Aqueous Conductive Silver Ink

Printed electronics is continuing to grow as more and more applications are developed and commercialised.  One of the key stumbling blocks continues to be the practicalities of printing narrow conductive tracks and the ease of use of the materials and processes involved.  One of the key system components, often taken for granted,  is the ink.  The majority of conductive ink compositions in use today are solvent-based thick film systems designed for low speed screen printing.  Water based conductive inks and coatings offer significant ecological advantages over solvent-based compositions, as the latter release solvents into the atmosphere on drying.  Aqueous conductive inks, however, have so far not offered the high conductivity, or low electrical resistivity, achievable with solvent-based formulas.

Sun Chemical Corp. have just been granted a patent for an aqueous ink with high conductivity and good printing properties.  This invention more specifically relates to an aqueous conductive silver ink suitable for use in RFID and other electronic technologies. The composition is highly conductive and requires reduced drying energy. In addition, it may be applied to low cost substrates via high speed printing processes. The key components of the ink formulation include: (meth)acrylic copolymer or salt thereof; conductive particles; an anionic surface wetting agent; defoamer and water.  The first claim suggests the ink can be up to 80% water depending on the amount of the other ingredients.

The patent is US8709288 and it was issued on 29 April 2014.  The Inventors are Jason Rouse and Dave Klein.

The independent claims from the patent are listed below:

1. A method of forming a conductive pattern on a substrate comprising applying a conductive composition comprising (a) metallic silver conductive particles, (b) water soluble styrene/(meth)acrylic copolymer, (c) an anionic wetting agent, (d) defoamer and (e) 10 to 80% water, the composition providing a sheet resistance of less than 0.83 ohms/sq, on the substrate and drying the composition.
34. A method of forming a conductive pattern on a substrate comprising applying a conductive composition consisting essentially of (a) metallic silver conductive particles, (b) water soluble styrene/(meth)acrylic copolymer, (c) an anionic surfactant, (d) defoamer and (e) 10 to 80% water, the composition providing a sheet resistance of less than 0.83 ohms/sq, on the substrate and drying the composition.
37. A method of forming a conductive pattern on a substrate comprising applying a conductive composition consisting of (a) metallic conductive particles, (b) water soluble styrene/(meth)acrylic copolymer, (c) an anionic surfactant, (d) defoamer, (e) 10 to 80% water, and optionally another solvent, the composition providing a sheet resistance of less than 0.83 ohms/sq, on the substrate and drying the composition.

The differences above are subtle but important, claim 34 describes the composition with an anionic surfactant rather than a wetting agent, and claim 37 describes a more general formulation with any metallic conductive particle and optionally includes another solvent.  Each claim includes the requirement that the composition provides a resistance of less that 0.83 ohms/sq on the substrate.

 

Phil’s Comments:

Good to see that research into inks suitable for printed electronics is providing environmentally acceptable formulations.  Reducing solvents and also reducing the energy requirements for drying are all good directions for the ink design.  We sometimes forget the huge demand put upon the inks we use, for example they need good abrasion and chemical resistance when dried so that they are not easily scratched or wiped off during subsequent uses, they need to have proper rheology and substrate wetting properties to obtain good ink transfer and graphic reproduction. Additionally, the ink should possess good flexibility and thermal stability to withstand the physical deformation to which the substrate may be subjected.  Let’s remember the research that goes into the inks when we next see a printed electronics design in use!

 

“Thinking Ink” – allows printed packaging to interact with smartphones

A recent patent lawsuit filed by Touchpac Holdings against TouchBase Technologies Inc. has highlighted the US Patent US8497850 which is owned by Printechnologics GmbH (T+Ink).  Details of the lawsuit were reported by Printed Electronics World in their article on 18 Feb 2014.

The patent was granted on 30 July 2013 and describes the technology that T+Ink have developed and called Touchcode.  This is a highly secure printed conductive ink signature embedded in packaging, labels and other objects that a smartphone or other sensing device can detect through capacitive touch.  The patent provides some detail of the system which is defined in independent claim 22:

A system comprising: a. an information carrier comprising a dielectric and/or conductive pattern which encodes information, and wherein the information carrier (1) comprises a substrate (2) with a first dielectric coefficient ∈1 and a second material with a second dielectric coefficient ∈2, the second coefficient ∈2 being different from the first coefficient ∈1, or wherein the information carrier (1) comprises a substrate (2) with a first conductivity σ1 and a second material with a second conductivity σ2, the second conductivity σ2 being different from the first conductivity σ1, b. a detection device having a capacitive touch screen; the detection device decodes the information upon interaction between the information carrier and the touch screen, wherein the interaction is based on a difference in the dielectric coefficient and/or the conductivity of the pattern, wherein the interaction is induced by relative motion between the information carrier and the touch screen.

A more visual explanation can be found on the T+Ink Touchcode website.  As explained on the website, Touchcode is an invisible electronic code printed on paper, cardboard, film or labels. Just put the product on the display of your smartphone/tablet/multitouch device to read the data.  In this way you can add interactivity to just about any product.  The system is similar to QR codes, barcodes or other tags but does not require activating a camera app on the phone or tablet to work.

Metal Nanoparticle Conductive inks Patented by Aculon Inc

Aculon Inc. were granted a patent on 15 October for “Electroconductive Inks made with Metallic Nanoparticles”.  Aculon are well known for their surface coatings using nanoparticle technology, featuring improved adhesion to a wide range of surfaces and adding functionality for numerous applications including printed electronics.  It is therefore no surprise that conductive inks are described with advantageous adhesion to a variety of substrates including silicon, glass and plastic.

The granted patent is US8558117 and the claims are quite broad but require the inks to have an omega substituted organophosphoric acid, this is the added ingredient which helps to improve the adhesion.

The first 10 claims are shown below:

What is claimed is: 1. An electroconductive ink comprising: (a) a dispersion of metal nanoparticles with a dispersant on the surface of the metal nanoparticles and as a separate component, (b) an omega substituted organophosphorus acid; the metal nanoparticles being present in amounts of 5 to 80 percent by weight and the omega substituted organophosphorus acid being present in amounts of 0.0005 to 5 percent by weight; the percentages by weight being based on total weight of the electroconductive ink.

2. The composition of claim 1 in which the metal is selected from copper, silver, palladium, gold or any combination thereof.

3. The composition of claim 2 in which the metal is silver.

4. The composition of claim 3 in which the dispersant is an organic carboxylic acid.

5. The composition of claim 4 in which the organocarboxylic acid contains from 4 to 36 carbon atoms.

6. The composition of claim 1 in which the metal nanoparticles have a particle size of 1 to 500 nanometers.

7. The composition of claim 1 in which the dispersion contains an organic diluent.

8. The composition of claim 7 in which the organic diluent is selected from hydrocarbons, alcohols, esters and ketones.

9. The composition of claim 1 in which the organo groups of the organophosphorus acid have omega substituted functional groups.

10. The composition of claim 1 in which the omega substituted functional groups are selected from phosphonic acid, carboxylic acid, hydroxyl, thiol and amine including various combinations thereof.

Phil’s Comments:

I have reported on a number of conductive ink patents over the past year or two and a common theme running through them is the need for good adhesion along with their conductivity.  The patents generally differ in the description of the adhesion promoting substances used in the nanoparticle inks and on other improved properties that enhance their application for printed electronics.  Successful conductive ink formulations are essential for printed electronics to become a mainstream technology for building new devices and specialist inks with patented formulations will continue to dominate the scene for a number of years before generic formulations will be cheaply available as we see for conventional inkjet printing.

Printed Electronics via Electrophotography

There are many different technologies used for printing electronic circuits such as screen printing, flexography, gravure, offset lithography and inkjet – a review of these can be found on the Wikipedia page for Printed Electronics.  One of the printing technologies which is not currently used is electrophotography although there have been various publications including patent applications that have proposed this route.

A recent patent alert I received indicated that Eastman Kodak has just been granted a method patent for producing a printed circuit using electrophotography.  The company is just emerging from chapter 11 bankruptcy and one of its stated aims is that it will focus on commercial printing.  This patent therefore fits well with its new strategy.  Although the patent title indicates a printed circuit board the description makes it clear that flexible devices are envisaged such as RFID, sensors and flexible displays.

The patent is US8497057 and was published on 30 July 2013.

Title: Method of Producing Electronic Circuit Boards using Electrophotography

Inventors: Thomas N Tombs; Donald S Rimal

Filed: 14 Sept 2012 and is a division of US 12/341099 filed on 22 Dec 2008

Abstract: The present invention provides a method producing printed electronic circuits using electrophotography.

The granted claims read as follows:

1. A method for producing a printed circuit, the method comprising performing the following steps in order:

charging a primary imaging member;
creating an electrostatic latent image by image-wise exposing the primary imaging member;
image-wise depositing thermoplastic particles onto the primary imaging member;
transferring the thermoplastic particles to an electrically insulating substrate;
a first fixing step of permanently fixing the transferred thermoplastic particles;
depositing conductive particles over the substrate;
a second fixing step of permanently fixing the conductive particles on the fixed thermoplastic particles so that conductive paths are formed between fixed conductive particles; and
removing conductive particles from portions of the substrate other than that coated with the fixed thermoplastic.

2. The method according to claim 1, wherein the conductive particles is permanently fixed by the application of heat.

3. The method according to claim 1, wherein the conductive particles is permanently fixed by the application of pressure.

4. The method according to claim 1, wherein the conductive particles is permanently fixed by the exposure to solvent vapor.

5. The method according to claim 1, wherein the conductive particles is metal.

 

Phil’s Comments

This method patent will shortly be complemented with the original patent which has been granted and is due to be published later this month (info obtained from the USPTO PAIR database).  I will post the details of this one when it is public.  I was interested to see if this was one of the first granted patents for electrophotography and had a quick look through the citations and examiner’s search results.  The USPTO PAIR database is a mine of information when you want to investigate the process by which a US patent is granted and see the comments made by the examiners.

Although a number of documents were found in the search none was considered relevant enough to impact the inventive step of the above claims as they are finally worded.  It should be noted that considerable changes were required to the original 2008 application.

I did a brief search for the keywords electrophotography and printed electronics in titles and abstracts came up with the following:

US7638252: Electrophotographic printing of electronic devices

and

US20070234918 A1:  System and method for making printed electronic circuits using electrophotography

This patent application was never granted and was abandoned.

From the references and citations it is clear that electrophotographic approaches for printed electronics have been documented well before 2008.  However, the Eastman Kodak patent has described a stepwise process which is somewhat different to the prior art and reviewing the prosecution it is clear that considerable detail has been gone over in agreeing the allowed claims over the prior art.

 

 

 

Copper Inks for Printed Electronics

I recently spotted a patent for copper based metallic inks jointly granted to Applied Nanotech Holdings, Inc. and Ishihara Chemical Co., Ltd.  The conductive inks I have previously reported on have been silver or graphene based, these can be very expensive and so there is always interest in a lower cost alternative.  Copper metal is a standard in the electronics industry and is about one tenth the cost of silver.

Patent summary:

US8404160   –  METALLIC INK

Inventor(s): LI YUNJUN [US]; ROUNDHILL DAVID MAX [US]; LI XUEPING [US]; LAXTON PETER B [US]; ARIMURA HIDETOSHI [US]; YANIV ZVI [US]

Applicant(s): APPLIED NANOTECH HOLDINGS INC [US]; ISHIHARA CHEMICAL CO LTD [JP]

Filing date: 31/03/2009
Issue date: 26/03/2013

Abstract
A metallic ink including a vehicle, a multiplicity of copper nanoparticles, and an alcohol. The conductive metallic ink may be deposited on a substrate by methods including inkjet printing and draw-down printing. The ink may be pre-cured and cured to form a conductor on the substrate.

Claim 1:

1. A conductive metal ink comprising: a vehicle comprising two or more oxygenated organic compounds; copper nanoparticles; and a medium length chain aliphatic alcohol, wherein the ink does not comprise a binder.

There are a further 23 claims giving more details of the components mentioned in claim 1.

Two figures from the patent give a flow chart for two processes which can be used:

Direct printing:

ink1

Patterning after the ink deposition:

ink2

Phil’s Comments

This is a key patent for copper inks and has a broad first claim which had to be modified from the original application by the addition of the restriction that the ink does not contain a binder and I suspect this was to get around some of the prior art identified by the patent examiner.

Six very detailed examples are given which illustrate the invention and provide further details of the printing process, the curing or sintering that is applied to produce the conductive tracks on a range of materials including flexible substrates.

Silver Nanoparticle Inks for Printed Electronics

Xerox are continuing to add to their patent portfolio of silver nanoparticle inks for printed electronics.  A new patent was granted on 4th Dec 2012 adding to the one already granted on 17th April 2012.  These two patents are summarised below:

US8158032  (B2)  –  SILVER NANOPARTICLE INK COMPOSITION FOR HIGHLY CONDUCTIVE FEATURES WITH ENHANCED MECHANICAL PROPERTIES

Abstract: A conductive ink composition comprising organic-stabilized silver nanoparticles and a solvent, and a polyvinyl alcohol derivative resin of Formula (1) wherein R1 is a chemical bond or a divalent hydrocarbon linkage having from about 1 to about 20 carbons; R2 and R3 are independently an alkyl, an aromatic or substituted aromatic group having from about 1 to about 20 carbon atoms; x, y and z represent the proportion of the corresponding repeat units respectively expressed as a weight percent, wherein each repeat unit is randomly distributed along polymer chain, and the sum of x, y and z is about 100 weight percent, and wherein the polyvinyl alcohol derivative resin is present in an amount of from 0.1 to about 5 weight percent of the ink composition.

US8324294  (B2)  –  SOLVENT-BASED INKS COMPRISING SILVER NANOPARTICLES

Abstract: High performing nanoparticle compositions suitable for printing, such as by inkjet printing, are provided herein. In particular, there is provided a conductive ink formulation comprising silver nanoparticles which has optimal performance, such as, reduced coffee ring effect, improved adhesion to substrates, and extended printhead de-cap time or latency time. The ink formulation comprises two or more solvents and a resin.

Phil’s Comments

The key technology improvements over the general prior art are for the development of inks which do not dry out and clog the printhead along with improvements in the adhesion of the ink to the substrate, reduction of the coffee ring effect and reduction of particle aggregation which leads to black dots in the printed lines.

The improved adhesion is as a result of the incorporation of a resin in the ink based on a polyvinyl alcohol derivative.  The earlier patent goes into some detail over the formulae for these resins.

The extended drying times to prevent printhead clogging are the result of incorporation of two or more ink vehicles (solvents) with vapour pressure properties as described in the second patent.  Typical compositions are based on aliphatic hydrocarbons comprising at least one cyclohexane ring, cyclic terpene, cyclic terpinene, terpineol, methyl naphthalene, and mixtures thereof.  Typical drying times are in excess of 5 hours and one claim states up to about 1 week.

The nanoparticles are stabilised with organo-amines, the first patent gives more detail on these structures,  a preferred compound is dodecylamine.

 

As always please leave a comment if you require further details or are interested in patent landscapes related to these conductive inks.

Printed Electronics using Graphene based Inks

When I spotted a recent granted patent with the title Printed Electronics, it was an obvious candidate for a Phil’s Patent Pick.  US8278757 was published on 2nd October 2012 and jointly assigned to Vorbeck Materials Corp. and Univ. of Princeton.  It was originally filed on 9th January 2009 and has a priority date of 5th February 2008.

The patent has quite broad claims and describes printed electronics devices made using a conductive ink comprising functionalized graphene sheets and a binder.  I don’t always reproduce the claims but in this case they are fairly simple to follow:

1. A printed electronic device, comprising a substrate comprising at least one surface, wherein a layer of an electrically conductive ink has been applied to a portion of the surface, and wherein the ink comprises functionalized graphene sheets and at least one binder.

2. The device of claim 1, wherein the substrate comprises paper and/or cardboard.

3. The device of claim 1, wherein the substrate comprises at least one polyolefin.

4. The device of claim 1, wherein the substrate comprises at least one polyimide.

5. The device of claim 1, wherein the ink further comprises at least one dispersant.

6. The device of claim 1, wherein the binder is a polymeric binder.

7. The device of claim 6, wherein the binder is one or more of poly(ethylene oxide), poly(propylene oxide), and ethylene oxide/propylene oxide copolymers.

8. The device of claim 1, wherein the ink further comprises at least one metal component.

9. The device of claim 8, wherein the metal is silver and/or copper.

10. The device of claim 1, wherein the ink further comprises at least one electrically conductive polymer.

11. The device of claim 1, wherein the ink further comprises at least one carbonaceous material other than the functionalized graphene sheets.

12. The device of claim 1, wherein the functionalized graphene sheets have a surface area of from about 300 to about 2630 m²/g.

13. The device of claim 1, wherein the functionalized graphene sheets have a surface area of from about 400 to about 2400 m²/g.

14. The device of claim 1, wherein the functionalized graphene sheets have a carbon to oxygen ratio of about 60:40 to 20:1.

15. The device of claim 1, further comprising one or more components selected from the group consisting of processors, memory chips, batteries, resistors, diodes, capacitors, and transistors.

16. The device of claim 1 in the form of an integrated circuit.

17. The device of claim 1 in the form of a printed circuit board.

18. The device of claim 1 in the form of a light-emitting diode display.

19. The device of claim 1 in the form of a radiofrequency identification device and/or a radiofrequency device antenna.

20. The radiofrequency identification device and/or radiofrequency device antenna of claim 19 in the form of a radiofrequency identification device and/or antenna for use with ultra-high frequencies.

21. A method for forming a printed electronic device, comprising the step of applying an electrically conductive ink to a substrate, wherein the ink comprises functionalized graphene sheets and at least one binder.

22. The method of claim 21, wherein the substrate is selected from one or more of paper, cardboard, polyolefin, and polyimide.

23. The method of claim 21, wherein the electrically conductive ink further comprises a carrier.

24. The method of claim 21, wherein the electrically conductive ink further comprises a dispersant.

25. The method of claim 21, wherein the functionalized graphene sheets have a surface area of from about 300 to about 2630 m²/g.

Phil’s Comments

It is worth looking at the examples in the patent.  The printing methods and samples generated for testing are pretty crude and suggest that only simple circuits have been generated such as RFID antennas.  The type of ink suggests that printing processes will be of the gravure or flexo type rather than ink-jet.  However, the wording of the allowed claims suggests that just about any printed electronic device containing a layer that includes graphene sheets with a functional group in a binder would infringe this patent.  The majority of the claims are all about the device and not the material.  The last five claims are about the method.  Vorbeck sell the graphene based inks and with this patent they will require anyone using the inks to enter into some form of agreement with them to make the devices.  It will be interesting to see how this works in practice and whether the patent will stand the test of time.