WO2008011223A1

WO2008011223A1 - Conductive paste with enhanced color properties

Info

Publication number: WO2008011223A1
Application number: PCT/US2007/069739
Authority: WO
Inventors: Daniel Franke Carroll
Original assignee: OMG Americas Inc
Current assignee: Borchers Americas Inc
Priority date: 2006-07-17
Filing date: 2007-05-25
Publication date: 2008-01-24
Anticipated expiration: 2009-01-17

Abstract

A cured, conductive paste having enhanced color properties. 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. The particle size of the first conductive particle is larger than the particle size of the second conductive particle, and the second conductive particle has substantially minimal effect on the conductivity of the cured conductive paste.

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 60/807,510 filed on July 17, 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 that provides performance within a specific range making it amenable for use in applications including defrosting plastic and glass panels or windows and RFID 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., T_c 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_SOften ^>:>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 circuitry 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] The second technology is tinting the color of the conductive paste with the additions 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 is 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 present invention is directed to a cured conductive paste having enhanced color properties, 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, and wherein the second conductive particle has substantially minimal effect on the conductivity of the cured conductive paste.

[0015] A further embodiment of the present 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, curing the conductive paste at a temperature of less than 150 ⁰C for less than about 120 minutes, and characterizing the conductive paste for voltage and color properties. Brief Description of the Drawings

[0016] FIG. 1 is a representative photograph showing the appearance (color) of three cured conductive paste mixtures of the present invention;

[0017] FIG. 2 is a representative photograph showing the appearance (color) of an additional three cured conductive paste mixtures; and

[0018] FIG. 3 is a representative photograph showing the appearance (color) of an additional three cured conductive paste mixtures.

Detailed Description of the Invention

[0019] 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.

[0020] 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.

[0021] 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.

[0022] The conductive particles, flakes or powders may be of a metal including, but not limited to, silver, copper, zinc, aluminum, magnesium, nickel, tin, 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 conductive particles, flakes, or powders may vary, in one aspect of the invention, a first conductive particle having a size of less than about 100 μm can be used with a second conductive particle having a size of less than about 100 nm. 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.

[0023] In an embodiment of the present 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.

[0024] 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 present 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.

[0025] Example #1

[0026] A conductive paste mixture was prepared by mixing a first conductive particle, for example a silver particle (flake), with an organic vehicle. The composition of the conductive paste is shown in Table 1 : [0027] Table 1

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] Examples #2 and #3

[0033] Two conductive silver films were prepared and characterized according to Example #1. In these examples, a portion of the Ag flake, the first conductive particle, was replaced with 30 wt.% nanosilver particles, the second conductive particle. The formulations are summarized in Table 2:

[0034] Table 2

[0035] The surface area of the nanopowders in Examples Wl 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 Wl. A summary of the film properties are summarized are summarized in Table 3.

[0036] Table 3

[0037] 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.

[0038] 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 shown to be detrimental to conductivity.

[0039] 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.

[0040] Example #4

[0041] 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.

[0042] 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. The cured conductive paste does not require a secondary overcoat layer of a paste or an ink. After curing, the film was characterized similarly to Examples #1, #2 and #3. The results are listed in Table 4.

[0043] Examples #5 and #6

[0044] 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 m²/g. Butyl carbitol was added to the conductive paste mixture to adjust the viscosity for the draw down. The cured conductive paste does not require a secondary overcoat layer of a paste or an ink. After curing, the films were characterized and the results summarized in Table 4.

[0045] Table 4

[0046] Examples #4 and #5 are consistent with the results in Examples #1, #2 and #3. The addition of the 5 m²/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.

[0047] 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. [0048J 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 m²/g nanosilver was the darkest shade of grey.

[0049] 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://colorpro.com/info/tools/convert.htm .

[0050] 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.

[0051] Example #7

[0052] 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. The cured conductive paste does not require a secondary overcoat layer of a paste or an ink. After curing, the film was characterized and results summarized in Table 5.

[0053] Examples #8 and #9

[0054] A cured conductive film was made according to Example #5. 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. The cured conductive paste does not require a secondary overcoat layer of a paste or an ink. After curing, the films were characterized with the results summarized in Table 5. [0055] Table 5

[0056] The results in Examples Wl, #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 nano silver was again found to have a higher resistance and lower conductivity.

[0057] 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 .

[0058] 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.

[0059] 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 m²/g or higher, in the cured silver film.

[0060] Based on the foregoing disclosure, it should be apparent that the modification of conductive pastes with nanoparticle materials, including nanoparticle silver, of the present 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 cured conductive paste having enhanced color properties, the 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 has substantially minimal effect on the conductivity of the cured conductive paste.

2. The cured conductive paste of claim 1, wherein the particle size of the first particle is less than about 100 μm and the particle size of the second particle is less than about 100 nm.

3. The conductive cured paste of claim 1, wherein the first conductive particle has a surface area of less than 5 m²/g and the second conductive particle has a surface area of at least about 20 m²/g.

4. The cured 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.

5. The method of claim 4, wherein the cured conductive paste does not require a secondary overcoat layer of a paste or an ink.

6. The method of claim 4, wherein the cured conductive paste is deposited onto substrates made of paper, polyester, polyvinyl acetate, polyvinylchloride, polycarbonate, polyurethane, polyimide or mixtures and copolymers of the like.

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

8. The cured conductive paste of claim 1, wherein the second conductive particle is a silver nanoparticle.

9. 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 at a temperature of less than 150 ⁰C for less than about 120 minutes.

10. The method of claim 9 further comprising the steps of characterizing the conductive paste for voltage and color properties.