WO2008024625A1

WO2008024625A1 - Conductive paste with enhanced color properties

Info

Publication number: WO2008024625A1
Application number: PCT/US2007/075466
Authority: WO
Inventors: Daniel Franke Carroll
Original assignee: Omg Americas, Inc.
Priority date: 2006-08-21
Filing date: 2007-08-08
Publication date: 2008-02-28

Abstract

A conductive paste having enhanced color properties for use in applications including defrosting plastic and glass panels or windows and RFID tags and antennas is provided. The conductive paste includes a first conductive particle having a predetermined particle size and a second conductive particle having a predetermined particle size dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle.

Description

CONDUCTIVE PASTE WITH ENHANCED COLOR PROPERTIES

Cross-Reference to Related Applications/Incorporation by Reference

[0001] This international patent application claims priority to and the benefit of U.S. Provisional Patent Application serial number 60/822,922 filed on August 21, 2006, which is incorporated herein by reference in its entirety.

Technical Field

[0002] This invention relates to a conductive paste. More particularly, this invention relates to a conductive paste having enhanced color properties for use in applications including defrosting plastic and glass panels or windows and RFlD tags and antennas.

Background of the Invention

[0003] Plastic materials, such as polycarbonate (PC) and polymethylmethyacrylate (PMMA), are currently being used in the manufacturing of numerous automotive parts and components, such as B-pillars, headlamps, and sunroofs. Automotive rear window (backlight) systems represent an emerging application for these plastic materials due to many identified advantages in the areas of styling/design, weight savings, and safety/security. More specifically, plastic materials offer the automotive manufacturer the ability to reduce the complexity of the rear window assembly through the integration of functional components into the molded plastic system, as well as to distinguish their vehicle from a competitor's vehicle by increasing overall design and shape complexity. The use of a light weight rear lift gate module may facilitate both a lower center of gravity for the vehicle, resulting in improved vehicle handling & safety, and improved fuel economy. Finally, enhanced safety is further recognized through a greater propensity for occupant or passenger retention with in a vehicle having plastic windows when involved in a roll-over accident. [0004] Although there are many advantages associated with implementing plastic windows, these plastic modules are not without limitations that represent technical hurdles that must be addressed prior to wide-scale commercial utilization. Limitations, relating to material properties, include the stability of plastics to prolonged exposure to elevated temperatures and the limited ability of plastics to conduct heat. In order to be used as a rear window or backlight on a vehicle, the plastic material must be compatible with the use of a defroster or defogging system. In this respect, a plastic backlight must meet the performance criteria established for the defrosting or defogging of rear glass windows. [0005] The difference in material properties between glass and plastics becomes quite apparent when considering heat conduction. The thermal conductivity of glass (T_c=22.39 cal/cm-sec-°C ) is approximately 4-5 times larger than that exhibited by a typical plastic (e.g., T₀ for polycarbonate=4.78 cal/cm-sec-°C). Thus a heater grid or defroster designed to work effectively on a glass window may not necessarily be efficient at defrosting or defogging a plastic window. The low thermal conductivity of the plastic may limit the dissipation of heat from the heater grid lines across the surface of the plastic window. Thus at a similar power output a heater grid on a glass window may defrost the entire viewing area of the window, while the same heater grid on a plastic window may only defrost the portion of the viewing area that is close to the heater grid lines.

[0006] A second difference between glass and plastics that must be overcome is related to the electrical conductivity exhibited by a printed heater grid. The thermal stability of glass as demonstrated by a relatively high softening temperature (e.g., T_Sor_ten ^>:>1000 ⁰C) allows for the sintering of a metallic paste to yield a substantially inorganic frit or metallic wire on the surface of the glass window. The softening temperature of glass is significantly larger than the glass transition temperature exhibited by a plastic resin (e.g., polycarbonate T_g=145 ⁰C). Thus for a plastic window, a metallic paste cannot be sintered, but rather must be cured at a temperature lower than the T_g of the plastic resin.

[0007] For curing, a conductive paste typically consists of metallic particles dispersed in a polymeric resin that will bond to the surface of the plastic to which it is applied. The curing of the metallic paste provides a conductive matrix consisting of closely spaced metallic particles dispersed through out a dielectric polymer. The presence of a dielectric layer (e.g., polymer) between dispersed conductive particles leads to a reduction in the conductivity or an increase in resistance exhibited by cured heater grid lines as compared to dimensionally similar heater grid lines sintered onto a glass substrate. This difference in conductivity between a heater grid printed on glass and one printed on a plastic window manifests itself in poor defrosting characteristics exhibited by the plastic window as compared to the glass window. [0008] Silver paste is the material of choice for printed applications which require a high conductivity. Typically, cured silver paste circuitry or films are white-silver in color. This color, when used in high end polycarbonate solar shading windows, which are dark green, and privacy windows, which are black, results in a substantial contrast difference that is undesirable for the consumer.

[0009] Another application where the contrast difference between printed circuitiy and the substrate may be important is Radio Frequency Identification (RFID) tags. RFID tags contain an antenna that is generally made of conductive silver or copper. The conductivity of the antenna is extremely important since it allows for longer read/write distances with the RFID tag. One method to make the RFID tags is to print the antennas onto flexible substrates such as paper, polyester, vinyl, etc. using conductive pastes. Great efforts are undertaken to make sure the RFID tags are not noticeable by the consumer. Generally, the RFID tags are hidden on the back side of labels, embedded inside substrates or covered by a protective layer. The ability to produce an RFID tag which could blend into the background of the substrate would provide more packaging options and potentially reduce the overall cost of the RFID tag.

[0010] Technology currently exists to tint the color of conductive inks in order to minimize the contrast difference with the substrate. The first method is an overcoat technology that utilizes an obscuration layer. In this process, a conductive ink is applied as a primary layer to the desired substrate and dried/cured into a film. A second ink layer, called an obscuration layer, is applied onto the top of the primary layer. The second ink layer or topcoat has the desired color and hides the cured conductive ink. This obscuration layer does not contribute to the conductivity of the cured conductive ink film. The disadvantage of this approach is that a two separate printing and curing steps are generally required. This process is more costly and undesirable for substrates which are sensitive to the curing process (i.e. temperature).

[0011] Another technology for tinting the color of the conductive paste is through the addition of pigments. The most common being the addition of conductive carbon black to darken or produce a grey/black colored conductive film. The main draw back with this approach is that the conductivity can be negatively affected. This approach is used mainly for low conductivity applications. This approach is not acceptable for applications where the voltage supply is limited and there is a need to carry maximum current. [0012] Many other types of pigments (organic, inorganic and metallic) can be used to adjust the color of the silver paste. This type of pigment technology is well known in the industry. However, in all cases, the conductivity of the pigment additions is lower than conductive silver material.

[0013] Therefore, there is a need to develop a composition and method to produce a conductive paste with enhanced color properties through the use of a pigment addition that acts synergistically with a polymer addition, without negatively affecting the conductivity of the cured conductive film or conductor. The advantage of this approach is that a tinted conductive film can be deposited onto a substrate in a one step process without the use of overcoat pastes and inks.

Brief Summary of the Invention

[0014] An embodiment of the invention is directed to a conductive paste, the paste including a first conductive particle having a predetermined particle size and morphology and a second conductive particle having a predetermined particle size and morphology dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle. In another embodiment of the invention, the conductive paste further includes a polymer layer applied and cured over the cured conductive paste to further enhance the color properties.

[0015] A further embodiment of the invention is directed to a method of enhancing the color properties of a cured conductive paste, the method including the steps of first preparing a conductive paste, wherein the conductive pastes comprises a first conductive particle having a predetermined particle size and morphology and a second conductive particle having a predetermined particle size and morphology dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle, depositing a primary layer of the conductive paste onto a surface, and curing the conductive paste. The method further includes curing the conductive paste at a temperature of less than 150 ⁰C for less than about 120 minutes. [0016] Yet a further embodiment of the invention is directed to a conductive paste. The paste includes a first conductive particle having a predetermined particle size and a second conductive particle having a predetermined particle size dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle, and wherein the second conductive particle is an alloy.

Brief Description of the Drawings

[0017] FIG. 1 is a representative photograph showing the appearance (color) of three cured conductive paste mixtures of the invention;

[0018] FIG. 2 is a representative photograph showing the appearance (color) of an additional three cured conductive paste mixtures;

[0019] FIG. 3 is a representative photograph showing the appearance (color) of an additional three cured conductive paste mixtures;

[0020] FIG. 4 is a representative photograph showing the appearance (color) of an additional three sets cured conductive paste mixtures, each set having a unique chemical composition and comparing the effect of an applied and cured polymer coating; [0021] FIG. 5 is a representative photograph showing the appearance (color) of an additional pair of cured conductive paste mixtures, each mixture having a unique chemical composition and comparing an uncoated and coated mixture with a cured polymer coating; and

[0022] FIG. 6 is a representative photograph showing the appearance (color) of an additional set of cured conductive paste mixtures containing varied amounts of at least one alloy conductive material.

Detailed Description of the Invention

[0023] It is know in the automotive industry that a conventional heater grid formed on a plastic panel using a metallic ink and subsequently cured according to the manufacturer's recommendations performs poorly in industry standardized defroster tests established for the evaluation of a heater grid on a glass window. The test protocol for the automotive industry requires 75% or greater defrosting of the visual area within a 30 minutes time frame. In order for a defroster formed on a plastic panel to achieve performance similar to a defroster formed on glass 10, the heater grid must defrost greater than or equal to 75% of the viewing area in less than about eight minutes. The test protocol utilized to characterize window defrosting is well known to those skilled in the art and is adequately described by SAE (Society of Automotive Engineers) standard J953 (April 93), as well as by many automotive manufacturer internal specifications, such as Volkswagen/ Audi specification #TL 820-45 or Ford Motor Company specification #01.1 l-L-401.

[0024] A window defroster assembly generally includes a defroster provided on a panel. The defroster includes a heater grid having a series of grid lines extending between generally opposed bus bars. Typically, the defroster additionally includes a transparent, conductive layer applied over the panel.

[0025] The heater grid may be formed from any conductive material including conductive pastes, inks, paints, or films known to those skilled in the art. If the conductive element is a paste, ink, or paint, it is preferred that they include conductive particles, flakes, or powders dispersed in a polymeric matrix. This polymeric matrix is preferably an epoxy resin, a polyester resin, a polyvinyl acetate resin, a polyvinylchloride resin, a polyurethane resin or mixtures and copolymers of the like.

[0026] The conductive particles, flakes or powders may be of a metal including, but not limited to, silver, silver oxide, copper, copper oxide, zinc, zinc oxide, aluminum, aluminum oxide, magnesium, magnesium oxide, nickel, nickel oxide, tin, tin oxide or mixtures and alloys of the like. These conductive particles, flakes, or powders may also be any conductive organic material known to those skilled in the art, such as polyaniline, amorphous carbon, and carbon-graphite. Although the particle size of any particles, flakes, or powders may vary, in one embodiment, the diameter of the particle is less than about 40 μm. In another embodiment, the diameter of the particle is less than about 10 μm. Any solvents, which act as the carrier medium in the conductive pastes, inks, or paints, may be a mixture of any organic vehicle that provides solubility for the organic resin. Representative solvents include, but are not limited to carboxylic acids, including aliphatic, aromatic or combinations thereof and glycol ethers, including ethylene glycol, propylene glycol or combinations thereof. [0027] In an embodiment of the invention, a conductive paste is modified with a conductive material to enhance the final color properties of the conductive paste without adversely affecting conductivity. The modified conductive paste, when deposited and cured onto privacy windows and tinted windows for solar shading, minimizes the color difference when compared to existing lighter colored conductive pastes.

[0028] The examples described below indicate how the individual constituents of the preferred conductive paste and the conditions of the process for testing provide the desired result. The examples demonstrate that by using the composition and processes of the invention, conductive pastes can be modified to enhance the color properties without sacrificing conductive capabilities. The examples will serve to further typify the nature of this invention, but should not be construed as a limitation in the scope thereof, which scope is defined solely in the appended claims.

[0029] Example #1

[0030] A conductive paste mixture was prepared by mixing silver particle (flake) with an organic vehicle. The composition of the conductive paste is shown in Table 1 :

[0031] Table 1

[0032] The ingredients were mixed together on a glass plate with a spatula until a paste was formed. The paste was then drawn down into a film on a Kapton substrate using a draw down bar. The thickness of the draw down was such that cured film was approximately 0.001" thick.

[0033] The Kapton substrate with silver paste film was then cured in air at 250 ⁰C for 15 minutes. Under these curing conditions, the Neodecanoic acid decomposes/volatilizes leaving a film comprised of entirely of the silver metal flake. The appearance of the silver film was white in color.

[0034] The conductivity of the cured silver film was measured using a GW Laboratory Power Supply Model GPR-1810 HD set up to run in a constant current mode. Two electrical probes 0.55" apart were placed onto the silver film. A constant current of 10 Amps was applied to the probes. If a voltage could be measured through the probe, the film was considered conductive. A relative comparison of conductivity between films can be made with this method. A film with a low measured voltage has a low internal resistance and a high overall conductivity. In contrast, a film with high measured voltage has a high internal resistance and a low overall conductivity. For the film produce in Example #1, the measured voltage at 10 amps was 0.76Volts, indicative of a highly conductive film.

[0035] The color of the conductive silver film was measured with a Minolta CR200

Chroma-meter. The Chromameter quantifies the color using the CIE L*a*b* color space method. The color of the cured silver film was measured to be L*=92.85, a*=-4.56 and b*=9.96.

[0036] Examples #2 and #3

[0037] Two conductive silver films were prepared and characterized according to

Example #1. In these examples, a portion of the silver flake was replaced with 30 wt.% nanosilver particles. In one embodiment, the nanosilver particles have a diameter that is less than about 10 μm. The formulations are summarized in Table 2:

[0038] Table 2

[0039] The surface area of the nanopowders in Examples #2 and #3 were 5 and 20 m /g, respectively. Examples #2 and #3 required additional neodecanoic acid compared to Example #1 in order to reduce the viscosity for a proper draw down. The cured film in Example #2 had a slight grey tint when compared to Example #1. The cured film in Example 3 had an even darker grey than Example #2. A summary of the film properties are summarized in Table 3. [0040] Table 3

[0041] Examples #1, #2 and #3 indicate that the additions of nanosilver particles to a silver flake paste degraded the conductivity of the cured silver film. The conductivity was the lowest for the film containing the nanosilver powder with the highest surface area (i.e. smallest the particle size). The decrease in conductivity with the nanoparticle additions can be explained by an increase in the internal resistance of the conductive film. It is well known that a flake or platelet morphology is the most desirable for electrical conductivity in pastes. The flake morphology minimizes the number of grain boundaries between metal particles. Grain boundaries have a higher resistance (lower conductivity) than bulk silver metal. The addition of nanoparticles dramatically increases the number of grain boundaries and disrupts the conduction paths formed by the flake morphology. To gauge the change in conductivity, a resistance value was calculated from the applied amps and measured voltage values. The calculated resistance increased as the surface area increased for the nanosilver addition. The calculated resistance for the films in Examples #2 and #3 was approximately 1.3 and 2.3 times greater, respectively than the Example #1. This increase in resistance cannot be tolerated for applications which require high conductivity.

[0042] The size of the nanopowder addition also affected the color of the cured silver film. As the surface area of the nanopowder increased, the color of the cured silver film became a darker grey. This result is supported by the L* measurements. One skilled in this art realizes that appearance of a film or coating darkens with decreased L* values. One skilled in the art also realizes that the darkness of a film can be tailored by varying the particle size of the pigment addition. The addition of nanosilver particles could be one effective way to darken cured silver paste films. However, this approach to controlling the darkness has been shown to be detrimental to conductivity.

[0043] For reference, FIG. 1 represents how the appearance (color) of the cured silver films in Examples #1 , #2 and #3 varied. These pictures were reproduced from the measured L*, a* and b* values using a color simulator found at http://colorpro.com/info/tools/convert.htm . [0044] Example #4

[0045] A conductive silver paste (6105 Polymer Thick Film Silver Conductor) was obtained from Methode Electronics Inc., Chicago IL. This conductive paste contains polymeric additives, such as vinyl-copolyester, as well as other organics. After curing at the recommended temperature of 125⁰C for 60 minutes, a small residual amount of the polymer remains in the cured film to improve the strength and adhesion properties of the film. It is estimated that the cured films contains approximately 5 wt.% polymer after curing at the above conditions.

[0046] A silver paste film from Methode Electronics was drawn down on to a Kapton substrate. The thickness of the draw down was such that the cured film had a thickness of approximately 0.001 ". The conductive silver film was cured in air at 125⁰C for approximately 60 minutes. After curing, the film was characterized similarly to Examples #1 , #2 and #3. The results are listed in Table 4. [0047] Examples #5 and #6

[0048] Two conductive films were prepared using the polymeric silver paste from Methode Electronics and the procedures described in Example #4. For Example #5, 30 wt.% of the silver paste was substituted with the nanosilver particles having a surface area of 5 m²/g. For Example #6, 30 wt.% of the silver paste was substituted with the nanosilver with a surface area of 20 m /g. Butyl carbitol was added to the conductive paste mixture to adjust the viscosity for the draw down. After curing, the films were characterized and the results summarized in Table 4. [0049] Table 4

[0050] Examples #4 and #5 are consistent with the results in Examples #1, #2 and #3. The addition of the 5 m /g nanosilver particles to the cured silver film with polymer addition negatively affected the conductivity. The calculated resistance increased by 1.6X over

Example #4. The slight increase in resistance when compared to Example #2 is attributed to the presence of the polymeric additive in the cured film.

[0051] The result for Example #6 was surprising and uniquely different from the previous examples. The addition of the 20 m²/g nanosilver powder did not adversely affect the conductivity. The calculated resistance of the silver film was only 1.1 X larger than Example

#4. This result indicates that the film in Example #6 nearly had the same level of conductivity as Example #4. This result is significantly different than the 2.3X increase in resistance calculated for Example #3.

[0052] In terms of the color properties, a similar trend was observed where the nanosilver additions darkened the color of the silver paste. The paste was the 20 m7g nanosilver was the darkest shade of grey.

[0053] FIG. 2 represents how the appearance (color) of the cured silver films in Examples

#4, #5 and #6 varied. These pictures were reproduced from the measured L*, a* and b* values using a color simulator found at the following website: http://coloφro.com/info/tools/convert.htm .

[0054] The results in Examples #4, #5 and #6 indicate an unexpected interaction between the residual polymer and the nanosilver addition in the cured film. At a surface area of 20 m²/g, the color of the cured silver film can be made dark grey without negatively affecting the conductivity. This result suggests a critical surface area or size exists for the nanosilver where the interaction with the polymer matrix enables the color to be adjusted without degrading the conductivity. The critical size corresponds to a surface area between 5 and 20 m²/g.

[0055] Example #7

[0056] A cured conductive film was made according to Example #4. The only change was a 4 wt.% addition of a two part marine epoxy (G/5 Five Minute Adhesive, Gougeon

Brothers, Bay City, MI) to the paste. After curing, the film was characterized and results summarized in Table 5.

[0057] Examples #8 and #9

[0058] A cured conductive film was made according to Example #5 and 6, respectively.

The only change was a 4 wt.% addition of a two part marine epoxy (G/5 Five Minute Adhesive, Gougeon Brothers, Bay City, MI) to the paste. After curing, the films were characterized with the results summarized in [0059] Table 5

[0060] The results in Examples #7, #8 and #9 are consistent with the results in Examples #4, #5 and #6. Again, Example #9, with the addition of the nanosilver with a surface area of 20 m²/g nearly had the same conductivity and calculated resistance as Example #7. Example #8 containing the 5 m²/g nanosilver was again found to have a higher resistance and lower conductivity.

[0061] FIG. 3 represents how the appearance (color) of the cured silver films in Examples #7, #8 and #9 varied. These pictures were reproduced from the measured L*, a* and b* values using a color simulator found at the following website: http://colorpro.com/info/tools/convert.htm.

[0062] The results in Examples #7, #8 and #9 indicate that the darkness of the cured films can be increased by adding more polymer to the cured film. This darkening affect is further enhanced with the addition of nanosilver. Realistically, there is an upper limit to the amount of polymer that can be added before the conductivity of the cured film is lost. [0063] This result indicates a method to control the darkness of a silver cured film without sacrificing the conductivity. The darkness is controlled by optimizing the amount of polymers (or organics) and high surface area nanosilver particles, preferably on the order of 20 m /g or higher, in the cured silver film.

[0064] In a further embodiment of the invention, a conductive paste is modified with a conductive material to enhance the final color properties of the conductive paste without adversely affecting conductivity which is then coated with a polymer-based or inorganic- based coating. This polymer-based or inorganic-based coating may include, but are not limited to, acrylics, polyurethanes, polyureas, SiO₂ coatings and mixtures thereof. The modified conductive paste, when deposited and cured onto privacy windows and tinted windows for solar shading followed by the application and curing of a polymer-based coating, minimizes the color (contrast) difference when compared to existing lighter colored conductive pastes.

[0065] The examples described below indicate how the individual constituents of the preferred conductive paste and the conditions of the process for testing provide the desired result. The examples demonstrate that by using the composition and processes of the invention, conductive pastes can be modified to enhance the color properties without sacrificing conductive capabilities. The examples will serve to further typify the nature of this invention, but should not be construed as a limitation in the scope thereof, which scope is defined solely in the appended claims.

[0066] Example #10

[0067] A conductive silver paste (6105 Polymer Thick Film Silver Conductor) was obtained from Methode Electronics Inc., Chicago IL. A silver paste film was formed by drawing down the silver paste onto a Kapton substrate. The thickness of the draw down was such that the cured film had a thickness of approximately 0.001".

[0068] The conductive silver film was cured in air at 125⁰C for approximately 180 minutes. After curing, one half of conductive film was coated with a clear, optically transparent, acrylic coating (Acrylic Crystal Clear, Krylon Products Group, Cleveland, OH).

The cured film with and without the acrylic coating was characterized using a Minolta CR200

Chroma-meter. The Chromameter quantifies the color of the cured film using the CIE

L*a*b* color space method. The results of the characterization are summarized in Table 6.

[0069] Example #11

[0070] A cured conductive film was made and characterized according to Example #10.

The only difference was a 1.5 wt.% addition of a marine epoxy (G/5 Five Minute Adhesive,

Gougeon Brothers, Bay City, MI) was added to the paste. The results are summarized in

Table 6.

[0071] Example #12

[0072] A cured conductive film was made and characterized according to Example #11.

The only difference was that a nanosilver powder, having a surface area of 20 m²/g, was added to the silver paste. The nanosilver powder addition was such that it accounted for 20.9 wt.% of the total silver in the silver paste/nanopowder mixture. Butyl Carbitol was added to the conductive paste mixture to adjust the viscosity. The results of the characterization are summarized in Table 6.

[0074] The results in Table 6 indicate the clear acrylic coating decreased the L* value of a cured conductive paste film. One skilled in this art realizes that a decreased L* value corresponds to a darkening of a film or coating. The L* values for Examples #10 and #11 decreased approximately 10.0 and 14.2%, respectively. The decrease in L* value was significantly greater for the cured film containing both nanosilver powder and epoxy additions. With an addition of 20.9 wt.% nanosilver and 1.5 wt.% epoxy, the L* value decreased by 26.4%. This enhancement in darkness when using the combination of nanosilver/epoxy additions and a clear, transparent overcoat is unexpected and viewed has unique.

[0075] For reference, FIG. 4 represents how the appearance (color) of the cured silver films in Examples #10, #11, and #12 varied. These pictures where reproduced from the measured L*, a* and b* values using a color simulator found at the following website: http://colθφro.com/info/tools/convert.htm. [0076] Example #13

[0077] A cured conductive film, without an acrylic coating, was made according to Example #12. In this case, the amount of epoxy and nanosilver was increased to 1.8 wt.% and 34.7 wt.%, respectively. The conductivity of the cured silver film was measured using a GW Laboratory Power Supply Model GPR-1810 HD set up to run in a constant current mode. Two electrical probes 0.55" apart were placed onto the silver film. A constant current of 5 Amps was applied to the probes. If a voltage could be measured through the probe, the film was considered conductive. A relative comparison of conductivity between films can be made with this method. A film with the low measured voltage has the low internal resistance and high overall conductivity. In contrast, a film with high measured voltage has a high internal resistance and a low overall conductivity. The characterization results are summarized in Table 7. [0078] Example #14

[0079] A cured conductive film was made and characterized according to Example #13 except the amounts of epoxy and nanosilver powder were decreased to 1.0 wt.% and 20.6 wt.%, respectively. After curing, the conductivity of the conductive film was measured. The cured film was then coated with a clear, optically transparent, acrylic coating (Acrylic Crystal Clear, Krylon Products Group, Cleveland, OH). The color of the coated film was then characterized. The results are summarized in Table 7. [0080] Table 7

[0081] The results in Table 7 demonstrate the significance of this invention. With the use of a clear, optical transparent coating, an equivalent darkness level (L* value) can be obtained with lower additions of epoxy and nanosilver. The benefit of this approach is to produce a cured conductive film with the same darkness level but a higher overall conductivity. For example, the measured voltage in Example #14 was 0.41V or 33% lower than Example #13. The ability to increase conductivity while maintaining an equivalent darkness is desirable for high power applications. In addition, the ability to darken conductive paste with reduced nanosilver additions will be a benefit in terms of a cost effective paste system. [0082] For reference, FlG. 5 represents how the appearance (color) of the cured silver films in Examples #12 and #13 varied. These pictures where reproduced from the measured L*, a* and b* values using a color simulator found at the following website: http : //colorpro . com/info/tool s/con vert .htm .

[0083] In yet a further embodiment of the invention, a conductive paste having a first conductive particle with a predetermined particle size is modified with a second conductive particle having a predetermined particle size, wherein the second conductive particle is a conductive alloy material to enhance the final color properties of the conductive paste without adversely affecting conductivity. The modified conductive paste, when deposited and cured onto privacy windows and tinted windows for solar shading, minimizes the color difference when compared to existing lighter colored conductive pastes.

[0084] The examples described below indicate how the individual constituents of the preferred conductive paste and the conditions of the process for testing provide the desired result. The examples demonstrate that by using the composition and processes of the invention, conductive pastes can be modified to enhance the color properties without sacrificing conductive capabilities. The examples will serve to further typify the nature of this invention, but should not be construed as a limitation in the scope thereof, which scope is defined solely in the appended claims.

[0085] Example #15

[0086] A cured conductive film was made according to Example #12 except a nanosilver/nanocopper alloy powder was utilized instead of the nano silver powder. The composition of the nano silver/copper alloy powder was about 75 wt.% silver and about 25 wt.% copper. The surface area was approximately 37 m²/g. The results of the characterization are summarized in Table 8.

[0087] Example #16

[0088] A cured conductive film was made according to Example #12 except a nanocopper powder was utilized instead of the nanosilver. The nano copper powder had a surface area of 80 m²/g. X-ray diffraction analysis indicated the nanocopper powder contained both copper and copper oxide. The characterization results are summarized in

Table 8.

[0089] Table 8

[0090] The results in Table 8 indicate that the use of metal or metal alloy powders other than silver can influence the darkness and color of the cured conductive paste. Example #15 indicates that an alloy powder where 25% of the silver is substituted by copper can produce a lower L* value or darken of the conductive film as compared to the film made with pure nano silver powder (Example #12). This enhancement in darkening is achieved with only a 25% decrease in the conductivity as compared to Example #12. The darkness level of the cured conductive film made with the nanosilver/copper alloy was also found to be significant enhanced with the used of the acrylic clear coating. Example #16 indicates that the color other than the level of darkness and brightness can be affected by the type of nanopowder addition. The additions of the nano copper powder in Example #16 produced a cure conductive film with a green tint. The conductivity of the cured film, however, was significantly lower than Example #12. The addition of the clear acrylic coating in Example #16 made with the nanocopper powder was also found to be effective means to enhance the darkness of the cured film.

[0091] For reference, FIG. 6 represents how the appearance (color) of the cured silver films in Examples 15 and 16 varied. These pictures where reproduced from the measured L*, a* and b* values using a color simulator found at the following website: http^/colorpro.com/info/tools/convert.htm.

[0092] Based on the foregoing disclosure, it should be apparent that the modification of conductive pastes with nanoparticle materials, including nanoparticle silver and nanoparticle alloy materials, of the invention will achieve the objectives set forth above. It is therefore understood that any evident variations will fall within the scope of the claimed invention. Thus, alternate specific component elements can be selected without departing from the spirit of the invention disclosed and described herein.

Claims

ClaimsWhat is claimed is:

1. A conductive paste comprising: a first conductive particle having a predetermined particle size and a second conductive particle having a predetermined particle size dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle.

2. The conductive paste of claim 1, wherein the particle size of the first particle is less than about 10 μm.

3. The conductive paste of claim 1, wherein the particle size of the second particle is less than about 100 nm.

4. The conductive paste of claim 3, wherein the second conductive particle has a surface area of at least about 20 m²/g.

5. The conductive paste of claim 1, wherein the polymer matrix is an epoxy resin, a polyester resin, a polyvinyl acetate resin, a polyvinylchloride resin, a polyurethane resin or mixtures and copolymers of the like.

6. The conductive paste of claim 1 , wherein the first conductive particle is a silver flake.

7. The conductive paste of claim 1, wherein the second conductive particle is silver nanoparticle or silver oxide particle or a composite mixture of both nanoparticles.

8. The conductive paste of claim 1 further comprising a cured transparent layer over the cured conductive paste, wherein the cured transparent layer further enhances the color properties of the cured conductive paste.

9. The conductive paste of claim 8, wherein the cured transparent layer comprises a polymer selected from the group consisting of acrylics, polyurethanes, and polyureas.

10. A method of enhancing the color properties of a cured conductive paste, the method comprising the steps of: preparing a conductive paste, wherein the conductive pastes comprises a first conductive particle having a predetermined particle size and a second conductive particle having a predetermined particle size dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle; depositing a primary layer of the conductive paste onto a surface; and curing the conductive paste, wherein the curing of the conductive paste darkens the color of the paste.

1 1. The method of claim 10, wherein the curing of the conductive paste is performed at a temperature of less than 150 ⁰C for less than about 120 minutes.

12. The method of claim 10, wherein the cured conductive paste does not require subsequent overcoat layers of a paste, an ink or mixtures thereof.

13. The method of claim 10, wherein the cured conductive paste is deposited onto substrates made of paper, polyester, polyvinyl acetate, polyvinyl chloride, polycarbonate, polyurethane, polyimide or mixtures and copolymers of the like.

14. The method of claim 10, wherein the particle size of the first particle is less than about 10 μm.

15. The method of claim 14, wherein the second conductive particle has a surface area of at least about 20 m²/g.

16. The method of claim 10, wherein the particle size of the second particle is less than about 100 nm.

17. The method of claim 10, wherein the first conductive particle is a silver flake.

18. The method of claim 10, wherein the second conductive particle is silver nanoparticle or silver oxide particle or a composite mixture of particles.

19. The method of claim 10, wherein the surface is a plastic window panel for an automobile.

20. The method of claim 19, wherein the window panel is comprised of plastic or glass.

21. The method of claim 20, wherein the plastic window panel is comprised of polycarbonate or polymethylmethyacrylate.

22. The method of claim 10, wherein the surface is a radiofrequency identification tag.

23. A conductive paste comprising: a first conductive particle having a predetermined particle size and a second conductive particle having a predetermined particle size dispersed in a polymeric matrix, wherein the particle size of the first conductive particle is larger than the particle size of the second conductive particle, and wherein the second conductive particle is an alloy.

24. The conductive paste of claim 23, wherein the wherein the particle size of the first particle is less than about 10 μm.

25. The conductive paste of claim 23, wherein the particle size of the second particle is less than about 100 nm.

26. The conductive paste of claim 25, wherein the second conductive particle has a surface area of at least about 35 m /g.

27. The conductive paste of claim 23, wherein the second conductive particle is a nanosilver/copper alloy powder.

28. The conductive paste of claim 23, wherein the second conductive particle is about 75 wt. % silver and about 25 wt.% copper.

29. The conductive paste of claim 23 further comprising a cured transparent layer over the cured conductive paste, wherein the cured transparent layer further enhances the color properties of the cured conductive paste.

30. The conductive paste of claim 29, wherein the cured transparent layer comprises a polymer selected from the group consisting of acrylics, polyurethanes, and polyureas.