COMPOSITION COMPRISING SILVER NANOWIRES AND STYRENE/(METH)ACRYLIC COPOLYMERS FOR THE PREPARATION OF ELECTROCONDUCTIVE TRANSPARENT LAYERS

Abstract

Described is a composition suitable for the preparation of an electroconductive transparent layer said composition comprising silver nanowires and dissolved styrene/(meth)acrylic copolymers.

Claims

1. A composition, comprising (A) water, (B) electroconductive nanoobjects, said electroconductive nanoobjects (B) having two external dimensions in the range of from 1 nm to 100 nm and a third external dimension in the range of from 1 μm to 100 μm, wherein a weight fraction of said electroconductive nanoobjects (B) is in the range of from 0.01 wt.-% to 1 wt.-% based on a total weight of the composition, and (C) one or more styrene/(meth)acrylic copolymers dissolved in water, wherein said dissolved copolymers (C) each has a number average molecular weight in the range of from 500 g/mol to 22000 g/mol, and a total weight fraction of said dissolved copolymers (C) is in the range of from 0.02 wt.-% to 5 wt.-%, based on the total weight of the composition.

2. The composition according to claim 1, wherein said electroconductive nanoobjects (B) have a length in the range of from 1 μm to 100 μm, and a diameter in the range of from 1 nm to 100 nm.

3. The composition according to claim 1, wherein said electroconductive nanoobjects (B) comprise one or more materials selected from the group consisting of silver, copper, gold, and carbon.

4. The composition according to claim 1, wherein said electroconductive nanoobjects (B) are selected from the group consisting of nanowires and nanotubes.

5. The composition according to claim 1, wherein a ratio of the total weight of said electroconductive nanoobjects (B) to the total weight of said dissolved copolymers (C) is in the range of from 1:20 to 20:1.

6. The composition according to claim 1, further comprising one or more additional binding agents, wherein a total weight fraction of said additional binding agents based on the total weight of the composition is equal to or less than the total weight fraction of said dissolved copolymers (C) based on the total weight of the composition.

7. The composition according to claim 1, comprising (A) water, (B) silver nanowires, wherein said silver nanowires (B) have a length in the range of from 10 μm to 50 μm and a diameter in the range of from 3 nm to 30 nm, and a weight fraction of said silver nanowires (B) is 0.5 wt.-% or less, based on a total weight of the composition, and (C) a styrene/(meth)acrylic copolymer dissolved in water, wherein said dissolved copolymer (C) has a number average molecular weight in the range of from 1700 g/mol to 15500 g/mol, and a weight fraction of said dissolved copolymer (C) is less than 2 wt.-% based on the total weight of the composition, wherein a ratio of the total weight of said silver nanowires (B) to the weight of said dissolved copolymer (C) is in the range of from 1:5 to 5:1.

8. A method for preparing an electroconductive layer on a substrate, the method comprising: applying said composition according to claim 1 to a surface of the substrate, and removing constituents which at 25° C. and 101.325 kPa are liquid from said composition to such an extent that a layer is formed on said surface of said substrate, wherein the electroconductive layer has a light transmission of 80% or more measured according to ASTM D1003 (procedure A).

9. The method according to claim 8, wherein said applying is carried out by a technique selected from the group consisting of a spin coating, a draw down coating, a roll-to-roll coating, a gravure printing, a microgravure printing, a screen-printing, a flexoprinting, and a slot-die coating.

10. The method according to claim 8, wherein said substrate comprises a material selected from the group consisting of glass and an organic polymer.

11. The method according to claim 8, wherein said removing is achieved by subjecting said composition to a temperature in the range of from 100° C. to 150° C. for a duration of 15 minutes or less.

12. An electroconductive layer, comprising: constituents of the composition according to claim 1 which at 25° C. and 101.325 kPa are solid, wherein the electroconductive layer has a light transmission of 80% or more measured according to ASTM D1003 (procedure A).

13. The electroconductive layer according to claim 12, which exhibits a haze of 2% or less as measured according to ASTM D1003 (procedure A), and a sheet resistance of 1000 Ohm/square or less as measured by the four point probe.

14. The electroconductive layer according to claim 12, which exhibits one or more of a haze of 1% or less as measured according to ASTM D1003 (procedure A), a sheet resistance of 100 Ohm/square or less as measured by the four point probe, and a light transmission of 90% or more as measured according to ASTM D1003 (procedure A).

15. An article, comprising a substrate having a surface and the electroconductive layer according to claim 12 arranged on at least a portion of said surface of said substrate.

16. The article according to claim 15, wherein said electroconductive layer has a thickness in the range of from 10 nm to 1000 nm.

17. A method for preparing the electroconductive layer according to claim 12, the method comprising: incorporating the composition into the electroconductive layer.

18. A method for preparing the article according to claim 15, the method comprising: incorporating the composition into the article.

Description

EXAMPLES

1. Examples of Electroconductive Layers on Glass Substrates Obtained by Spin-Coating

1.1 Compositions Comprising No Additional Binding Agent

[0159] An aqueous dispersion of silver nanowires (nanoobjects (B) as defined above) and an aqueous solution of a styrene/acrylic copolymer (C) as defined above (Joncryl 60, commercially available from BASF) are mixed so as to obtain an ink having a concentration of silver nanowires and a weight ratio of silver nanowires (B) to dissolved copolymer (C) as indicated in table 1.

[0160] The ink is spin-coated (Smart Coater 100) on glass substrates at various spin speeds (see table 1) for 60 sec to generate layers with different wet thickness. The layers are then dried at 130° C. for 5 min.

[0161] The sheet resistance Rsh given in Ohms/square (OPS) of the dried layer is measured by a four-point probe station (Lucas lab pro-4) and the optical properties are measured according to ASTM D1003, procedure A-Hazemeter by a haze-gard plus hazemeter (BYK Gardner). The results are compiled in table 1.

[0162] With regard to the optical properties, T refers to the light transmission and H refers to the haze of the substrate coated with the electroconductive layer. H(substrate subtracted) refers to the difference between the haze of the substrate coated with the electroconductive layer and the haze of the blank substrate (not coated with the electroconductive layer).

TABLE-US-00001 TABLE 1 Concentration Weight ratio of silver silver nanowires/ Spin H (substrate Example nanowires copolymer Speed Rsh T H subtracted) No (mg/ml) (C) (rpm) (OPS) (%) (%) (%) 1 1.2 1:2 1000 35 91.8 0.93 0.76 2 1.2 1:2 1500 36 92.1 0.89 0.72 3 2.5 1:2 3000 53 92.1 0.68 0.51 4 1.9 1:2 3000 57 92.4 0.66 0.49 5 1.2 1:2 2500 107 92.7 0.54 0.37 6 1.2 1:2 2000 85 92.5 0.61 0.44 7 2.5 1:3 1000 34 90.5 1.23 1.06 8 2.5 1:3 1500 43 91.2 0.99 0.82 9 2.5 1:3 2000 46 91.4 0.91 0.74 10 2.5 1:3 2500 73 91.6 0.82 0.65 11 2.5 1:3 3000 91 91.8 0.73 0.56 12 2.5 1:4 3000 34 91 1.09 0.92 13 1.9 1:4 1000 61 91.4 0.89 0.72 14 1.9 1:4 1500 55 91.6 0.81 0.64 15 1.9 1:4 2000 79 92 0.74 0.57 16 1.9 1:4 2500 99 92.2 0.63 0.46 17 1.9 1:4 3000 88 92.2 0.59 0.42 18 1.6   1:1.7 750 35 91.3 0.99 0.82 19 1.6   1:1.7 1000 42 91.9 0.75 0.58 20 1.6   1:1.7 1500 55 92 0.68 0.51 21 1.6   1:1.7 3000 83 92.3 0.53 0.36 22 1.4   1:1.7 3000 90 92.4 0.52 0.35

1.2 Compositions Comprising an Additional Binding Agent

[0163] An aqueous dispersion of silver nanowires (nanoobjects (B) as defined above) and an aqueous solution of a styrene/acrylic copolymer (C) as defined above (Joncryl 60, commercially available from BASF), said aqueous solution further comprising an additional binding agent, are mixed so as to obtain an ink having a concentration of silver nanowires and a weight ratio of silver nanowires (B) to dissolved copolymer (C) and additional binding agent as indicated in table 2.

[0164] The additional binding agent in examples 23-28 is hydroxypropyl methyl cellulose (HPMC, available from Aldrich) dissolved in water, i.e. an additional binding agent (F) as defined above. The additional binding agent in examples 29-32 is a copolymer of 2-ethylhexyl acrylate and methyl methacrylate having a number average molecular weight in the range of from 25000 g/mol to 200000 g/mol in the form of polymer beads having an average diameter of 80 nm dispersed in water (Acronal LR9014 from BASF), i.e. an additional binding agent (D) as defined above

[0165] The ink is spin-coated (Smart Coater 100) on glass substrates at various spin speeds (see table 2) for 60 sec to generate layers with different wet thickness. The layers are then dried at 130° C. for 5 min.

[0166] The sheet resistance (as defined above) of the dried layer is measured by a four point probe station (Lucas lab pro-4) and the optical properties (as defined above) are measured according to ASTM D1003 procedure A-Hazemeter by a haze-gard plus hazemeter (BYK Gardner). The results are compiled in table 2.

[0167] The applied amount of ink is the same in all spin coating examples. The thickness of the dried layer depends on the spin speed when using an ink of a fixed concentration. At high spin speeds there is more ink flowing away from the substrate. Thus, variation of the spin speed can be used to vary the sheet resistance and optical properties (as defined above), so as to match the requirements of different applications of transparent electroconductive layers. High spin speeds allow for generating very thin layers having high light transmission and low haze, but rather high sheet resistance. In turn, low spin speeds allow for generating thicker layers having a low sheet resistance, but a lower light transmission and a higher haze.

TABLE-US-00002 TABLE 2 Weight ratio silver nanowires/ Concentration copolymer of silver (C)/additional Spin H (substrate Example nanowires binding Speed Rsh T H subtracted) No (mg/ml) agent (rpm) (OPS) (%) (%) (%) 23 2.5 2.5:4:1 750 25 90 1.59 1.42 24 2.5 2.5:4:1 1000 44 91 1.2 1.03 25 2.5 2.5:4:1 1500 60 91.6 0.98 0.81 26 2.1 2.5:4:1 1000 70 91.6 0.94 0.77 27 1.8 2.5:4:1 1000 78 91.7 0.88 0.71 28 1.6 2.5:4:1 1000 125 92.3 0.73 0.56 29 3 3:4:2 1000 25 90.1 2.01 1.84 30 3 3:4:2 2000 51 91.3 1.3 1.13 31 3 3:4:2 3000 80 91.8 1.08 0.91 32 3 3:4:2 4000 208 92.7 0.62 0.45

2. Examples of Electroconductive Layers on Polymer Substrates Obtained by Draw-Down Coating

[0168] An aqueous dispersion of silver nanowires (nanoobjects (B) as defined above) and an aqueous solution of a styrene/acrylic copolymer (C) as defined above (Joncryl 60, commercially available from BASF) are mixed so as to obtain an ink having a concentration of silver nanowires and a weight ratio of silver nanowires (B) to dissolved copolymer (C) as indicated in table 3. The ink is ball milled for 30 minutes to improve homogenization.

[0169] The ink is applied to a polymer substrate using a draw-down bar (wet thickness t=6 μm, coating speed v=2″/sec) to obtain a layer on said substrate. The layer is then dried at 135° C. for 5 min. In examples 33 and 34 the substrate is an optical polyacarbonate foil (e. g. commercially available under the product specification Makrofol DE 1-1 175 μm from Bayer Material Science). In example 35 the substrate is an optical polyethylene terephthalate foil (Melinex 453/400, Teijing Films).

[0170] The sheet resistance (as defined above) of the dried layer is measured by a four point probe station (Lucas lab pro-4) and the optical properties (as defined above) are measured according to ASTM D1003 procedure A-Hazemeter by a haze-gard plus hazeometer (BYK Gardner). The results are compiled in table 3.

TABLE-US-00003 TABLE 3 Concentration Weight ratio of silver silver nanowires/ Example nanowires copolymer Rsh H H (substrate No (mg/ml) (C) (OPS) T (%) (%) subtracted) (%) 33 4   1:1.6 48.9 89.35 1.26 1.13 34 4 1:2 47.63 89.75 1.12 0.99 35 3.6 1:2 130 89.5 2.01 1.51

3. Measurement of the Adhesion of the Electroconductive Layer to the Substrate

3.1 Spin Coating on Glass Substrate

[0171] An aqueous dispersion of silver nanowires (nanoobjects (B) as defined above) and an aqueous solution of a styrene/acrylic copolymer (C) as defined above (Joncryl 60, commercially available from BASF) are mixed so as to obtain an ink having a concentration of silver nanowires of 2.5 mg/ml and a weight ratio of silver nanowires (B) to dissolved copolymer (C) of 1:2.

[0172] The ink is spin coated (Smart Coater 100) on a glass substrate at 3000 rpm for 60 sec to generate a layer. The layer is then dried at 130° C. for 5 min.

[0173] The adhesion of the electroconductive layer to the substrate is examined by the Scotch tape test as follows: A fresh piece of commercial 3M Scotch tape (e. g. commercially available under the product specification Scotch D. Art. Nr. 11257 from 3M) is pressed onto the surface of the electroconductive layer and then peeled off. This procedure is repeated so that the total number of peeling steps is 10. The sheet resistance (as defined above) of the dried electroconductive layer step is measured by a four point probe station (Lucas lab pro-4) before the first peeling step and after the 10th peeling step. As can be seen from table 4 the sheet resistance has not significantly changed after completion of 10 peeling steps. Accordingly, the layer has a strong adhesion to the substrate.

TABLE-US-00004 TABLE 4 Number of peeling steps Rsh (OPS) 0 54 10 54

3.2 Draw-Down Coating on a Plastic Substrate

[0174] The electroconductive layer of example 35 described above was subject to the adhesion test described above. The peeling procedure is repeated so that the total number of peeling steps is 8. The sheet resistance (as defined above) of the dried electroconductive layer step is measured by a four point probe station (Lucas lab pro-4) before the first peeling step and after the 8th peeling step. As can be seen from table 5 the sheet resistance has not significantly changed after completion of 8 peeling steps. Accordingly, the layer has a strong adhesion to the substrate.

TABLE-US-00005 TABLE 5 Number of peeling steps Rsh (OPS) 0 130 8 131

4, Conclusion

[0175] The examples as presented above show that the compositions according to the present invention (as defined above) are suitable for the preparation of an electroconductive transparent layer having [0176] a light transmission of 80% or more, in preferred embodiments of 90% or more measured according to ASTM D1003, and [0177] a sheet resistance of 1000 Ohm/square or less, in preferred embodiments less than 100 Ohm/square, and [0178] in preferred embodiments a haze more measured according to ASTM D1003 of 2% or less, in further preferred embodiments of 1.5% or less and in particularly preferred embodiments of 1% or less [0179] a strong adhesion to the substrate.

[0180] The above examples show that the haze of the obtained electroconductive layers increases, when the concentration of solid components in the ink used for layer preparation increases, provided that all other coating parameters remain unchanged. This is evident from examples 7, 29 and 33-35 each using an ink having a rather high concentration of solid constituents (but still within the range according to the present invention). Accordingly, it is concluded that the haze of the obtained electroconductive layer is significantly higher when using the ink according to US 2008/0182090, which has a significantly higher concentration of solid constituents, compared to the composition of the present invention (as defined above). However, for applications like touch panels a high haze is detrimental.