Method of preparing a product comprising surface modified silver nanowires, and use of the product

Abstract

Described is a method of preparing a product comprising an amount of surface modified silver nanowires, the method comprising the following steps: preparing or providing a first surface agent compound having one or more functional groups selected from the group consisting of thiol and amine, for attaching the first surface agent compound to the surface of a silver nanowire, preparing or providing an amount of silver nanowires, attaching said first surface agent compound prepared or provided in step (a) to the surface of said silver nanowires prepared or provided in step (b), so that surface modified silver nanowires result, and optionally (d) adding one or more further constituents and/or conducting one or more further steps so that said product results. Also described are a corresponding product, corresponding uses and a method of coating the surface of an article.

Claims

1. A method of preparing a product comprising an amount of surface modified silver nanowires dispersed in a product dispersion medium, the method comprising (a) preparing or providing a first surface agent compound having one or more functional groups selected from the group consisting of thiol, for attaching the first surface agent compound to the surface of a silver nanowire, wherein said first surface agent compound comprises a poly(ethylene glycol) methyl ether thiol (PEGT), (b) preparing or providing an amount of silver nanowires which is dispersed in an intermediate dispersion medium, attaching a second surface agent compound different from the first surface agent compound to the surface of said silver nanowires, and/or the amount of silver nanowires is prepared using a polyol process and a second surface agent compound different from the first surface agent compound is attached to the surface of said silver nanowires, (c) attaching said first surface agent compound to the surface of said silver nanowires to obtain the surface modified silver nanowires, after (b) and before, in or after (c), said intermediate dispersion medium is partially or completely exchanged against an additive dispersion medium which is different from the intermediate dispersion medium, and said additive dispersion medium comprises one or more compounds selected from the group consisting of a substituted or an unsubstituted, a branched or a linear, and an aliphatic or an aromatic organic compound having a total number of carbon atoms of at least 4 and optionally (d) adding one or more further constituents and/or conducting one or more further steps to obtain said product.

2. The method according to claim 1, wherein the product dispersion medium consists of an additive dispersion medium or a mixture of an additive dispersion medium and an intermediate dispersion medium.

3. The method according to claim 1, wherein in (c) said second surface agent compound is partially or completely removed from the surface of said silver nanowires prepared or provided in (b).

4. The method according to claim 1, wherein in (b) and in one or more further steps after (c) said intermediate dispersion medium is partially or completely exchanged against an additive dispersion medium which is different from the intermediate dispersion medium.

5. The method according to claim 1, wherein in (b) the amount of silver nanowires is dispersed in the intermediate dispersion medium in a concentration of from 0.001 wt.-% to 2 wt.-%, based on a total amount of the intermediate dispersion, and the intermediate dispersion has a homogeneity index of at least 80%, and/or said product is a product dispersion which comprises an amount of the surface modified silver nanowires dispersed in the product dispersion medium in a concentration of from 0.001 wt.-% to 2 wt.-%, based on a total amount of the product dispersion and the product dispersion has a homogeneity index of at least 80%.

6. The method according to claim 1, wherein said intermediate dispersion medium and/or said additive dispersion medium has a boiling point >30° C. at 1013 hPa.

7. The method according to claim 1, wherein a ratio of a total weight of surface agent compounds attached to the surface of said surface modified silver nanowires to a total weight of said surface modified silver nanowires is in the range of from 0.01 to 0.3 and is determined by thermogravimetry.

8. The method according to claim 1, wherein said first surface agent compound has a weight average molecular weight of from 500 to 500 000 g/mol.

9. The method according to claim 1, wherein the product dispersion medium comprises one or more compounds selected from the group consisting of water, ethanol, 2-propanol, dipropylene glycol methyl ether acetate, butyl acetate and THF.

10. The method according to claim 1, wherein the intermediate dispersion medium comprises one or more compounds selected from the group consisting of water, acetone, methanol, ethanol, propan-1-ol, propan-2-ol, 1-hydroxybutane, 2-hydroxybutane, 1-hydroxy-2-methylpropane, 2-hydroxy-2-methylpropane, formic acid, acetic acid, propionic acid and aniline.

11. The method according to claim 1, wherein the additive dispersion medium comprises one or more compounds selected from the group consisting of 1-hydroxypentane, 2-hydroxypentane, 1-hydroxy-2-methylpentane, 2-hydroxy-2-methylpentane, diethyl ether, tetrahydrofuran, diisopropyl ether, dipropylene glycol methyl ether acetate, ethyl acetate, butyl acetate, ethyl lactate, butyl hydroxyacetate, 2-butanone, 2-pentanone, 2-hexanone isophorone and an aromatic hydrocarbon.

Description

DESCRIPTION OF FIGURES

(1) FIG. 1: In FIG. 1, surface modified silver nanowires having PVP attached to their surface, dispersed in dipropylene glycol methyl ether acetate are shown. FIG. 1 shows that the surface modified silver nanowires having attached to their surface PVP agglomerate and are thus not randomly dispersed in the dipropylene glycol methyl ether acetate. The microscopy image shown in FIG. 1 was measured by dark-field microscopy.

(2) FIG. 2: In FIG. 2, a dispersion of surface modified silver nanowires having PEGT with a weight average molecular weight of 6000 g/mol attached to their surface in dipropylene glycol methyl ether acetate are shown. FIG. 2 shows that the surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol are homogeneously dispersed in the dipropylene glycol methyl ether acetate. The microscopy image shown in FIG. 2 was measured by dark-field microscopy.

(3) FIG. 3: In FIG. 3, a dispersion of surface modified silver nanowires having PEGT with a weight average molecular weight of 800 g/attached to their surface mol in a mixture of dipropylene glycol methyl ether acetate and vinyl chloride co- and acrylate as a binder are shown. FIG. 3 shows that the surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 800 g/mol are homogeneously dispersed. The image shown in FIG. 3 has been measured by scanning electron microscopy.

(4) FIG. 4: In FIG. 4, a dispersion of surface modified silver nanowires having PEGT with a weight average molecular weight of 6000 g/mol attached to their surface in a mixture of dipropylene glycol methyl ether acetate and vinyl chloride co- and acrylate as a binder is shown. FIG. 4 shows that the surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol are homogeneously dispersed. The image shown in FIG. 4 has been measured by scanning electron microscopy.

(5) FIG. 5: In FIG. 5, a dispersion of surface modified silver nanowires having PEGT with a weight average molecular weight of 6000 g/mol and PEGT with a weight average molecular weight of 800 g/mol attached to their surface in a mixture of dipropylene glycol methyl ether acetate and vinyl chloride co- and acrylate as a binder are shown. FIG. 5 shows that said surface modified silver nanowires are homogeneously dispersed. FIG. 5 was measured by scanning electron microscopy.

(6) FIG. 6: A sketch of a surface agent compound attached to the surface of a surface modified silver nanowire is shown in FIG. 6. The surface bound functional group of the surface agent compound is thiolate (deprotonated thiol group). The Ag surface is depicted by a bold horizontal line, the “backbone” of the surface agent compound is depicted by ˜˜˜.

(7) FIG. 7: A sketch of a surface agent compound attached to the surface of a surface modified silver nanowire is shown in FIG. 7. The surface bound functional group of the surface agent compound is amine. The Ag surface is depicted by a bold horizontal line, the “backbone” of the surface agent compound is depicted by ˜˜˜.

(8) FIG. 8: A sketch of an exchange of second surface agent compounds attached to the surface of a surface modified silver nanowire by first surface agent compounds is shown in FIG. 8.

(9) FIG. 9 A graph showing the haze and sheet resistance of the dried ink layers prepared in example 2.

(10) FIG. 10 A graph showing the contact angle of dried ink layers prepared in example 3.

(11) The invention is further described by the experimental examples below:

EXAMPLES

(12) Determination Methods:

(13) 1. Thermogravimetric analysis Thermogravimetric analysis was conducted on a TA instrument.

(14) 2. Optical microscopy Optical microscopy images have been obtained by dark-field microscopy on Olympus BX 51 instrument.

(15) 3. Scanning electron microscopy Scanning electron microscopy images have been obtained on a Phenom, Pro-X instrument.

(16) 4. Sheet resistance The sheet resistance of a dried layer has been measured by a four point probe station on a Lucas lab pro-4.

(17) 5. Optical properties The transmission and the haze have been measured according to ASTM D1003 by a haze-gard plus hazemeter of the supplier BYK-Gardener.

(18) 6. Contact angle experiments The contact angle of different silver nanowire layers on a polymer substrate have been measured by an OCA measuring instruments from dataphysics.

(19) 7. Homogeneity index A typical and preferred intermediate or product dispersion has a concentration of silver nanowires in the range of from 0.001 wt.-% to 2 wt.-%. In particular the homogeneity index of such intermediate or product dispersions can be determined according to the following routine: To determine the homogeneity index, a dark-field microscopic image is taken of 0.05 ml of a dispersion by an Olympus BX 51 microscope of the company Olympus. The magnification is 200×, where the ocular lens magnification of the microscope is 10× and the objective lens magnification is 20×. The size of a taken image corresponds to an area of 0.12 mm.sup.2 (which can be determined with the scale bar on the image). Subsequently, the dark-field microscopic image is divided into 400 identical square units. In a following step, the ratio of the number of square units comprising silver nanowires to the total number of overall square units is determined. For each sample, the average is calculated of three ratios obtained from a corresponding number of microscopic images of the same dispersion sample, taken at randomly chosen, different positions in the dispersion sample. This average of three ratios is defined as the homogeneity index. The homogeneity index for the perfectly homogeneously dispersed sample is 100%. A sample having a homogeneity index above 80% is considered as ‘stable’.

EXPERIMENTS

Experiment 1

Experiment 1.1

(20) Nitrogen was bubbled through 500 ml of ethanol with a flow rate of 5 ml N.sub.2/min for half an hour in order to remove the oxygen dissolved in the ethanol.

(21) Step (a):

(22) Seven solutions, each comprising one specific first surface agent compound, were prepared in the oxygen-free ethanol with different concentrations as shown in table 1a.

(23) TABLE-US-00002 TABLE 1a Different stirring times of the prepared solutions 1 to 7 solution 1 2 3 4 5 6 7 concentration of the first surface 200 50 50 200 50 200 50 agent compound (μL)

(24) As examples of first surface agent compounds, in each of the seven solutions, one of the following compounds was added: solution 1: PVP (CAS-number: 9003-39-8) having a weight average molecular weight of 40,000 g/mol (hereinafter designated as PVP), solution 2: 1-dodecanethiol, solution 3: PEGT having a weight average molecular weight of 800 g/mol, solution 4: PEGT having a weight average molecular weight of 6000 g/mol, solution 5: 1-aminohexadecane (hereinafter designated as hexadecylamine), solution 6: polyetheramine (CAS-number: 83713-01-3) having a weight average molecular weight of 2000 g/mol (hereinafter designated as polyetheramine), or solution 7: 4-methylbenzenethiol.

(25) Step (b):

(26) An intermediate dispersion with a concentration of 0.5 wt.-% of surface modified silver nanowires having PVP attached to their surface was prepared.

(27) Step (c):

(28) To 200 ml of each of the prepared surface agent compounds containing solutions, 16 ml of the prepared intermediate dispersion are added so that, in each case, a reaction dispersion is obtained, respectively (reaction dispersions 1, 2, 3, 4, 5, 6 and 7).

(29) Under a nitrogen atmosphere, each of the reaction dispersions is stirred for a period as specified in Table 1 so that the PVP attached to the surface of the surface modified silver nanowires is exchanged by the corresponding first surface agent compound (as described under step (a) above). In each case, an intermediate product dispersion comprising surface modified silver nanowires having attached to their surface the corresponding first surface agent compounds results.

(30) TABLE-US-00003 TABLE 1 Different stirring times of the reaction dispersions reaction dispersions 1 2 3 4 5 6 7 stirring time (min.) 3 3 3 120 3 120 60

(31) Step (d):

(32) Each of the intermediate product dispersions is centrifuged for 15 min at 4000 rpm and, in each case, the intermediate product dispersion medium is separated from the surface modified silver nanowires which are subsequently redispersed in 30 ml of a 1:1 ethanol/acetone solution resulting, in each case, in an ethanol/acetone dispersion.

(33) Subsequently, the centrifugation, separation, and redispersion step have been repeated for each of the seven ethanol/acetone dispersions followed by another centrifugation and another separation step so that the following six samples of solid surface modified silver nanowires resulted:

(34) sample 1: surface modified silver nanowires having PVP attached to their surface,

(35) sample 2: surface modified silver nanowires having 1-dodecanethiol attached to their surface (as an example for the method of the invention),

(36) sample 3: surface modified silver nanowires having PEGT having a weight average molecular weight of 800 g/mol attached to their surface (as an example of a product of the invention),

(37) sample 4: surface modified silver nanowires having PEGT having a weight average molecular weight of 6000 g/mol attached to their surface (as an example of a product of the invention),

(38) sample 5: surface modified silver nanowires having hexadecylamine attached to their surface (as an example of a product of the invention),

(39) sample 6: surface modified silver nanowires having polyetheramine attached to their surface (as an example of a product of the invention),

(40) and

(41) sample 7: surface modified silver nanowires having 4-methylbenzenethiol attached to their surface (as an example of a product of the invention).

(42) Subsequently, the resulting samples 1-7 were redispersed in various product dispersion media as depicted in Table 2 below and, in each case, a product dispersion resulted. The concentration of the surface modified silver nanowires having attached to their surface the corresponding surface agent compound in the resulting product dispersion was 0.1 wt.-%, based on the total weight of the product dispersion.

(43) In a next step, the homogeneity indices of the resulting product dispersions comprising (i) surface modified silver nanowires (i-comparative) having PVP attached to their surface or (i-inventive) having surface agent compounds attached to their surface and (ii) different dispersion media

(44) have been measured by optical microscopy. The dispersions comprising surface modified silver nanowires

(45) (i-inventive) having surface agent compounds attached to their surface (samples 2 to 6)

(46) are examples of products of the invention. The results are summarized in Table 2.

(47) TABLE-US-00004 TABLE 2 Cells showing the letter Y correspond to dispersions with a homogeneity index above 80% (sufficient for an industrial application in coating or printing processes). Cells of Table 2 showing the letter N correspond to dispersions with a homogeneity index below 80% (insufficient for an industrial application in coating or printing processes). product dispersion sample 1 sample 2 sample 3 sample 4 sample 5 sample 6 sample 7 medium comparative according to the invention water Y N Y Y Y Y N ethanol Y Y Y Y Y Y Y 2-propanol Y Y Y Y Y Y Y toluene N N N Y N N N dipropylene  N* Y Y  Y** Y Y Y glycol methyl ether acetate butyl acetate N N Y Y Y N N butanone N N N Y N Y N acetone N N N Y N Y N THF N Y Y Y Y Y Y *A micospcopy image of the dispersion of surface modified silver nanowires having PVP attached to their surface in dipropylene glycol methyl ether acetate is shown in FIG. 1 below. **A micospcopy image of the dispersion of surface modified silver nanowires having PEGT having a weight average molecular weight of 6000 g/mol attached to their surface in dipropylene glycol methyl ether acetate is shown in FIG. 2 below.

(48) The results depicted in table 2 show that silver nanowires having attached to the surface 1-dodecanethiol, PEGT having a weight average molecular weight of 800 g/mol and PEGT having a weight average molecular weight of 6000 g/mol, hexadecylamine, polyetheramine having a weight average molecular weight in the range of 2000 g/mol and 4-methylbenzenethiol can be dispersed in different organic compounds.

Experiment 1.2

(49) Dispersions comprising water, ethanol or 2-propanol were stored for two weeks and four weeks and their homogeneity indices were determined after storage (see table 2 i) and table ii) below, respectively).

(50) The results depicted in table 2 i) show that silver nanowires having attached to the surface 1-dodecanethiol, PEGT having a weight average molecular weight of 800 g/mol and PEGT having a weight average molecular weight of 6000 g/mol, hexadecylamine polyetheramine having a weight average molecular weight in the range of 2000 g/mol and 4-methylbenzenethiol can be homogeneously dispersed in water, ethanol or 2-propanol and are stable over a period of two weeks.

(51) In contrast thereto, silver nanowires having attached to the surface PVP were not anymore homogeneously dispersed in 2-propanol after storage over a period of two weeks.

(52) TABLE-US-00005 TABLE 2 i) Cells showing the letter Y correspond to dispersions with a homogeneity index above 80% after storage over a period of two weeks. Cells of Table 2 i) showing the letter N correspond to dispersions with a homogeneity index below 80% after storage over a period of two weeks. product dispersion sample 1 sample 2 sample 3 sample 4 sample 5 sample 6 sample 7 medium comparative according to the invention water Y N Y Y Y Y N ethanol Y Y Y Y Y Y Y 2-propanol N Y Y Y Y Y Y

(53) The results depicted in table 2 ii) show that silver nanowires having attached to the surface 1-dodecanethiol, PEGT having a weight average molecular weight of 800 g/mol and PEGT having a weight average molecular weight of 6000 g/mol hexadecylamine, polyetheramine and 4-methylbenzenethiol can be homogeneously dispersed in water, ethanol or 2-propanol and are stable over a period of four weeks.

(54) In contrast thereto, silver nanowires having attached to the surface PVP were not anymore homogeneously dispersed in 2-propanol after storage over a period of four weeks.

(55) TABLE-US-00006 TABLE 2 ii) Cells showing the letter Y correspond to dispersions with a homogeneity index above 80% after storage over four weeks. Cells of Table 2 ii) showing the letter N correspond to dispersions with a homogeneity index below 80% after storage over a period of four weeks. product dispersion sample 1 sample 2 sample 3 sample 4 sample 5 sample 6 medium comparative according to the invention sample 7 water Y N Y Y Y Y N ethanol Y Y Y Y Y Y Y 2-pro-panol N Y Y Y Y Y Y

Experiment 1.3

(56) Different Dispersions Comprising surface modified silver nanowires having PVP attached to their surface in different dispersion media (sample 1), surface modified silver nanowires having 1-dodecanethiol attached to their surface in different dispersion media (sample 2), surface modified silver nanowires having PEGT having a weight average molecular weight of 800 g/mol attached to their surface in dipropylene glycol methyl ether acetate (sample 3), surface modified silver nanowires having PEGT having a weight average molecular weight of 6000 g/mol attached to their surface in different dispersion media (sample 4), surface modified silver nanowires having polyetheramine having a weight average molecular weight in the range of 2000 g/mol attached to their surface in different dispersion media (sample 6) or surface modified silver nanowires having 4-methylbenzenethiol attached to their surface in dipropylene glycol methyl ether acetate (sample 7)

(57) were stored for two weeks and their homogeneity indices were determined after storage (see table 2 iii) below).

(58) TABLE-US-00007 TABLE 2 iii) Cells showing the letter Y correspond to dispersions with a homogeneity index above 80% after storage over a period of two weeks. Cells of Table 2 iii) showing the letter N correspond to dispersions with a homogeneity index below 80% after storage over a period of two weeks. product sample 1 sample sample sample sample sample dispersion compar- 2* 3 4* 6** 7 medium ative according to the invention dipropylene N Y Y Y Y Y glycol methyl etheracetate *Having been stored for a storage period of two weeks, sample 2 was still homogeneously dispersed when tetrahydrofuran was the dispersion medium (in contrast to a corresponding dispersion comprising silver nanowires having attached to the surface PVP). **Having been stored for a storage period of two weeks, sample 4 was still homogeneously dispersed when toluene or acetone was the dispersion medium (in contrast to a corresponding dispersion comprising silver nanowires having attached to the surface PVP). ***Having been stored for a storage period of two weeks, sample 6 was still homogeneously dispersed when butanone, tetrahydrofuran or acetone was the dispersion medium (in contrast to a corresponding dispersion comprising silver nanowires having attached to the surface PVP).

(59) The results depicted in table 2 iii) show that silver nanowires having attached to the surface 1-dodecanethiol, PEGT having a weight average molecular weight of 800 g/mol, PEGT having a weight average molecular weight of 6000 g/mol, and polyetheramine having a weight average molecular weight in the range of 2000 g/mol can be homogeneously dispersed in dipropylene glycol methyl ether acetate and that the corresponding dispersions are stable over a period of two weeks.

(60) In contrast to the results above, silver nanowires having attached to the surface PVP could not be homogeneously dispersed in the corresponding dispersion media.

Experiment 1.4

(61) Dispersions Comprising surface modified silver nanowires having PVP attached to their surface in different dispersion media (sample 1), surface modified silver nanowires having 1-dodecanethiol attached to their surface in different dispersion media (sample 2), surface modified silver nanowires having PEGT having a weight average molecular weight of 800 g/mol attached to their surface in dipropylene glycol methyl ether acetate (sample 3), surface modified silver nanowires having PEGT having a weight average molecular weight of 6000 g/mol attached to their surface in different dispersion media (sample 4), surface modified silver nanowires having polyetheramine having a weight average molecular weight in the range of 2000 g/mol attached to their surface in dipropylene glycol methyl ether acetate (sample 6), or surface modified silver nanowires having 4-methylbenzenethiol attached to their surface in dipropylene glycol methyl ether acetate (sample 7)

(62) were stored for four weeks and their homogeneity indices were determined after storage (see table 2 iv) below).

(63) TABLE-US-00008 TABLE 2 iv) Cells showing the letter Y correspond to dispersions with a homogeneity index above 80% after storage over a period of four weeks. Cells of Table 2 iv) showing the letter N correspond to dispersions with a homogeneity index below 80% after storage over a period of four weeks. product sample disper- 1 sample sample sample sample sample sion compar- 2* 3 4** 6 7 medium ative according to the invention dipropylene N Y Y Y Y Y glycol methyl ether acetate *Having been stored for a storage period of four weeks, sample 2 was still homogeneously dispersed in tetrahydrofuran (in contrast to a corresponding dispersion comprising silver nanowires having attached to the surface PVP). **Having been stored for a storage period of four weeks, sample 4 was still homogeneously dispersed in acetone or toluene (in contrast to a corresponding dispersion comprising silver nanowires having attached to the surface PVP).

(64) The results depicted table 2 iv) show that silver nanowires having attached to the surface 1-dodecanethiol, PEGT having a weight average molecular weight of 800 g/mol, PEGT having a weight average molecular weight of 6000 g/mol and polyetheramine can be homogeneously dispersed in dipropylene glycol methyl ether acetate and that the corresponding dispersions are stable over a period of four weeks.

(65) In contrast to the results above, silver nanowires having attached to the surface PVP could not be homogeneously dispersed in the corresponding dispersion media.

Example 2

Example 2.1

(66) 2 ml of a product dispersion comprising dipropylene glycol methyl ether acetate and surface modified silver nanowires having attached to their surface PVP as prepared in experiment 1 (see table 2 above) was mixed with 0.01 g of vinyl chloride co- and acrylate polymers (vinyl chloride co- and terpolymersacrylate polymers is a product from Wacker Chemie AG with the material number 291132 VINNOL® E 15/40 A TF) so that a silver ink composition resulted.

(67) The concentration of said surface modified silver nanowires in the silver ink composition resulting in Example 2.1 was 0.5 wt.-%. The concentration of said vinyl chloride co- and acrylate polymer in the silver ink composition was 0.5 wt.-%.

(68) The resulting silver ink composition could not successfully be printed onto a substrate due to the aggregation of surface modified silver nanowires having attached to their surface PVP in said silver ink.

Example 2.2

(69) Surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 800 g/mol were prepared as described above in experiment 1 (sample 3).

(70) 2 ml of the product dispersion comprising dipropylene glycol methyl ether acetate and surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 800 g/mol as prepared in experiment 1 (see table 2 above) was mixed with 0.01 g of vinyl chloride co- and acrylate polymers is a product from Wacker Chemie AG with the material number 291132 VINNOL® E 15/40 A TF) so that a silver ink composition resulted.

(71) The silver ink composition was completely mixed by LAU Disperser DAS 200 (DIN 53 238) for 15 min.

(72) The concentration of said surface modified silver nanowires in the silver ink composition resulting in Experiment 2.2 was 0.5 wt.-%. The concentration of said vinyl chloride co- and acrylate in the silver ink composition was 0.5 wt.-%.

(73) The silver ink composition was applied to a substrate by blade coating (coating speed: 5 cm/s) to give a wet ink layer on the polymer substrate having a thickness of 6 μm. The blade coating was conducted by K303 Modell 625 from Erichsen at room temperature. More information on the blade coating can be found in the application with the international application number PCT/EP2015/068421 and ASTM D823.

(74) The wet ink layer on the polymer substrate was dried at 130° C. for 5 minutes to give a dried ink layer on the polymer substrate. During this heat treatment, the PEGT with a weight average molecular weight of 800 g/mol remained attached to the surface of said surface modified silver nanowires and said surface modified silver nanowires remained homogeneously dispersed in the wet ink layer on the polymer substrate until it was completely dried.

(75) The haze and sheet resistance of the dried ink layer on the polymer substrate was determined and is shown in FIG. 9. A scanning electron microscopy image of the dried ink layer on the polymer substrate is shown in FIG. 3.

Example 2.3

(76) Surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol were prepared as described above in experiment 1 (sample 4).

(77) 2 ml of the product dispersion comprising dipropylene glycol methyl ether acetate and surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol as prepared in experiment 1 (see table 2 above) was mixed with 0.01 g of vinyl chloride co- and acrylate polymer is a product from Wacker Chemie AG with the material number 291132 VINNOL® E 15/40 A TF) so that a silver ink composition resulted.

(78) The silver ink composition was mixed by LAU Disperser DAS 200 for 15 min.

(79) The concentration of said surface modified silver nanowires in the silver ink composition resulting in Experiment 2.3 was 0.5 wt.-%. The concentration of said vinyl chloride co- and acrylate in the silver ink composition was 0.5 wt.-%.

(80) The silver ink composition was applied to a substrate by blade coating (coating speed: 5 cm/s) to give a wet ink layer on the polymer substrate having a thickness of 6 μm.

(81) The wet ink layer on the polymer substrate was dried at 130° C. for 5 minutes to give a dried ink layer on the polymer substrate. During this heat treatment, the PEGT with a weight average molecular weight of 6000 g/mol remained attached to the surface of said surface modified silver nanowires and said surface modified silver nanowires remained homogeneously dispersed in the wet ink layer on the polymer substrate until it was completely dried.

(82) The haze and sheet resistance of the dried ink layer on the polymer substrate was determined and is shown in FIG. 9. A scanning electron microscopy image of the dried ink layer on the polymer substrate is shown in FIG. 4.

Example 2.4

(83) Step (a):

(84) A first solution containing PEGT with a weight average molecular weight of 6000 g/mol with a concentration of 200 μM was prepared in the oxygen-free ethanol.

(85) A second solution containing PEGT with a weight average molecular weight of 800 g/mol with a concentration of 5 mM was prepared in the oxygen-free ethanol.

(86) Step (b):

(87) An intermediate dispersion with a concentration of 0.5 wt.-% of surface modified silver nanowires having PVP attached to their surface was prepared.

(88) Step (c):

(89) 200 ml of the first solution were mixed with 16 ml of the intermediate disperison with a concentration of 0.5 wt.-% of surface modified silver nanowires having PVP attached to their surface for 120 min under a nitrogen atmospheres, so that the PVP attached to the surface of the surface modified silver nanowires is exchanged by PEGT with a weight average molecular weight of 6000 g/mol. Then 5 mL of the second solution were added into the mixture under stirring. An intermediate product dispersion comprising surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 800 g/mol and PEGT with a weight average molecular weight of 6000 g/mol results.

(90) Step (d):

(91) The intermediate product dispersion is centrifuged for 15 min at 6000 rpm and the intermediate product dispersion medium is separated from the surface modified silver nanowires which are redispersed in 30 ml of fresh 1:1 ethanol/acetone solution resulting in an ethanol/acetone dispersion.

(92) Subsequently, the centrifugation, separation, redispersion steps have been repeated for the ethanol/acetone dispersion followed by another centrifugation and separation step so that surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 800 g/mol and PEGT with a weight average molecular weight of 6000 g/mol result.

(93) The surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 800 g/mol and PEGT with a weight average molecular weight of 6000 g/mol were dispersed in dipropylene glycol methyl ether acetate resulting in a dispersion having a concentration of said surface modified silver nanowires of 0.5 wt.-%, based on the total weight of the surface modified silver nanowires and the PEGT attached to their surface.

(94) Step (e):

(95) To the resulting dispersion, vinyl chloride co- and acrylate polymer was added to give a silver ink composition. The silver ink composition was completely mixed by LAU Disperser DAS 200 (DIN 53 238) for 15 min.

(96) The concentration of said surface modified silver nanowires in the silver ink composition was 0.5 wt.-%. The concentration of said vinyl chloride co- and acrylate in the silver ink composition was 0.5 wt.-%.

(97) The silver ink composition was applied to a substrate by blade coating (coating speed: 5 cm/s) to give a wet ink layer on the polymer substrate having a thickness of 6 μm.

(98) The wet ink layer on the polymer substrate was dried at 130° C. for 5 minutes to give a dried ink layer on the polymer substrate. During this heat treatment, the PEGT with a weight average molecular weight of 800 g/mol and PEGT with a weight average molecular weight of 6000 g/mol remained attached to the surface of said surface modified silver nanowires and said surface modified silver nanowires remained homogeneously dispersed in the wet ink layer on the polymer substrate until it was completely dried.

Example 2.5

Results of Examples 2.2, 2.3 and 2.4

(99) A scanning electron microscopy image of the dried ink layer on the polymer substrate is shown in FIG. 5.

(100) For each example (i.e. examples 2.2, 2.3 or 2.4), the haze and sheet resistance of four dried ink layers on a polymer substrate (samples) prepared according to the corresponding example were determined and the results of all four samples for each example are shown in FIG. 9. Due to the varying layer thickness of the four layers prepared according example 2.2, 2.3 or 2.4, the haze and the sheet resistance is slightly different. In general, the lower the haze and the sheet resistance, the more suitable the corresponding dispersion is for use as a silver ink composition.

(101) It can be concluded from FIG. 9 that both PEGT with a weight average molecular weight of 6000 g/mol or PEGT with a weight average molecular weight of 800 g/mol can be successfully used to modify the surface of silver nanowires for use in a silver ink composition. Moreover, the dispersion comprising surface modified silver nanowires (AgNWs) having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol and PEGT with a weight average molecular weight of 800 g/mol gives the best performance.

Experiment 2.6

(102) To a dispersion as resulting in step (d) of Experiment 2.4, 0.01 g of vinyl chloride co- and acrylate was added to give a silver ink composition.

(103) The concentration of said surface modified silver nanowires in the silver ink composition was 0.2 wt.-%.

(104) The silver ink composition was completely mixed by LAU Disperser DAS 200 (DIN 53 238) for 15 min.

(105) The silver ink composition was applied to a polymer substrate using a slot bead dye with a 15 cm broadness at varying flow from 2 mL/min to 10 mL/min and a web speed between 4 m/min and 8 m/min so that the thickness of the resulting wet ink layer on the polymer substrate was in between 3 μm and 21 μm. The each of the wet ink layers was dried at 130° C. for 5 minutes to give a dried ink layer on a polymer substrate.

(106) Results are summarized in the table 2a.

(107) TABLE-US-00009 TABLE 2a Haze and sheet resistance of different dried ink layers on the polymer substrates as prepared in experiment 2.6 Thickness Total Haze of Net Haze of of wet dried ink layer dried ink layer Sheet Resistance of ink layers (μm) (%) (%) dried ink layer (OPS) 8 0.88 0.45 112 10 1.05 0.62 82 12 1.13 0.7 63 14 1.71 1.28 41 17 2.17 1.74 38 21 2.34 1.91 32

(108) Only when the thickness of the resulting wet ink layer on the polymer substrate is in between 8 μm and 21 μm, the wetting is homogeneous enough to provide a quality transparent electrode (resulting after drying the wet ink layer on the polymer substrate). For a quality transparent electrode, the sheet resistance of the dried ink layers at different points of the transparent electrode must not vary by more than 10%, based on the average value of the sheet resistances measured at different points on the same transparent electrode.

Example 3

(109) A dispersion of surface modified silver nanowires having the surface agent compound 1-dodecanethiol attached to their surface in ethanol (concentration of said surface modified silver nanowires in ethanol: 0.5 wt.-%) is being mixed polyvinyl butyral (0.5 wt.-%, CAS number: 63148-65-2) having a weight average molecular weight in the range from 50,000 g/mol to 60,000 g/mol to give a silver ink composition. The concentration of surface modified silver nanowires and polyvinyl alcohol in the silver ink composition are shown in Table 3. The total weight of 1-dodecanethiol attached to the surface modified silver nanowires was 10 mg per g surface modified silver nanowires determined by TGA.

(110) TABLE-US-00010 TABLE 3 compound surface modified silver nanowires polyvinyl butyral concentration 0.5 0.5 (wt.-%)

(111) The resulting silver ink composition is coated onto a polymer substrate by blade coating (coating speed: 5 cm/sec) so that a wet ink layer results on the polymer substrate. The thickness of the wet ink layer was 6 μm. The wet ink layer on the polymer substrate was dried at 130° C. for 5 minutes. The resulting dried ink layer had a transmission of 92.9%, a haze of 1.13% and a sheet resistance of 99Ω/sq.

(112) As a reference, a wet reference ink layer has been prepared in the same way as described above with a dispersion comprising surface modified silver nanowires having PVP attached to their surface instead of 1-dodecanethiol.

(113) The results of the contact angle measurements depicted in FIG. 10 are as follows: Graph A: the contact angle of the wet ink reference layer comprising surface modified silver nanowires having PVP attached to their surface is 55° and Graph B: the contact angle of the wet ink layer comprises surface modified silver nanowires having 1-dodecanethiol attached to their surface is 66°.

(114) The larger contact angle of the wet ink layer comprising surface modified silver nanowires having 1-dodecanethiol attached to their surface indicates that said wet ink layer is more hydrophobic than the corresponding wet reference ink layers comprising surface modified silver nanowires having PVP attached to their surface.

(115) As a consequence the wet ink layer comprising surface modified silver nanowires having 1-dodecanethiol attached to their surface does not smear over the surface of the hydrophilic surface of the polymer substrate and can thus be more precisely coated on the surface of the polymer substrate.

(116) The results show that the surface modification with 1-dodecanethiol can enhance the hydrophobic properties of the coatings. Therefore, a better water-resistance of the coating can also be expected in a humid environment.

Experiment 4

(117) A dispersion of surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol (concentration of said surface modified silver nanowires in ethanol: 0.5 wt.-%) is being mixed with poly(methyl methacrylate) having a weight average molecular weight of 7000 g/mol (0.5 wt.-%) to give a silver ink composition. The concentration of surface modified silver nanowires and poly(methyl methacrylate) in the silver ink composition are shown in Table 4.

(118) TABLE-US-00011 TABLE 4 surface modified compound silver nanowires poly(methyl methacrylate) concentration 0.5 0.5 (wt.-%)

(119) The resulting silver ink composition is coated onto a polymer substrate by blade coating (coating speed: 5 cm/sec) so that a wet ink layer results on the polymer substrate. The thickness of the wet ink layer was 6 μm.

(120) The wet ink layer on the polymer substrate was dried at 130° C. for 5 minutes. The resulting dried ink layer had a transmission of 92.2%, a haze of 0.70% and a sheet resistance of 65Ω/sq.

(121) The above results show that the haze and sheet resistance of a dried silver ink layer comprising surface modified silver nanowires having attached to their surface PEGT with a weight average molecular weight of 6000 g/mol and poly(methyl methacrylate) and poly(methyl methacrylate) as a binder are similar compared to products comprising poly(ethylene glycol) methyl ether thiol and vinylchloride co- and acrylate polymers as binder.

(122) Consequently, different binders can be used with product dispersions comprising surface modified silver nanowires having surface agent compounds with one or more functional groups selected from the group consisting of thiol and amine attached to their surface, to formulate a silver ink.

(123) The skilled person is thus able to combine different constituents with a variety of product dispersions to give a product of the invention adjusted to the specific needs of the coating or printing method used for applying said product of the invention onto the surface of an article.

Example 5

(124) The silver ink composition prepared in example 2.4 was applied on a substrate by screen printing (printing speed: 30 cm/s) to give a wet ink layer on the polymer substrate.

(125) The screen printing is conducted on AT-HUF-701 from Alraun Technik GmbH at room temperature.

(126) During the screen printing, the PEGT with a weight average molecular weight of 800 g/mol and PEGT with a weight average molecular weight of 6000 g/mol remained attached to the surface of said surface modified silver nanowires.

(127) The wet ink layer on the polymer substrate was dried at 130° C. for 5 minutes to give a dried ink layer on the polymer substrate. The surface modified silver nanowires remained homogeneously dispersed in the wet ink layer on the polymer substrate until the wet ink layer was completely dried.

(128) The resulting dried ink layer on the polymer substrate showed a transmission of 87.5%, a haze of 5.24% and a sheet resistance of 160Ω/sq.

(129) Therefore, screen printing of a product of the invention on the surface of a substrate (as an example of a method of coating the surface of an article according to the invention) was successful and met the demands of industrial standards in view of the transmission, the haze and the sheet resistance of the resulting dried ink layer.