEMC aqueous solution from which coarse gel-like foreign substances were removed (this is referred to as a “raw material HEMC aqueous solution”).
When acetone (manufactured by Wako Pure Chemical Industries, Ltd.) in a volume ratio of 8 times the volume of the raw material HEMC aqueous solution was added to the obtained "raw material HEMC aqueous solution" at room temperature, HEMC was precipitated. After precipitation, stirring was performed for 30 minutes. A precipitate was obtained by this operation. The liquid containing the precipitate was filtered through a metal mesh having an opening of 150 μm, and HEMC was recovered as a solid content. The obtained solid content was vacuum-dried, and the obtained powder was added to acetone (manufactured by Wako Pure Chemical Industries, Ltd.) in an amount of twice the volume of the above "raw material HEMC aqueous solution" and stirred for 30 minutes. .. After stirring, the liquid containing the solid content was filtered again with a metal mesh having an opening of 150 μm, and HEMC was recovered as the solid content. This solid content was vacuum dried to obtain HEMC powder (hereinafter, this is referred to as "purified HEMC powder").
37.5 g of the above-mentioned purified HEMC powder was put into 2500 g of pure water at 99 ° C., which was strongly stirred with a stirrer, and the strong stirring was continued for 24 hours, and then cooled to 10 ° C. The gel-like insoluble component was removed by filtering the cooled liquid with a metal mesh having an opening of 100 μm to obtain a water-soluble cellulose ether aqueous solution C2 having a water-soluble cellulose (HEMC) concentration of 1.00% by mass.

(Gelation temperature of water-soluble cellulose ether T ₁ )
As an aqueous solution sample for measuring the gelation temperature T ₁ , a water-soluble cellulose ether aqueous solution X2 having a water-soluble cellulose ether (HEMC) concentration of 2% by mass was used in the same process as the above-mentioned water-soluble cellulose ether aqueous solution C2. Obtained. Here, a preliminary experiment was conducted on the step of obtaining a water-soluble cellulose ether aqueous solution from the above-mentioned purified HEMC powder, and the relationship between the input amount of the purified HEMC powder and the water-soluble cellulose ether concentration obtained after filtration was grasped. Based on the preliminary experiment data, a water-soluble cellulose ether aqueous solution X2 having an adjusted water-soluble cellulose ether concentration of 2% by mass was obtained. Using this water-soluble cellulose ether aqueous solution X2, the _{gelation temperature T 1} was determined _{according to the above-mentioned "Method for determining the gelation temperature T 1} (° C.) of the water-soluble cellulose ether". The viscosity was measured using a rheometer (model number MCR92) manufactured by Anton Pearl Co., Ltd. in a vibration mode.
_{The gelation temperature T 1} of the water-soluble cellulose ether (HEMC) used in this example was 75 ° C.

(Coating liquid for forming a conductive film)
To 5000 g of the metal nanowire dispersion liquid A, 2633 g of pure water and 967 g of the water-soluble cellulose ether aqueous solution C2 were added, and the mixture was stirred with a stirrer for 2 hours. Then, 2150 g of a 50% aqueous solution of 2-propanol was added, and the mixture was stirred for 12 hours to obtain a coating liquid for forming an aqueous conductive film. When the metallic silver concentration of the coating liquid for forming a conductive film was measured by ICP emission spectroscopic analysis, the metal nanowire content was 0.18% by mass in terms of metallic silver. In this example, the liquid medium constituting the coating liquid consists of a mixed solvent composed of water and 2-propanol, and a water-soluble cellulose ether dissolved in the solvent. The metal nanowire content in the total mass of the coating liquid is 0.18% by mass, the water-soluble cellulose ether content is 0.09% by mass, the 2-propanol content is 10% by mass, and the remainder of these is water. is there. This coating liquid does not contain propylene glycol.

Using this coating liquid, a conductive film was prepared by the same procedure as in Comparative Example 1, and the frequency of occurrence of concave defects was investigated. As a result, the frequency of concave defects in this example was 1.2 pieces / m ² .

[Example 1]
The experiment was carried out under the same conditions as in Comparative Example 3 except that the coating liquid for forming the conductive film was prepared as follows.
(Coating liquid for forming a conductive film)
To 5000 g of the metal nanowire dispersion liquid A, 375 g of pure water and 860 g of the water-soluble cellulose ether aqueous solution C2 were added, and the mixture was stirred with a stirrer for 2 hours. Then, 4300 g of a 50% aqueous solution of 2-propanol and 215 g of propylene glycol were added and stirred for 12 hours to obtain an aqueous coating film for forming a conductive film. When the metallic silver concentration of the coating liquid for forming a conductive film was measured by ICP emission spectroscopic analysis, the metal nanowire content was 0.18% by mass in terms of metallic silver. In this example, the liquid medium constituting the coating liquid consists of a mixed solvent consisting of water and 2-propanol, propylene glycol, and a water-soluble cellulose ether dissolved in the liquid components thereof. The metal nanowire content in the total mass of the coating liquid is 0.18% by mass, the water-soluble cellulose ether content is 0.08% by mass, the propylene glycol content is 2.0% by mass, and the 2-propanol content is 20. By mass%, these residues are water.

Using this coating liquid, a conductive film was prepared by the same procedure as in Comparative Example 1, and the frequency of occurrence of concave defects was investigated. As a result, the frequency of concave defects in this example was less than ^{0.01 / m 2.}

[Example 2]
The experiment was carried out under the same conditions as in Comparative Example 3 except that the coating liquid for forming the conductive film was prepared as follows.
(Coating liquid for forming a conductive film)
To 5000 g of the metal nanowire dispersion liquid A, 2418 g of pure water and 967 g of the water-soluble cellulose ether aqueous solution C2 were added, and the mixture was stirred with a stirrer for 2 hours. Then, 2150 g of a 50% aqueous solution of 2-propanol and 215 g of propylene glycol were added and stirred for 12 hours to obtain an aqueous coating film for forming a conductive film. When the metallic silver concentration of the coating liquid for forming a conductive film was measured by ICP emission spectroscopic analysis, the metal nanowire content was 0.18% by mass in terms of metallic silver. In this example, the liquid medium constituting the coating liquid consists of a mixed solvent consisting of water and 2-propanol, propylene glycol, and a water-soluble cellulose ether dissolved in the liquid components thereof. The metal nanowire content in the total mass of the coating liquid is 0.18% by mass, the water-soluble cellulose ether content is 0.09% by mass, the propylene glycol content is 2.0% by mass, and the 2-propanol content is 10. By mass%, these residues are water.

Using this coating liquid, a conductive film was prepared by the same procedure as in Comparative Example 1, and the frequency of occurrence of concave defects was investigated. As a result, the frequency of concave defects in this example was less than ^{0.01 / m 2.}

[Example 3]
The experiment was carried out under the same conditions as in Comparative Example 3 except that the coating liquid for forming the conductive film was prepared as follows.
(Coating liquid for forming a conductive film)
To 5000 g of the metal nanowire dispersion liquid A, 2095 g of pure water and 1290 g of the water-soluble cellulose ether aqueous solution C2 were added, and the mixture was stirred with a stirrer for 2 hours. Then, 2150 g of a 50% aqueous solution of 2-propanol and 215 g of propylene glycol were added and stirred for 12 hours to obtain an aqueous coating film for forming a conductive film. When the metallic silver concentration of the coating liquid for forming a conductive film was measured by ICP emission spectroscopic analysis, the metal nanowire content was 0.13% by mass in terms of metallic silver. In this example, the liquid medium constituting the coating liquid consists of a mixed solvent consisting of water and 2-propanol, propylene glycol, and a water-soluble cellulose ether dissolved in the liquid components thereof. The metal nanowire content in the total mass of the coating liquid is 0.13% by mass, the water-soluble cellulose ether content is 0.12% by mass, the propylene glycol content is 2.0% by mass, and the 2-propanol content is 10. It is by mass% and these residues are water.

Using this coating liquid, a conductive film was prepared by the same procedure as in Comparative Example 1, and the frequency of occurrence of concave defects was investigated. As a result, the frequency of concave defects in this example was less than ^{0.01 / m 2.}
The above results are shown in Table 1.

The conductive films of Examples 1 to 3 using the coating liquid in which the water-soluble cellulose ether having a gelation temperature T _{1 of 70 ° C. or higher and propylene glycol coexist had an extremely low frequency of concave defects.}
On the other hand, in Comparative Example 1, a _{water-soluble cellulose ether having a low gelation temperature T 1} was applied as a thickening substance, and a conventional general coating liquid to which propylene glycol was not added was used. In this case, the conductive film had many concave defects.
In Comparative Example 2, a coating liquid in which water-soluble cellulose ether and propylene glycol coexist was used, but the effect of suppressing the occurrence of concave defects was insufficient because _{the gelation temperature T 1 of the water-soluble cellulose ether was low.} ..
In Comparative Example 3 _{, a water-soluble cellulose ether having a gelation temperature T 1} of 70 ° C. or higher was applied as a thickening substance, but since propylene glycol was not added to the coating liquid, concave defects were generated. The deterrent effect was insufficient.

Claims (9)

A water-based coating liquid in which metal nanowires are dispersed in a liquid medium containing water, water-soluble cellulose ether, and propylene glycol. The water-soluble cellulose ether is gelled with a cellulose ether aqueous solution having a concentration of 2% by mass. A coating liquid for forming a conductive film having a temperature T _{1 of 70 ° C. or higher.} The coating film for forming a conductive film according to claim 1, wherein the content of the water-soluble cellulose ether is 0.01 to 1.00% by mass. The coating film for forming a conductive film according to claim 1 or 2, wherein the content of propylene glycol is 0.1 to 5.0% by mass. The coating film for forming a conductive film according to any one of claims 1 to 3, wherein the water-soluble cellulose ether is HEMC (hydroxyethyl methyl cellulose). The coating liquid for forming a conductive film according to any one of claims 1 to 4, which contains a monohydric alcohol having 4 or less carbon atoms as a component of the liquid medium. The coating film for forming a conductive film according to claim 5, wherein the content of the monohydric alcohol having 4 or less carbon atoms is 60% by mass or less. The coating liquid for forming a conductive film according to any one of claims 1 to 6, wherein the metal nanowire is a silver nanowire. A step of applying the coating liquid according to any one of claims 1 to 7 onto a substrate,
The step of drying the coating film after coating,
A method for producing a conductive film including. The method for producing a conductive film according to claim 8, wherein the coating film after coating is dried under the condition that the coating film temperature satisfies the following formula (1).
Coating film temperature (° C) ≤ T ₁ +50 ... (1)