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!

 

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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.