Transparent electrically conductive substrate and manufacturing method thereof

Abstract

Method of manufacturing a transparent electrically conductive substrate having an application process whereby a wet layer is formed by applying onto a substrate film a coating liquid comprising metallic nanowires dispersed in a solvent, and a drying process whereby the solvent contained in the abovementioned wet layer is removed by drying, characterised in that the abovementioned drying process includes a process whereby the orientation of the abovementioned metallic nanowires is altered by introducing a forced draft facing towards the substrate from a direction that is different from the longitudinal direction of the substrate film.

Claims

1. A method of manufacturing a transparent electrically conductive substrate comprising: a step of applying a coating liquid containing metallic nanowires dispersed in a solvent onto a substrate to form a wet layer thereon, a step of drying the wet layer on the substrate to remove the solvent therefrom: wherein the step of drying includes introduction of a forced draft facing towards the wet layer from a direction that is different from the longitudinal direction of the substrate on which the wet layer is formed, and the forced draft strikes the metallic nanowires protruding from a surface of the wet layer and alters an orientation of the metallic nanowires, such that the metallic nanowires become oriented non-uniformly.

2. The method of manufacturing a transparent electrically conductive substrate according to claim 1, wherein the forced draft is introduced facing towards the wet layer after the thickness of the wet layer has been reduced to 13 μm or less.

3. The method of manufacturing a transparent electrically conductive substrate according to claim 1, wherein the forced draft is introduced facing towards the wet layer in a direction substantially perpendicular to the longitudinal direction of the substrate film after the thickness of said wet layer has been reduced to 13 μm or less.

4. The method of manufacturing a transparent electrically conductive substrate according to claim 1, wherein the forced draft is introduced facing towards the wet layer at a wind speed of 4 to 20 m/s after the thickness of said wet layer has been reduced to 13 μm or less.

5. The method of manufacturing a transparent electrically conductive substrate according to claim 1, wherein the forced draft, regulated to a temperature of 30 to 60° C., is introduced facing towards the wet layer after the thickness of said wet layer has been reduced to 13μm or less.

6. A method of manufacturing a transparent electrically conductive substrate comprising: an unwinding step whereby a substrate film wound in the form of a roll is unwound, an application step whereby a wet layer is formed by applying onto said substrate film a coating liquid that contains metallic nanowires dispersed in a solvent and that is squeezed out from a slot die, a transport step whereby the substrate film on which the wet layer has been formed is transported to a drying step, the drying step whereby solvent contained in the wet layer is removed by drying, and a rewinding step whereby the resulting transparent electrically conductive substrate is rewound, wherein: the drying step includes introduction of a forced draft facing towards the wet layer from a direction that is different from the longitudinal direction of the substrate film on which the wet layer is formed, and the forced draft strikes the metallic nanowires protruding from a surface of the wet layer and alters an orientation of the metallic nanowires, such that the metallic nanowires become oriented non-uniformly.

7. The method of manufacturing a transparent electrically conductive substrate according to claim 6, wherein the forced draft is introduced facing towards the wet layer after the thickness of the wet layer has been reduced to 13 μm or less.

8. The method of manufacturing a transparent electrically conductive substrate according to claim 6, wherein the forced draft is introduced facing towards the wet layer in a direction substantially perpendicular to the longitudinal direction of the substrate film after the thickness of said wet layer has been reduced to 13 μm or less.

9. The method of manufacturing a transparent electrically conductive substrate according to claim 6, wherein the forced draft is introduced facing towards the wet layer at a wind speed of 4 to 20 m/s after the thickness of said wet layer has been reduced to 13 μm or less.

10. The method of manufacturing a transparent electrically conductive substrate according to claim 6, wherein the forced draft, regulated to a temperature of 30 to 60° C., is introduced facing towards the wet layer after the thickness of said wet layer has been reduced to 13 μm or less.

Description

BRIEF EXPLANATION OF THE FIGURES

(1) FIG. 1 is a diagram explaining the relationship between the metallic nanowires and the forced draft in a preliminary test shown in FIG. 6.

(2) FIG. 2 is a diagram explaining the relationship between the metallic nanowires and the forced draft in an embodiment of the present invention.

(3) FIG. 3 is a diagram explaining the relationship between the film and the forced draft inside the drying oven in an embodiment of the present invention.

(4) FIG. 4 is a diagram explaining the relationship between the film and the forced draft in a process in which the orientation of the metallic nanowires is modified, in an embodiment of the present invention. (a) is a diagram explaining that the forced draft is introduced in the TD, and (b) is a diagram explaining that the draft blows in a direction parallel to the substrate directly above the surface of the wet layer (the centre of the draft is 1 to several cm above the surface of the wet layer).

(5) FIG. 5 is a flow diagram explaining why surface resistance anisotropy occurs.

(6) FIG. 6 is a diagram explaining a preliminary test of the present invention in which a forced draft was introduced from the substrate film TD in order to modify the orientation of the metallic nanowires after application of a coating liquid (nanowires+aqueous solvent).

(7) FIG. 7 is a diagram explaining the positional relationship between four divided zones in a drying oven used in the process in which the orientation of the metallic nanowires is modified.

MODE OF EMBODYING THE INVENTION

(8) {Metallic Nanowires}

(9) In the present invention metallic nanowires function as the main conductor, comprising a transparent conductive material. The metallic element employed in the metallic nanowires can be an element whose conductivity in the bulk state is 1×10.sup.6S/m or more. Specific examples include Ag, Cu, Au, Al, Rh, Ir, Co, Zn, Ni, In, Fe, Pd, Pt, Sn, Ti and the like. Combinations of two or more types of metallic nanowire can also be used, but from the point of view of conductivity it is preferable to use elements selected from Ag, Cu, Au, Al and Co.

(10) There is no particular limitation to the means of manufacture of the metallic nanowires, and conventional means such as liquid phase or gas phase methods can for example be used. For example, reference can be made to non-patent literature 1 and 2 for methods of manufacturing Ag nanowires, to patent literature 3 for a method of manufacturing Au nanowires, to patent literature 4 for a method of manufacturing Cu nanowires, and to patent literature 5 for a method of manufacturing Co nanowires. In particular, the methods of manufacturing Ag nanowires reported in the abovementioned non-patent literature 1 and 2 are water based and allow a large amount of Ag nanowires to be manufactured easily, and as Ag has the largest conductivity of all metals, these methods can be applied preferably as methods of manufacturing metallic nanowires relating to the present invention.

(11) With the present invention a three-dimensional conductive network is formed by the metallic nanowires coming into contact with each other, and a high conductivity is achieved, while light can be transmitted through apertures in the conductive network where there are no metallic nanowires, and thus both high conductivity and high transmittance can coexist.

(12) From considerations of transparency, the diameter of the metallic nanowires in the present invention is preferably 200 nm or less and more preferably 100 nm or less. The average length of metal nanowires is preferably 1 μm or more from considerations of conductivity, and 100 μm or less in consideration of the effect on transparency due to agglomeration. More preferably it is between 1 and 50 μm and most preferably between 3 and 50 μm.

(13) {Solvent, Preferably Aqueous Solvent}

(14) In the present invention “aqueous solvent” refers to a solvent comprising at least 50% by weight of water. Of course pure water that does not contain any other solvent may also be used, and taking into account the dispersion stability of the metallic nanowires during drying, it is preferable for the amount of other solvents to be small. There is no particular limitation to components other than water in the aqueous solvent provided that they are solvents that are miscible with water, but alcohol-based solvents can preferably be used, and of these isopropyl alcohol, whose boiling point is relatively close to that of water, is preferable.

(15) {Substrate}

(16) Any substrate can be used without any particular limitation provided that it is a transparent resin film. Preferable examples include polyester resin films such as polyethylene terephthalate (PET), polyethylene naphthalate, and modified polyester; polyolefin resin films such as polyethylene (PE) resin film, polypropylene (PP) resin film, polystyrene resin film, and cyclic olefin resin; vinyl resin films such as polyvinyl chloride and polyvinylidene chloride; polyetheretherketone (PEEK) resin film, polysulfone (PSF) resin film, polyether sulfone (PES) resin film, polycarbonate (PC) resin film, polyamide resin film, polyimide resin film, acrylic resin film, and triacetyl cellulose (TAC) resin film; but any resin film can be preferably employed as the transparent resin film in the present invention provided that it has a transmittance of at least 80% with respect to wavelengths in the visible range (380 to 780 nm). Of these, in terms of transparency, heat resistance, ease of handling, strength and cost, polyethylene terephthalate film and polycarbonate film are preferable.

(17) {Preparation of Coating Liquid}

(18) The coating liquid comprising metallic nanowires dispersed in a solvent, preferably an aqueous solvent, may contain additives and binders to adjust the viscosity, corrosion characteristics, adhesion force and the dispersion of nanowires. Examples of suitable additives and binders include, but are not limited to: carboxymethylcellulose (CMC), 2-hydroxyethyl cellulose (HEC), hydroxypropyl methylcellulose (HPMC), methylcellulose (MC), polyvinyl alcohol (PVA), tripropylene glycol (TPG), and xanthan gum (XG); surfactants such as ethoxylates, alkoxylates, ethylene oxide and propylene oxide and copolymers thereof; and sulfonates, sulfates, disulfonic acid, sulfosuccinates, phosphate esters, and fluorosurfactants (for example Zonyl (registered trademark), DuPont).

(19) {Application Method}

(20) Die coating methods and gravure coating methods may for example be employed, but since gravure coating methods leave engraved marks on the substrate, while die coating methods do not damage the substrate, die coating methods are most suitable.

(21) {Manufacturing Method for Transparent Electrically Conductive Substrate}

(22) Application process: The prepared coating liquid is applied onto the substrate using for example a die coater. There is no particular limit to the thickness of the coating liquid, but it is preferably about 10 to 30 μm. Transport process: The substrate film on which a wet layer has been formed in the application process is transported to a drying process. There is no particular limit to the transportation means, but a roll conveyor is typical. If the transport process is long, the thickness of the wet layer may be reduced somewhat during said process. In such cases the forced draft used to modify the orientation of the nanowires may be introduced immediately in the drying process described hereinbelow. Further, if the transport process is short then it is preferable for a certain amount of drying to be performed to reduce the thickness of the wet layer before introducing the forced draft to modify the orientation of the nanowires. Drying process: There is no particular limit to the drying method, and examples include heating methods using an infra-red heater or the like, or methods that heat by means of a drying draft, but if air stagnates within the drying oven then the concentration of solvent vapour will increase and drying will take longer so it is preferable to use a drying draft. In order to prevent the coating surface from becoming rough it is preferable for the drying draft to blow in the opposite direction to the direction of progress of the film. Further, it is preferable for the drying draft to flow at a distance of ten-odd to several tens of cm from the surface of the film. It should be noted that a drying draft is not essential in the drying process, and drying can be performed using only the draft in the process that is provided in the drying process to modify the orientation of the nanowires. In this case, the draft used to modify the orientation of the nanowires has a greater drying effect that the drying draft.
{Process for Modifying the Orientation of the Metallic Nanowires}

(23) The process for modifying the orientation of the metallic nanowires is provided in the drying process.

(24) Specifically, the means involves introducing a draft facing towards the wet layer on the substrate from a direction different to the MD. The anisotropy can be improved most effectively if the direction of the draft is the TD. When the substrate film is viewed from above, it is most preferable for the draft to blow from one side to the other sides (in the TD) (refer to FIG. 4(a)). The centre of this draft preferably blows in a direction parallel to the substrate directly above (1 to several cm above) the surface of the wet layer (refer to FIG. 4(b)). If the draft blows such that it blows onto the wet layer then there is a danger that the smoothness of the substrate surface may be disturbed. The height of the draft is adjusted appropriately such that the draft strikes nanowires protruding from the substrate surface without disturbing the smoothness of the substrate surface.

(25) The wind speed of the draft is preferably 4 to 20 m/s, and most preferably 8 to 12 m/s. If the wind speed is less than 4 m/s then the effect of changing the state of orientation of the nanowires is poor, and if it exceeds 20 m/s there is a danger that the smoothness of the substrate surface will be disturbed.

(26) The temperature of the draft is preferably 30 to 60° C. If the temperature of the draft is higher then there is a danger that the appearance may be degraded, for example by the coating layer turning white. Further, if the temperature of the draft is higher the drying time is shortened, and the point at which the orientation of the nanowires can be modified becomes narrower. Conversely, if the temperature of the draft is lower, then the effect on the drying of the wet layer will be less and thus the point at which the orientation of the nanowires can be modified will become longer, but drying time will become longer so a temperature above ambient temperature, in particular 30° C. or higher, is preferable. It should be noted that if a drying draft is used in the drying process, then if the temperature of this draft (draft used to alter the orientation of the nanowires) and the temperature of the drying draft are the same then a single temperature control device is sufficient.

(27) In order to reduce the forced draft time or increase the line speed it is in particular preferable to introduce the draft to alter the orientation of the nanowires after the thickness of the wet layer has been reduced to 13 μm or less by means of a drying draft or the like, and it is particularly preferable to introduce the draft after the thickness of the wet layer has been reduced to 10 μm or less. Further, introduction of the forced draft may be continued until the wet layer is completely dry, but it is also acceptable to stop the forced draft when the thickness of the wet layer has been reduced to an extent such that the nanowires have lost their freedom, even if some aqueous solvent still remains. Specifically, the forced draft may be stopped when the thickness of the wet layer is in the region of 5 μm or less. Drying may then be performed if necessary using a drying draft or the like.

(28) {Surface Resistance of Transparent Electrically Conductive Substrate}

(29) The surface resistance of transparent electrically conductive substrate of the present invention is represented by Formula 1 mentioned below, where R.sub.MD is defined as the surface resistance in the MD of the substrate film and R.sub.TD is defined as the surface resistance in the TD.
R.sub.TD/R.sub.MD=0.8 to 1.2 (In particular 0.9 to 1.1 is preferable.)  (1)[Formula 1]

(30) The present invention will now be described more specifically with reference to embodiments, but the technical scope of the present invention is not limited by these examples. Further, in the embodiments % refers to % by weight in all cases unless specified otherwise.

(31) [Embodiments]

(32) {Embodiments 1-6}

(33) <Coating Liquid>

(34) Coating liquid: Mixture of 0.10 wt % metallic nanowires (fibre length: 1 to 100 μm) and 99.90 wt % solvent (pure water).
<Method of Manufacture> Application process: Using die coating method, the coating liquid is squeezed out from a slot die onto the substrate film which is being transported by roll conveyor. The thickness of the wet layer immediately after application is 15.0 μm. Transport process: The substrate film onto which the coating liquid has been squeezed out is transported by roll conveyor to a drying oven. Drying process: Performed using a drying draft in a drying oven. Specifically, drying is performed by introducing drying air (40° C., 1 m/s) in the direction opposite to the direction of progress of the film, 30 cm above the surface of the wet layer. It should be noted that the drying draft was introduced in a zone in which a forced draft was not introduced from the TD. In other words, in the present embodiment a drying draft is not blown in zones in which a forced draft is introduced from the TD. Process to modify the orientation of the metallic nanowires: As shown in the following figure, the drying process (drying oven) is divided into four zones, and as shown in FIG. 1 a draft (40° C., 10 m/s) is introduced from the TD direction. (Embodiments 1-6)

COMPARATIVE EXAMPLE 1

(35) The same draft as in the embodiments was introduced from the TD before the substrate film was transported to the drying process, immediately after the coating liquid had been applied onto the substrate film from a slot die, and this is defined as Comparative example 1.

(36) It should be noted that the film transport speed in Embodiments 1 to 6 and Comparative example 1 was 15 m/s.

(37) <Measurement of Surface Resistance>

(38) Two metal electrodes of length 30 mm and width 7 mm were prepared and were affixed such that the distance between the electrodes was 24 mm, the surfaces of the electrodes were connected to the clips of a tester (digital multimeter AD-5536 manufactured by A and D) using conducting wires, and this device was use to measure the resistances of the transparent electrically conductive substrates that had been obtained. The results are shown in Table 3.

(39) TABLE-US-00003 TABLE 3 Comp. Embod. 1 Embod. 2 Embod. 3 Embod. 4 Embod. 5 Embod. 6 example 1 Location of 4.sup.th zone 3.sup.rd zone 3.sup.rd and 4.sup.th 2.sup.nd zone 1.sup.st zone 1.sup.st and 2.sup.nd Immediately forced draft zones zones after from TD application Variation in 6.8 to 0.1 7.4 to 6.8 7.4 to 0.1 10.5 to 7.4 13.1 to 10.5 13.1 to 7.4 15 thickness of wet layer when forced draft introduced from TD (μm) Surface 87 95 81 107 109 103 109 resistance (R.sub.TD) (Ω) Surface 83 90 81 90 92 95 84 resistance (R.sub.MD) (Ω) R.sub.TD/R.sub.MD 1.05 1.06 1.00 1.18 1.19 1.09 1.31

(40) Improvements in R.sub.TD/R.sub.MD were observed if a draft was blown from the TD when the thickness of the wet layer was 13 μm or less, but the improvement effects were observed particularly at 10 μm or less. It should be noted that in embodiments 5 and 6 a forced draft was immediately introduced from the TD during the drying process, and R.sub.TD/R.sub.MD improvement effects were confirmed, but it is thought that this was because the thickness of the wet layer on the substrate film transported to the drying process had already been reduced to 13.1 μm.

(41) {Embodiments 7-9}

(42) Experiments were next performed varying the speed of the draft from the TD. It should be noted that the proportion of metallic nanowires was 0.18%, and solvent (pure water) was 99.82%, the film transport speed was 10 m/s, and the draft from the TD was blown in the first and second zones. The results are shown in Table 4.

(43) TABLE-US-00004 TABLE 4 Embodiment 7 Embodiment 8 Embodiment 9 Wind speed (m/s) 4.4 6.9 10.4 Surface resistance (R.sub.TD) 53 51 51 (Ω) Surface resistance (R.sub.MD) 48 54 59 (Ω) R.sub.TD/R.sub.MD 1.10 0.94 0.86

(44) Improvements in R.sub.TD/R.sub.MD were observed by increasing the wind speed of the draft from the TD. Depending on the conditions, in some cases R.sub.MD even exceeded R.sub.TD (R.sub.TD/R.sub.MD was less than 1).

(45) A transparent electrically conductive substrate was manufactured under the same conditions as in embodiment 8, with only the film transport speed returned to 15 m/s. R.sub.TD/R.sub.MD was 1.1. If the amount of reduction in the thickness of the wet layer is within the same range, then if a draft to modify the orientation of the nanowires is employed, the slower the film transport speed, the greater is the R.sub.TD/R.sub.MD improvement effect.