CA2509608A1 - Aqueous printable electrical conductors - Google Patents

Abstract

An aqueous printable electrical conductor (APEC) is defined as a dispersion comprising metal powder (with specific surface properties) dispersed into an aqueous acrylic, urethane/acrylic mix, natural polymers vehicle (gelatine, soy protein, casein, starch or similar) or in a film forming reactive fatty acids mixture without a binder resin. The aqueous printable dispersion can be applied to substrates through different printing processes such as flexography, gravure, screen, dry offset or others. The preferred substrates include: (1) coated paper, (2) uncoated paper, and (3) a variety of plastics with treated and untreated surfaces. When printed at a thickness of 1-8µm, curing is not required as the dispersion cures at ambient temperatures. When the dispersion is used for any of the above applications it will provide sufficient electrical conductivity to produce electrical circuits for intelligent and active packaging, sensors, radio frequency identification (RFID) tag antennae, and other electronic applications.

Description

AQUEOUS PRINTABLE ELECTRICAL CONDUCTORS
Description The present invention relates to the preparation of electrical conductors which can be printed on substrates and used as electrical circuits in intelligent and active packaging, sensors, RFID antennae, etc.
These electrical conductors are identified as Aqueous Printable Electrical Conductors or "APEC's".
Background of the Invention Printable inks that can be used in different applications, such as those envisaged herein are well known. Such inks are set forth, for example, in US Patent No. 6,379,745 of Parelec and in a US Patent Application of Flint Ink (No. 2003/0151028).
Such inks have serious limitations which limitations are discussed hereinbelow.
The printable electrical conductive materials of the '745 patent are comparable, and in some cases superior with regard to conductivity, to aqueous printable electrical conductive materials. However, they are not aqueous and the substrates with which such inks are used are generally limited to plastics, which are heat resistant and are not recyclable. In addition, these printable electrical conductive materials require high temperature curing, thus using more heat energy. They require a special heating chamber, which produces gas emissions during heating.
The Flint Ink materials are not able to reach the conductivity of the technology described herein, although they are also aqueous electrical conductive materials (i.e. inks) and are designed to print both on paper and cardboard. They are not printable on plastics. The Flint Ink conductive ink materials are less shelf-stable, separate easily, require heat for dying and curing, are more difficult to control during printing press applications, and are much more sensitive to pH variations.
Neither of the aforementioned applicants describes how they could provide a commercially applicable means to ensure electrical conductivity using an aqueous media combined with ambient temperature air curing, resulting in the production of industry-useable devices on a number of existing printing press set-ups common in world markets.
Other patents, such as US 5,492,653; 5,286,415; 4,715,989 etc., describe aqueous silver, other metallic and carbon flakes-based compositions as well as water-based conductive thick film inks that are mostly used as coating compositions with primary applications being sprays, paints or printable screens.
They are always used with organic co-solvents with possible uses in EMI
(electromagnetic interference) and RFI (radio frequency interference) shielding. Their electrical conductivity is much lower than that which is described by the methodology in this patent application and they are not applicable in the ways so described for aqueous printable electrical conductors.

Summary of the Invention The present invention involves a dispersion comprised of powder or flakes of highly conductive material suspended in an aqueous acrylic, urethane/acrylic mix, or other vehicle. This dispersion is capable of being deposited onto a substrate using different printing methods for production of various devices. The preferred morphology of the conductive material is flaky. The metal of choice is silver (Ag) with particle sizes ranging from 0.001Nm to 50Nm (normal is 1-8pm). The surface of the metal flakes is preferably chemically treated but this does not exclude untreated flakes in which case the surface treatment is performed in-house as a first step before preparation of the dispersion. Fatty acid treated flakes from large batch producers currently work the best.
The first step of the method for making APEC's is preparation of the vehicle (i.e. the printing media) for the metal powder or flakes. The vehicle for this type of printing application has surface properties that are different when compared to conventional printing ink vehicles. The requirement for different surface properties is the result of the aqueous printing processes needing higher than normal surface pressures, lower resin content, low molecular weight polymers, and the use of polymers not capable of intensive cross-linking, which characteristic minimizes the influence of dielectric binders and increases electrical conductivity. Additionally, the preparation of APEC's requires very low or no use of additives (e.g. adhesion promoters, antifoaming agents, waxes, etc.). The dispersion used to make APEC's retains acceptable printing characteristics but requires more frequent on-press additive addition in negligible amounts (less than 1 %) compared to conventional printing inks.
An APEC requires 2 to 4 times more frequent addiction of adjusters. The APEC's also possess mechanical resistance, shelf life and other required properties without compromising the conductivity of the deposited electrical conductor.
The second step in creating APEC's is selecting the correct metal powder or flake. The metal's surface treatment needs to be selected to ensure compatibility of the metal with the vehicle system, thus ensuring dispersion stability. Fatty acid treated particle surfaces are preferred. The treatment of the metal particle's surface ensures a very thin, low molecular weight layer that does not affect the conductivity significantly. The surface treatment also regulates the surface pressure of the metal particles. Surface treatment substances migrate towards the film part of the deposited conductor as part of the film-forming process. The surtace treatment substances are critical to ensuring the printability of conductive traces for different applications.
The third step is the process of dispersing the metal into the printing vehicle. This process requires slow mixing of the metal with the selected vehicle. A dispersion mixer with specially shaped mixing heads is used to ensure good and visually laminar flow. In preparing conventional inks, grinding aids and surfactants are added during this process to reduce the surface pressure between mixing surfaces. To avoid possible influences on the conductive properties, such additives are not used in the current process. Generally, the quantity of metal added is two to four (or more) times the weight of the vehicle. As a result of this high load, heat is produced during the mixing process because of friction and increased mixing speed. This is avoided by regulating the mixing speed and adding small aliquots of the vehicle during mixing. When the metal has been incorporated into the base, the dispersion is mixed at a higher speed for a short time with care being taken not to introduce air into the mixture. The temperature of the dispersion should not exceed 30-35°C during mixing. In some cases, the final product will need to be filtered using an appropriate sized silk mesh filter. The mixing process produces a visually homogeneous mixture in liquid form that is stored in sealed bottles at ambient and room temperature (5°C
to 30°C).
The viscosity of the APEC's varies inversely with the metal load. As an example, for flexography process printed applications, the viscosity ranges from 25 to 85 seconds measured with a Zahn 3 cup (approximately 500 to 3600 centipoises, based on the APEC's specific gravity of 2.4-4). Gravure, screen, and some other dry offset applications can be achieved by adjusting the viscosity of the base composition.
The viscosity and printing properties of APEC's should be adjusted prior to printing on the press and held constant during printing through the appropriate use of additives.
This can be achieved by adding ammonia water and minimal amounts of specially selected antifoaming agents if needed.
Depending on the printing method and duration of the print run, the addition of 1.0 - 10% ammonia water and 0.01 - 0.2% antifoaming agent (both by volume) may be necessary. Such addition should be done immediately prior to printing and the combination mixed well. The amounts and frequency of later additions are dependent on the ink properties, structure of the press, exposed surfaces of the emulsion, ambient temperature, humidity, printing speed and other factors. Closed ink box systems, coupled to a slow ink circulating pump, are best for keeping the physical properties of the printed electronic conductors constant.
Detailed description of the process The conductivity of APEC's depends on a number of factors. The most important is the position, arrangement, and physical connection between the powder or flake metal particles that are deposited onto the printed film. Conductivity and stability are influenced by the film drying process, the origin of the heating/curing media, and external treatment processes such as applied pressure and high energy light treatments.
It is desirable to achieve proper orientation and close positioning between the metal particles without the formation of a thick polymer particle surface-wetting layer. This is accomplished by selection of raw materials and correct preparation of the vehicle. It is also necessary to prepare the mixing surface through the use of techniques such as particle surface treatment.
It is well known that the proper orientation of the particles is facilitated by surface property adjusters. These include: (1) surfactants, (2) adhesion promoters, and (3) stabilizers. These chemicals generally result in good particle wetting and stability of the dispersion process. However, they also cause a stable surface layer to form on top of the particles (metal particles in the present case) thereby decreasing electrical conductivity. A trade-off exists when using surface property adjusters. For these additives to be used, they must be highly efficient and work in extremely low quantities. It is also preferable to use non-ionic stabilizers to reduce the forces of the particle surface layers. The particle stabilization process, including stabilization layer charging, increases the shelf life of the dispersion but reduces conductivity. The advantage of the conductive dispersion is also related to the conductive properties of water, which allows for the elimination of the electrical charges resulting from friction created by the mixing process.
To achieve improved conductivity with lower mixing time, low particle surface wetting is used.
Low particle surface wetting also allows mixing to be achieved by shaking.
Shaking is limited to small volumes and is not necessarily scalable for commercial serial printing.
Thin surface layers are required to ensure a long shelf life. With the current process, shelf lives of several months (e.g greater than 6 months) are achieved. Incompatibility exists between the ammonia water or any water media and the fatty acids coating the metal particles. This incompatibility prevents the metal particles from being completely stable. To achieve the greatest conductive properties, the metal particles must not be permanently wetted within the vehicle polymer layers.
The formation of a polymer layer on top of metal particle surfaces is necessary to achieve proper particle deposition during the printing process. The polymer layer also facilitates adherence of the particles to the substrate material, and enhances inter-particle consolidation after the drying process.
APEC's tend to dry quickly because of the low percentage of liquid in the metal dispersion. The pH of the APEC is typically 7.5 to 8.5. The pH drops when the ammonia water compound evaporates.
During this process the polymer system goes from a water-soluble state to a water-insoluble state. The drying process can be facilitated by forced convection with ambient or slightly elevated temperature air.
This process plays a major role in determining the conductive properties of the final product. While the APEC is drying the polymers shrink. This is occurs when the polymers lose their water bridges, which are composed of hydrogen bond chemical connections.
The post printing drying process continues for 24 hours at ambient temperature, until most of the liquid has evaporated. This process results in a conductivity increase of up to 50 percent. Several sub processes take place during the drying process. Oxidization polymerization may result in the formation of oxygen cross-linking bridges. Other sub processes are characterized by additional cross-linking of bonds between the reactive polymer chains in the vehicle. Further cross-linking of bonds results from Diels-Alder reactions but only when conjugated double bond structures exist within the vehicle system.
The final process of chemical drying results in a significantly reduced volume of the printed film, which affects film thickness and metal particle orientation. The metal particles move closer to each other due to the reduced volume of the printed film. At the same time, more cross-linked polymers increase the dielectric properties, thereby decreasing conductivity. The largest effect is due to the orientation of the metal particles, resulting in an overall increase in conductivity. Post-drying or post-drying under pressure gives increased conductivity, resulting in stable resistance readings and a film that is less dependent on ambient temperature and humidity. These post-printing operations should only be applied in special cases, as they are not part of the standard printing process.
Normally, with flexographic printing, APEC's do not require special heating.
Room temperature blown air is enough to obtain usable prints. However, with gravure and screen printing, APEC's require extra heat for thorough curing and better performance, especially when materials of differing viscosities are used in the printing process.
The preferable heating and curing methods employ IR heat sources that have the ability to deliver energy into the printed layer allowing drying to commence not from the surface (as is the case when using hot air to dry) but from within. Although these APECs are not UV
curable, application of UV drying during flexographic printing offers two benefits: (1) UV sources produce additional heat, and (2) UV light leads to a slight destruction of binder polymers which increases electrical conductivity.
A major advantage of APEC's is their ability to be printed on paper substrates. Pricing benefits (compared to printing applications requiring other substrates) and ecological gains (via de-inking procedures and recycling benefits) are other advantages of APEC's.
EXAMPLES
Example 1. APEC formulated as an ink for flexography to print UHF antennae.
1 ) Predominantly oleic fatty acid treated silver (particle size 1 to 5 pm flakes shaped) is combined and blended in a regular open air mixer with aqueous solution of 50% solids acrylic resins in water (with ammonia traces to keep pH in the pH window of 7.5-8.5) in a proportion 3.4 to 1 weight portions with no additives.
2) After 20 minutes mixing with an average mixing speed of 1500 rpm with Hi-Vane mixing head (which does not allow the heat to exceed 30°C) 10% pure ammonia preliminary diluted in water is added via continuous mixing in 1.7 wt. % to the ink. The dispersion is mixed for 5 more minutes. Oleic fatty acids react with ammonia to form ammonia soap according to the following basic chemical reaction:
CH3(CHz)~CH=CH(CHZ)~COOH + NH40H = CH3(CHz)~CH=CH(CH2)~COONH4 + H20 The reaction is similar for other oils included with oleic acid - linolic and linoleic acids respectively with two and three double bonds. It is widely known that unsaturated 2 and 3 double bond fatty acids are responsible for the formation of an elastic film after printing and oleic acid does not dry at all. That acid forms a nano layer on the top of the metal particles, helping to make the silver flakes compatible with the alkyl parts used in resins added to dispersions to provide stability and printability. Such oleic acid nano layers reduce the conductivity of the APEC, however, when oleic acid is removed from the surface during exposure to heat shock or other drying methods the printed structure will exhibit an increase in sheet conductivity.
S

3) The obtained ammonia soap is an active surfactant which helps achieve partial wetting of the silver particles by washing part of the oil layer off the top of the metallic flake particles. The reaction essentially produces a lubricant that eases mixing.
Further, acrylic resins experience simultaneously the well-known influence of ammonia on poly acids to regulate viscosity (change from coil polymer chains to stretched chains), allowing adjustment of pH and drying time and thus contributing to final conductivity.
4) A freshly prepared dispersion of the APEC is printed on semi-gloss paper and the resulting prints tested for conductivity, printing properties, adhesion, and other properties, with the following results:
Before any heat treatment the measured conductivity is 0.4 Ohms/square on a fresh printed sample (3-4 Nm thick films) and increases to 0.2 Ohms/square over the following 24 hours. After a heat shock is applied the conductivity is increased by 100% to less than 0.1 Ohms/square at the same thickness.
To print UHF RFID antenna patterns using the APEC described in this example, a Mark Andy 2200 flexography press is used with 13 BCM Praxair anilox drum to print by flexography method with Du Pont Cyril photopolymer plates (hardness 42) at speeds up to 350 feet per minute with preliminary addition of 0.01 % antifoam just before the press starts printing. The results are kept stable with the addition of ammonia water to re-establish the initial volume every 20 minutes.
Stable in-line conductivity values are verified with the proprietary high-speed in-line resistance feedback device. This process results in fully working RFID transponder labels made in-line on a standard flexogrpahic press with a chip attachment device, using chips from Texas Instruments and Alien Technologies.
The transponders provide read ranges of 14-25 feet.
Example 2. A nanotechnology for increasing APEC conductivity.
The process comprises:
1) The ink is made as described in Example 1, but without or with only a minimal amount of binder resin and 80-100 % water emulsion of pre-reacted fatty acids with different metal salts ( Ag, Pd, Pt, Au, Cr, Ni, Cu, Na, K, Mg etc). to form metal soaps and corresponding surfactants. The main reaction is:
CH3(CHZ)~CH=CH(CH2)~COOH + AgCI = CH3(CHZ)~CH=CH(CHZ)~COOAg + HCI
This is a method for making silver soap of oleic acids. The process is similar for other fatty acids.
At high energy treatment (e.g. thermal shock at 200° C for 1-3 seconds) fatty acid chains are destroyed but the Ag metal remains to form a nano dispersion. Nano dispersed metals (Ag in the example) create conductive bridges between the silver flakes, thus increasing the conductivity of the printed traces. A
further benefit of this process is that some hydrochloric acid (HCI) is also released. HCL reduces the pH, which helps keep the polymer at low viscosity (coil configuration) at the same solids content.

Example 3. APEC formulated as an ink for flexography to print smart packages 1) Predominantly oleic fatty acid treated silver (particle size 1 to 5 pm flakes shaped) is combined and blended in a regular open air mixer with aqueous solution of 50% solids acrylic and resins water emulsion in a proportion 2.4 to 1 by weight with 0.1 %
polyethylene/polypropylene wax, a silicon based adhesion promoter 1 %, an antioxidant 0.1 % and antifoam 0.01 %. A plasticizer is added if needed as half the minimal amount recommended by the manufacturer.
2) Mixing at an average speed of 1500 rpm with a Hi-Vane mixing head (which does not allow the heat to exceed 30° C) results in an APEC with a minimum shelf life of six months.
Fatty acids react with ammonia to form ammonia soaps as described in Example 1. No additional ammonia is added when making smart packaging ink, but it is necessary to add ammonia on the press to recover the initial volume. The initial ammonia comes from the ammonia existing in the pH adjuster of the acrylic resins. It is well known that unsaturated 2 and 3 double bond fatty acids are responsible for the formation of an elastic film after the printing. There is no requirement for special heating above room temperature blown air. If extra heat is applied the unsaturated fatty acids polymerize to form a thin dielectric layer on top of the prints, reducing conductivity. If reduction of resistance is desirable a combination of IR, UV and/or heat shock can be used. However this must be applied under strict control to avoid the opposite effect of the heat treatment, which can release dielectric compounds into the APEC
structure and increase resistance.
3) The small amount of ammonia soap is still an active surfactant that lubricates surfaces to ease mixing and improves printability. The acrylic resins experience simultaneously the well-known influence of ammonia on poly acids to regulate viscosity (change from coil polymer chains to stretched chains), and to adjust pH and drying time. This also contributes to the final conductivity.
4) A dispersion of the APEC is printed (see below) on label stock 55 Cast Gloss Elite (Avery benison) and the prints tested for conductivity, stability on bending, printing properties, adhesion, and other properties, giving the following results:
The measured conductivity is 1.5-2 Ohms/square on a fresh printed sample (1-2 Nm thick films) and this increases over the subsequent 24 hours to 0.8-1.0 Ohms/square.
Commercial example: A SOHN 4 colour 8 inch flexography label printing press is used with 10.9 BCM anilox drum to print by flexography method with Du Pont Cyril photopolymer plates (hardness 42) at speeds up to 100 feet per minute. Results are kept stable by the addition of extra ink and extender every 1 hour of printing. A lamination and die cutting station is used in-line to create pharmaceutical smart package inlays (commercially produced under the Med-ic~ trademark) with an additional RFID sensor chip attachment. Such printed smart packages are able to record removal of medication doses from a blister package and display the results on a computer screen using an associated RFID reader and software.

Similar technology but with different viscosity and resin binders is used to make gravure, screen, dry offset etc. APEC's.
In summary of the foregoing it will be seen that the present invention provides a number of significant advantages and improvements having regard to the known prior art and the general practice of preparing electrically conductive inks for printing on different substrates.
Thus the present invention may be seen to contemplate the following:
1. A method for creating aqueous printable electrical conductors (APEC's) which includes preparation of a relatively stable dispersion but with intentionally partially-wetted silver flake particles to increase conductivity. To achieve the greatest conductivity, the metal particles should not be permanently wetted within the vehicle polymer layers. The basic chemistry of the process is demonstrated (example 1 ).
2. A method for making (APEC's) which includes (as a means to increase the electrical conductivity) a vehicular system using (a) low molecular weight polymers, (b) polymers incapable of intensive cross-linking and (c) other polymer systems that minimize the influence of dielectric binders.
3. A method for making APEC's using ammonia treatment during dispersion preparation and during printing on commercial presses. Although chemically treated flakes (e.g. with fatty acids and salts) are generally known to be incompatible with ammonia, in the proposed method this is used to create partial silver particle wetting. This is unrelated to the common use of ammonia as a pH and viscosity adjuster for water based printing inks (see example 1 ).
4. A method for producing APEC's that requires little or no use of additives (e.g. adhesion promoters, antifoaming agents, waxes, etc.) during preparation but which achieve and maintain desirable printing properties including but not limited to mechanical resistance, shelf life, etc. without compromising the conductivity of the deposited electrical conductor.

5. A method for creating APEC's comprising a way to achieve proper orientation and close positioning between the metal particles without the formation of a thick polymer particle surface-wetting layer. This is accomplished by special selection of raw materials and correct preparation of the vehicle. It is also necessary to prepare the mixing surface through the use of techniques such as particle surface treatment.

6. A method for making an APEC that cures at ambient temperature with or without air blowing (depending on the printing speed) when printed in the thickness range of 1-8 Nm using flexography process.

7. A method for creating an APEC suitable for thick film printing (> 8 Nm) by screen or gravure that is cured with a combination of infrared (IR) and ultraviolet (UV) light sources to achieve optimum conductivity. IR drying is more uniform than hot air drying because the process does not start at the surface but from the bulk of the film and UV light sources are also heat sources whereby the UV light leads to destruction of oil films and sometimes of dry polymer chains in the binder, giving increased conductivity.
8. A method for creating an APEC suitable for application to substrates by different printing processes such as flexography, gravure, screen, or dry offset printing, and to substrates including but not limited to coated paper, uncoated paper, and plastics with treated and untreated surfaces.

9. A method for making APEC's optimized to print on paper substrates and having the highest quality and pricing benefits compared to existing inks and coatings and with ecological gains (via de-inking procedures and recycling benefits).

10. A method for increasing the conductivity of an APEC by exposing the printed surfaces (preferably on the printed side) to a thermal shock during printing (e.g. by touching the surface for 1 to 3 seconds with a hot metal drum at 120°C - 300°C mounted on the printing machine). This increases the conductivity of the printed matter more than 50 percent while reducing the film thickness by 25 percent or more depending on the thickness of the initially printed film. Although such shock drying is not necessary to cure thin-film inks (1-8 pm thickness), it can contribute to maximizing the conductivity of the APEC.

11. A method for increasing the conductivity of an APEC by exposing the printed surfaces to the combined effect of ultrasound and heat to further increase the conductivity of the printed traces.

12. A method for creating an APEC that facilitates chemical control over the conductive properties by means of selecting the type of fatty acids to treat the particle surface (forming nano layers) and removing these layers partially and in controlled fashion via chemical reactions through soap (surfactant) forming and subsequent suppression of the foam.

13. A method for creating an APEC that allows increasing the conductivity chemically by means of forming metal soaps of fatty acids that are susceptible to destruction by oxidation and high temperature shock, releasing a metal in nano dimensional form. This process creates extra bridges between silver flakes and increases conductivity.

14. A nanotechnology for increasing the conductivity of APEC's by forming nanoscale dispersed metals (Ag in the example) to create conductive bridges between the main components (silver flakes).

15 A device and method for controlling the mechanical lay-down of APEC's on a printing press by utilizing an array of devices controlled by an in-line resistance measurement toot (taking and reporting between 10,000 and 1 million resistance measurements per second) connected to a network of pump, injector, pressure, speed and drying equipment controllers in real time (while printing).

16. A graphical display that gives immediate feedback to the press operator of improper APEC
laydown and thus allows both manual and automatic correction of the printing process. Poorly printed APEC structures can also be marked (e.g using ink jet markers or printers inline) to avoid them being utilized downstream (e.g. to skip chip attachment if the APEC structure is an antenna).

Claims (5)

1. A method of creating aqueous printable electrical conductors which comprises the steps of preparing an aqueous polymeric vehicle for the conductors, preparing a volume of metallic conductive particles, and mixing the particles within the vehicle, said vehicle being selected from the group consisting of an aqueous acrylic vehicle, an urethane/acrylic mix and a natural polymer vehicle, said particles having a size within the range of 0.001 µm to 50 µm.

2. The method of claim 1 wherein said metal particles have a particle size in the range of 1-8 µm.

3. The method of claim 1 or claim 2 wherein said metal particles are treated with a fatty acid prior to being mixed with said vehicle.

4. The method of any one of claims 1 to 3 wherein said metal particles are of silver.

5. The method of any one of claims 1 to 4 wherein said polymeric vehicle is a tow molecular weight polymer, is incapable of intensive cross-linking and minimize the influence of dielectric binders.