WO2018134411A1

WO2018134411A1 - Stretchable conductive ink

Info

Publication number: WO2018134411A1
Application number: PCT/EP2018/051475
Authority: WO
Inventors: Beatriz MEDINA RODRÍGUEZ; Mireia BLANES GUÀRDIA; Francisco Manuel RAMOS PÉREZ
Original assignee: Francisco Albero S.A.U.
Priority date: 2017-01-23
Filing date: 2018-01-22
Publication date: 2018-07-26

Abstract

The present invention relates to a stretchable conducive ink suitable for printing electric circuits on a stretchable, thermoformable and flexible substrates and textiles. The ink is water-based and comprises a conductive material, an EVA copolymer, a co-solvent, and a dispersing agent. It relates also to the use of such ink composition for manufacturing a conductive polymer thick film, to a process for manufacturing a conductive polymer thick film, to a conductive polymer thick film, to the use of such conductive polymer thick film for manufacturing an electric circuit, and to the electric circuit comprising the conductive polymer thick film.

Description

STRETCHABLE CONDUCTIVE INK

Technical Field

The present invention relates to stretchable conductive inks for the manufacturing of electronic circuits on stretchable substrates.

Technical Background

Flexible electronic devices can be produced by depositing single or multiple layers of functional materials, such as conductive, resistive, semi-conductive or dielectric inks, onto a flexible substrate. Today, the application of some of these materials is being performed by high cost, small capacity processes such as physical and chemical vapour deposition. Although these methods give very controlled and smooth functional layers, they require a dust free, controlled temperature environment, which is expensive to build and maintain, and not viable for high speed production.

Other techniques, such as spin coating, have a significant lower-cost.

However, the waste of functional material becomes an important drawback. Furthermore, all these technologies are wider used for large-area electronics for layer coating, and they are not a suitable solution for transferring a concrete pattern to the substrate, since chemical or physical etching for "drawing" electronic circuits is needed.

On the other hand, printing tends to be exclusively an additive process and offers a low-cost, high throughput manufacturing process of lesser quality, combined with the possibility of patterning circuitries of high complexity. In addition, along with the advantages of low costs and waste, printing also offers a wider variety of flexible and rigid substrate choices. Computer keyboards, hand calculators, photovoltaic cells, and telephones are examples of high-volume devices that benefit from this economical method of circuit manufacture, as well as smart packaging and smart textiles.

However, there are many challenges to printing functional electronic devices, such as print quality, smoothness and registration accuracy requirements well beyond what is required for graphic printing. Substrate properties (smoothness, compressibility, porosity and ink receptivity, wettability, etc.) and ink properties (ink chemistry, viscosity, rheological behaviour, solvent evaporation rate, drying, etc.) are influencing factors.

In general, printed electronics are based on structures formed by a polymer thick film (PTF) circuitry pattern, which is then transferred onto a particular substrate. There are several methods used to apply both conductive and dielectric layers of PTF. Screen printing, gravure and flexographic (Flexo) techniques are conventional technologies, wherein screen printing has stood out among them in the electronics manufacturing sector. Currently, inkjet printing is emerging as an alternative for thin film electronics.

The basic constituents of a PTF conductive ink are metallic particles, resin and solvents. But several limitations typically exist in these materials including low conductivity, poor stretchability, and resistance increase with applied strain.

In the prior art a considerable number of technical solutions have been disclosed, such as the following examples.

In US patent US4371459 it is disclosed a screen-printable conductor composition comprising: a conductive phase containing silver and base metal powders dispersed in a solution of a multipolymer prepared by copolymerization of vinyl acetate, vinyl chloride, and an ethylenically unsaturated dicarboxylic acid and a linear aromatic polyester resin dissolved in volatile nonhydrocarbon solvent.

In US patent application US-A-2008/182090 it is disclosed an ink composition containing: conductive particles; an anionic wetting agent; styrene/(meth)acrylic copolymer or salt thereof and blends thereof with other (meth)acrylic copolymer resins or salts; defoamer; and water, which is suitable for high speed printing to produce electronic circuitrty such as radio frecuency identification antennas.

In the article Xu et at., Highly Conductive and Stretchable Silver Nanowire Conductors, Adv. Mat., 2012, 24(37), 51 17-5122, it is disclosed a highly conductive and stretchable conductor with silver nanowires embedded in the surface layer of poly(dimethylsiloxane) (PDMS).

In the article Shen et at., Preparation of solid silver nanoparticles for inkjet printed flexible electronics with high conductivity, Nanoscale, 2014, 6, 1622-1628, it is disclosed the preparation of polyacrylic acid coated silver particles for inkjet printing of flexible electronics.

In International patent application WO-A-2014/1 12683 it is disclosed A conductive ink composition comprising: conductive metal particles including submicron- sized copper (Cu) particles and nano-sized silver (Ag) particles at a specific ratio of copper:silver; and based on 100 parts by weight of the conductive metal particles, 2-10 parts by weight of a binder and 5-20 parts by weight of an organic solvent. It is also disclosed that an electrode can be formed by printing that ink on a flexible substrate made of polyimide resin. In International patent application WO-A-2014/1 13937 it is disclosed a flexible conductive ink composition comprising: a resin binder, silver-plated core conductive particles, and conductive particles having a surface area of at least 1 .0 m²/g.

In the article Woo et at., Highly conductive and stretchable Ag nanowire/carbon nanotube hybrid conductors, Nanotechnology, 2014, 25(28), 285203, it is disclosed that the manufacturing of stretchable conductors through simple, cost- effective and scalable methods is a challenge, and authors report on an approach used to develop nanowelded Ag nanowire/singlewalled carbon nanotube (AgNW/SWCNT) hybrid films to be used as high-performance stretchable conductors.

In the review N. Li Pira, Smart integrated systems and circuits using flexible organic electronics: automotive applications, in Handbook of Flexible Organic Electronics Materials, Manufacturing and Applications, Ed. S. Logothetidis, Elsevier, Cambridge, 2015, it is disclosed that the most common types of electrically conductive inks are based on conductive nano- or microparticle components that have been dispersed into a polymer matrix, and that these particles metallic based, such as silver, copper and gold as well as conductive oxides. It is disclosed also that are commercial silver water- and solvent-based inkjet inks. As for this technique, fast drying at room temperature is not crucial.

In International patent application WO-A-2015/073654 it is disclosed a polymer thick film conductor composition comprising: 30 to 70% by weight silver flakes; 20 to 50% by weight first organic medium comprising 10 to 50% by weight of thermoplastic urethane resin dissolved in a first organic solvent, and 5 to 20% by weight second organic medium comprising 10 to 50% by weight thermoplastic polyhydroxyether resin dissolved in a second organic solvent.

In the article Rao et at., Conductive Silver Inks and its Applications in Printed and Flexible Electronics, RSC Adv., 2015, 5, 77760-77790, it is disclosed that solvent based silver nanoparticles conductive inks are capable to substitute the printed circuit boards technology while reducing the cost of manufacturing. It is also disclosed that the binder is a necessary component for the formulation of a conductive ink, because it is used to bind the particles and works as an effective agent in the synthesis of conductive inks, being hydroxyethyl cellulose the best known binder.

In the review article Rajan et at., Silver nanoparticle ink technology: state of the art, Nanotechnol. Sci. Appl., 2016, 9, 1 -13, it is disclosed the advantages offered by print electronics based on inkjet printing to the consumer and that the most promising tool to achieve the target depends upon the availability of nanotechnology- based functional inks, being silver nanoparticle-based inks the most widely diffused product, settled technology, and the highest sales volumes are related to the market.

KR 101 433 639 B1 discloses a copper nano-gel composition, which comprises a copper nano-gel composition dispersed in an organic solvent, wherein the copper nano-gel composition is obtained by dissolving a copper conductor in a mixed solvent (water and ethylene glycol), adding a polymer binder, and a reducing agent. Polyvinylpyrrolidone (PVP) is further added to the ink for preventing oxidation. In a preferred embodiment, the binder is selected from a group including EVA.

US 2014/290350 A1 refers to a specific metal compound, to a polymer article containing such metal compound and method for preparing the polymer article as well as selective metallization of a surface of the polymer article are also provided. In addition, US 2014/290350 A1 also discloses an ink composition and the selective metallization for a surface of the insulative substrate using the ink composition.

WO 2013/097729 A1 discloses an ink composition, which comprises a metal compound and an ink vehicle, wherein the metal compound is at least one selected from a group consisting of a compound of formula (I) and a compound of formula (II): Τί02-σ (I), M1 M2pOq (II), wherein 0.05 < σ < 1 , 8, M1 is at least one element selected from a group consisting of groups 2, 9-12 of the periodic table according to lUPAC nomenclature, M2 is at least one element selected from a group consisting of groups 3-8, 10 and 13 of the periodic table according to lUPAC nomenclature, and 0 < p < 2, and 0 < q < 4.

Despite of the many contributions described in the prior art, there remains the need of having a stretchable conductive ink composition showing a good stretchable behaviour by maintaining a very low electrical resistance variation when the substrate is subjected to a strain, and being easy to implement industrially the manufacture of polymer thick films by deposition of this ink on a stretchable substrate.

Object of the invention

The object of the present invention is an ink composition.

Also part of the object of the invention is the use of such ink composition for manufacturing a conductive polymer thick film conductor composition.

Also part of the object of the invention is a process for manufacturing a conductive polymer thick film.

Also part of the object of the invention is a conductive polymer thick film. Also part of the object of the invention is the use of such conductive polymer thick film for manufacturing an electric circuit. Also part of the object of the invention is an electric circuit comprising the conductive polymer thick film.

Brief description of the figure

Figure 1 illustrates the variation of the resistance (AR/R₀), being R₀ the initial resistance, versus the elongation [(L-L₀)/U], being L₀ the initial length and L the length during the elongation test, for different conductive inks. "A" is the stretchable conductive ink prepared in Example 1 ; "B" is the stretchable conductive ink prepared in Example 2; and "C", "D", "E", "F", "G" and "H" are commercially available conductive inks. "C", "D" and "E" are described as flexible inks, "F" and "G" are thermoformable inks and "H" is sold as a stretchable ink.

Detailed description of the invention

The object of the present invention is an ink composition comprising: 1 ) a conductive material,

2) an ethylene/vinyl acetate copolymer,

3) a co-solvent selected from Ci-C₄ alcohols, glycols, and mixtures thereof,

4) a dispersing agent, and

5) water.

The authors of the present disclosure have developed an ink composition that, when printed, confers to the substrate the property of conducting electricity and, if the substrate is stretchable, it shows an excellent stretchable behaviour by maintaining a very low electrical resistance variation and hence, low conductivity losses, when the substrate is subjected to an elongation. Furthermore, the ink composition, being water-based and containing water-miscible co-solvents, allows an easy cleaning of the screen printing tools, and solves the problem of the premature drying of the ink, providing the possibility of a sequential industrial process.

In the present description as well as in the claims, the singular forms "a" and "an" include also the plural reference unless the context clearly indicates otherwise.

In this description, the percentages (%) are expressed in weight/weight unless otherwise indicated, and in the compositions, the sum of the percentages of the different components is adjusted to add up to 100%.

A stretchable conductive ink is an ink that when printed on a stretchable substrate, after drying of the ink results in a polymer thick film, which conducts electricity and is stretchable. In this description the wording "ink" refers to a stretchable conductive ink.

Conductive material

The stretchable conductive ink composition of the invention comprises a conductive material.

This stretchable conductive ink composition comprises a conductive material generally selected from silver, gold, copper, nickel, platinum, aluminium or carbon. Metal material can be in the form of, for example, particles, preferably spherical, flakes, or nanowires. Carbon material can be, for example, carbon black, carbon particles, carbon nanofibers, carbon nanotubes, graphite, or graphene. Preferably the composition of the invention comprises a conductive material selected from silver, copper, gold, carbon, and mixtures thereof; more preferably from silver particles, silver flakes, silver nanowires, carbon black, and mixtures thereof; and more preferably silver flakes, silver nanowires, and mixtures thereof.

Conductive material suitable for the stretchable conductive ink composition of the invention shows usually a particle size in the range of 1 to 100 μηι, preferably 1 to 50 μηι, and more preferably 1 to 25 μηι.

In one embodiment the conductive material consists of silver flakes. Silver possesses high electrical conductivity (6.3 x 10⁷/ΩΛη for silver nanoparticles) and is resistant to oxidation.

In another embodiment, the conductive material consists of a combination of silver flakes and silver nanowires. In a more preferred embodiment the combination comprises from 95% to 99% by weight of silver flakes and 1 % to 5% by weight of silver nanowires, in a yet more preferred embodiment, from 96% to 97% by weight of silver flakes and from 3% by weight to 4% by weight of silver nanowires.

This combination mixture of silver flakes and silver nanowires in the form of an intimate mixture of particles can be obtained by a process, which, for example, comprises the step of suspending the silver flakes and the silver nanowires in a solvent selected, for example, from the group comprising water, alcohols (such as e.g. ethanol, isopropyl alcohol, n-butyl alcohol, /-butyl alcohol, n-pentanol), glycols (such as e.g. ethylene glycol, propylene glycol, PEG 200, PEG 400), adding a dispersing agent, homogenizing in a planetary ball mill, drying at about 300⁵ C, and milling the solid.

Silver flakes to be incorporated in the ink of the invention usually show a diameter below 4 μηι. Silver nanowires can be used in the form of a suspension in a solvent, for example, ethanol, isopropyl alcohol, ethylene glycol, propylene glycol, PEG 200, or PEG 400, comprising from about 0.1 % to 5% by weight of silver nanowires. In a preferred embodiment, silver nanowires are suspended in ethanol in a concentration comprised from 0.1 % to 2% by weight, more preferably from 0.5% to 1 .5% by weight.

The content of the conductive material in the stretchable conductive ink composition is comprised between 30% to 95% by weight based on the total weight of the composition, preferably between 50% to 85%, more preferably between 60% to 80%, and yet more preferably between 65% and 75%.

To improve the mechanical or chemical properties of the conductive material, small amounts of other metals may be added to the ink composition. Some examples of such metals include: antimony, barium, boron, cerium, cobalt, europium, gadolinium, gallium, indium, lanthanum, lead, magnesium, molybdenum, palladium, platinum, ruthenium, silicon, strontium, tantalum, tin, titanium, tungsten, yttrium, zinc, and mixtures thereof. To enhance electrical properties of the conductive material, small amounts of other metals, such as copper or different shaped-conductor particles such as nanowires or nanotubes of conductive material, of silver, copper, carbon of mixture thereof. The additional metal(s) may comprise up to about 1 % by weight based on the total weight of the composition.

Conductive materials are commercially available to the skilled person, for example, through Metalor International, Grupo Antolin, Cambrios Technologies, Sigma-Aldrich, Novarials, or Cabot Corporation.

Binder

The ink composition of the invention includes a binder, which is an ethylene/vinyl acetate copolymer (EVA copolymer). The binder provides adhesion to the substrate, cohesion of the conductive material, and it protects the conductor from external effects.

In a preferred embodiment, the EVA copolymer shows a Tg comprised from -18⁵ C to 8⁵ C, more preferably comprised from -10⁵ C to 0⁵ C, and more preferably from -8⁵ C to -4^Q C.

Preferably, the EVA copolymer is added to the ink composition in an aqueous dispersion form, finding the solid content comprised in the range of 30% to 80% by weight, preferably 35% to 70%, more preferably 40% to 60%, and yet more preferably 45% to 55%. Usually the EVA copolymer dispersion shows a viscosity from 2000 to 10000 mPa.s, preferably from 4000 to 5000 mPa.s, measured according to DIN EN ISO 2555, RVT, Spindle no. 3, 20 rpm, at 23⁵ C.

Usually the EVA copolymer has a particle size comprised from 0.1 to 3 μηι, preferably from 1 to 1 μηι.

Generally the content of the EVA copolymer in the stretchable conductive ink composition of the invention is comprised from 2% to 15% by weight, expressed as dried matter (i.e. as 100% solids), based on the total weight of the ink composition, preferably from 4% to 10%, and more preferably from 6% to 9%. In the case of using a commercially available EVA copolymer supplied having a solid content of 45% by weight, in the ink of the invention it is used from 4.4% to 33.3% by weight of the EVA copolymer tel que/ to obtain the required solid content of binder in the ink (i.e. 2% to 1 5%). The skilled person can calculate easily the amount of commercially available product to obtain the required solid content of the copolymer in the ink.

EVA copolymers are commercially available from companies such as, for example, DuPont, Celanese, Arkema or ExxonMobil Chemical.

The use of the EVA copolymer in the ink composition provides an excellent stretchable behaviour of the substrate printed therewith by maintaining a very low electrical resistance variation when the substrate is subjected to an elongation.

Co-solvent and water

The stretchable and conductive ink composition of the invention includes a co-solvent selected from Ci-C₄ alcohols, glycols, and mixtures thereof. Preferably the co-solvent is a glycol selected from ethylene glycol, propylene glycol, 1 ,2-butanediol, and 1 ,3-butanediol, and more preferably the co-solvent is propylene glycol.

This co-solvent is miscible in water and helps to control the viscosity, to dissolve the binder, and to maintain wet the ink on the screen during its application for sequentially printings before the ink is transferred to the substrate.

Usually the content of the co-solvent in the ink is comprised up to 25% by weight based on the total weight of the ink composition, preferably from 5% to 20%, more preferably from 7% to 15%, and yet more preferably from 9% to 1 2%.

The ink of the invention also includes water as solvent. The content of water in the ink is comprised up to 50% by weight based on the total weight of the ink composition, preferably from 5% to 20%, and more preferably from 7% to 15%. The water present in the ink can proceed also from the binder, which is generally available dispersed in water. The presence of water and a water-miscible co-solvent allows an easy cleaning of the screen printing tools, and solves the problem of the premature drying of the ink, providing the possibility of a sequential industrial process and reducing manufacturing costs due to the saving in cleaning materials and time.

Dispersing agent

The ink composition of the invention comprises a dispersing agent.

Dispersing agents are products that help the stabilization of dispersions by separating the particles of the dispersed material avoiding settling or clumping. Dispersing agents also may reduce significantly the viscosity of the dispersion

Dispersing agents are generally selected from surfactants and polymers. In the ink of the invention, the dispersing agent is preferably a surfactant. Surfactants can be anionic, cationic, non-ionic or amphoteric. In a preferred embodiment the surfactant is anionic or non-ionic, and more preferably is anionic.

Anionic surfactants are compounds that dissociate in aqueous solution into a negatively charged ion (anion) carrying the surface active properties and a positively charged ion (cation). Cationic surfactants also dissociate in aqueous solution into an anion and a cation, but in this case the cation is the carrier of the surface active properties. Non-ionic surfactants cannot dissociate into ions in aqueous solution, but are solubilized in water due to the presence of polar groups. Amphoteric surfactants are substances whose hydrophilic group can have a positive, negative or both a positive and negative charge in aqueous solution, depending on the pH.

In the ink of the invention, the most common anionic surfactants can be used as dispersing agent, among them: soaps (salts of organic carboxylic acids), alkylbenzene sulfonates, a-olefin sulfonates, alcohol sulfates, alcohol ether sulfates, dialkylsulfosuccinates, alkyl and alkyl ethoxy sulfosuccinates, mono sulfosuccinates, alkyl ether carboxylates, alkyl and alkyl ether phosphates, petroleum sulfonates, alkane sulfonates, alkyl phenol and alkyl phenol ether sulfates, a-sulfo methyl esters, fatty acid isethionates, acyl sarcosinates, taurides, and mixtures thereof. In a preferred embodiment, the dispersing agent is selected from the group of CrCi₈ alkyl phosphates and CrCi₈ alkyl ether phosphates. This group of anionic surfactant can be seen as phosphoric esters of CrCi₈ alcohols or alkoxylated (i.e. ethoxylated, propoxylated or butoxylated) CrCi₈ alcohols.

If present, usually the content of the dispersing agent is comprised from 1 % to 5% by weight based on the total weight of the ink composition, preferably from 2% to 4%, and more preferably from 2.5% to 3.5%. Surfactants can be obtained commercially from companies such as, form example, BASF, Huntsman, Kao, Evonik, Croda, or Stepan.

Additional components

The ink composition of the invention may comprise one or more additional components to modulate features such as viscosity, rheology, and foaming behaviour. These additional components are, for example, defoamers, thickening agents, biocides, and mixtures thereof.

In a preferred embodiment, the ink composition further comprises a defoamer to prevent the formation of foam or the remove it. Defoamers work by penetrating the foam lamella, destabilizing it and making it burst. Defoamers contain active substances such as polysiloxanes (silicones), mineral oils, vegetable oils and/or polymers. Particularly effective defoamers tailored to specific applications can be formulated by combining the substances with each other and also by adding fine- particle hydrophobic solids such as silica. Polysiloxanes and modified polysiloxanes belong to the most widely used group of defoaming substances. An enormous range of defoaming agents is accessible via modification with polyethers or other polymers. Mineral oils (aliphatic or aromatic), with their high spreading power and high incompatibility, have long been used as defoamers. As renewable raw materials, vegetable oils are increasingly important in the formulation of defoamers. They exhibit high incompatibility and have very similar properties to those of mineral oils. Examples of polymer-based defoamers include modified fatty acids, polyethers or modified amides. The polarity of the defoamers can be adjusted via the composition of the polymers. In a preferred embodiment, the defoamer is selected from polysiloxanes (silicones), mineral oils, vegetable oils, polymers, and mixtures thereof. In a more preferred embodiment, the stretchable conductive ink of the invention comprises a polymer-based defoamer such as CrCi₈ alkyl polyalkyleneglycol ether.

If present, the content of the defoamer in the ink composition is generally comprised from 0.05% to 0.5% by weight based on the total weight of the composition, preferably from 0.08 to 0.2%, and more preferably from 0.09% to 0.1 %.

Defoamers are commercially available to the skilled person in the art through companies, such as, for example, Nopco, Evonik, Buckman, Byk, Dow Corning, or Emerald Foam Control.

In a preferred embodiment, the ink composition comprises a thickening agent suitable to regulate the rheological behaviour. The thickening agent is selected from those being compatibles with the ink composition from the following group compounds: poly(meth)acrylates, acylated cellulose polymers, polyvinyl acetates, partially hydrolysed polyvinyl acetates, polyvinyl alcohols, polyvinylpyrrolidones, polyoxylates, polycaprolactones, polyurethanes, polycyanoacrylates, vinyl acetate copolymers, and copolymers of lactic acid and caprolactone.

The final ink or paste properties are controlled by the % solids, viscosity, resistivity, adhesion, and flexibility.

The solid content of the stretchable conductive ink is usually comprised from 35% to 97% by weight, preferably from 55% to 95%, more preferably from 65% to 90%, and yet more preferably from 75% to 85%.

Generally the viscosity of the ink is in the range of from 1000 to 100000 cP measured at 80 s^~1 , preferably from 4000 to 40000 cP measured at 80 s^~1.

In a preferred embodiment the stretchable conductive ink shows thixotropy, more preferably up to 150000, and more preferably in the range of from 5000 to 30000.

A particularly preferred conductive ink composition comprises:

1 ) from 30% and 95% by weight of conductive material, preferably from 50% to

85%, more preferably from 60% to 80%, and yet more preferably from 65% to

75%, preferably the conductive material is selected from silver flakes, silver nanowires, and mixtures thereof,

2) from 2% to 15% by weight of EVA copolymer, preferably 4% to 10%, and more preferably from 6% to 9%, preferably the EVA copolymer has a Tg of from -18⁵

C to 8°- C,

3) from 5% to 20% by weight of a co-solvent selected from C1 -C4 alcohols, glycols, and mixtures thereof, preferably from 5% to 15%, and more preferably from 9% to 12%, preferably the co-solvent is propylene glycol,

4) from 1 % to 5% by weight of a dispersing agent, preferably from 2% to 4%, and more preferably from 2.5% to 3.5%, preferably the dispersing agent is selected from the group of CrCi₈ alkyl and CrCi₈ alkyl ether phosphates,

5) from 0.05% to 0.5% by weight of a defoamer, preferably from 0.08% to 0.2%, more preferably from 0.09% to 0.1 %, preferably the defoamer is a CrCi₈ alkyl polyalkyleneglycol ether, and

6) water to 100%.

Another particularly preferred conductive ink composition comprises: 1 ) from 30% and 95% by weight of conductive material selected from silver flakes, silver nanowires, and mixtures thereof, 2) from 2% to 15% by weight of EVA copolymer having a Tg of from -18⁵ C to 8⁵ C,

3) from 5% to 20% by weight of a glycol as co-solvent,

4) from 1 % to 5% by weight of an anionic surfactant as dispersing agent,

5) from 0.05% to 0.5% by weight of a polymeric defoamer, and

6) water to 100%.

A specially preferred conductive ink composition comprises: 1 ) from 65% to 75% by weight of conductive material selected from silver flakes, silver nanowires, and mixtures thereof,

2) from 6% to 9% by weight of EVA copolymer having Tg of from -8⁵ C to -4^Q C,

3) from 9% to 12% by weight of a propylene glycol,

4) from 2.5% to 3.5% by weight of a dispersing agent selected from the group of C1 -C18 alkyl and C1 -C18 alkyl ether phosphates,

5) from 0.09% to 0.1 % by weight of C1 -C18 alkyl polyalkyleneglycol ether as defoamer, and

6) water to 100%.

This ink composition shows a particularly high performance due to the combination of the EVA copolymer, propylene glycol and the specific dispersing agent.

Surprisingly, it has been found that the inclusion of the EVA copolymer in the ink composition confers to the printed substrate the property of conducting electricity and, if the substrate is stretchable, it shows an excellent stretchable behaviour by maintaining a very low electrical resistance variation when the substrate is subjected to an elongation. Furthermore, the ink composition, being water-based and containing water-miscible co-solvents, allows an easy cleaning of the screen printing tools, and solves the problem of the premature drying of the ink, providing the possibility of a sequential industrial process.

Preparation method

Generally the method for preparing the conductive ink composition of the invention comprises the addition of the dispersing agent to the co-solvent under stirring. Afterwards conductive material is added to the above solution and the resulting suspension is dispersed in a planetary ball mill for example, for about 10 minutes at 300 rpm. The binder (EVA copolymer) is added to the dispersion and is further dispersed in the planetary ball mill for example, for about 30 minutes at 300 rpm. Optionally, it is possible to perform a final refining in a roll mill. The skilled in the art may include variations to this method depending on the particular characteristics of the different components used in the conductive ink.

Screen printing process

The stretchable conductive ink composition of the invention is suitable to be used in a screen printing process for preparing a conductive polymer thick film. It is a proven easy to use technology for the manufacture of electronics, which can be used for a large variety of substrates.

In the screen printing process the ink is pushed through the open area of a stainless steel or polyester screen, and it allows ink to be patterned on substrate with the help of a squeegee. In this process usually high-viscosity inks with thixotropic (shear thinning) properties are required, because low-viscosity inks will simply run through the screen by gravity.

The wet-layer thickness is therefore usually high (about 10-300 μηι, preferably 10 -200 μηι), providing a sufficient layer thickness for optimal performance.

Also part of the object of the invention is the use of the stretchable conductive ink composition of the invention for manufacturing a conductive polymer thick film by screen printing.

Generally the ink of the invention is deposited by screen printing on a substrate to obtain a conductive polymer thick film. The substrate can be any flexible or stretchable substrate such as polyesters (PET, polyethylene terephthalate; PEN, polyethylene naphthalate; PC: polycarbonate), imides (PI, polyimide; PEI, polyetherimide), fluoropolymers (FEP, hexafluoroprene/tetrafluoroethylene copolymer), polyvinyl chloride), polyurethanes (PU), paper or textiles.

Also part of the object of the invention is a conductive polymer thick film, comprising a dried ink composition of the invention. Thus, this conductive polymer thick film comprises:

1 ) a conductive material, preferably a mixture of silver flakes and silver nanowires, and

2) an ethylene/vinyl acetate copolymer.

This conductive polymer thick film is obtainable by depositing the ink composition of the invention on a substrate by means of screen printing.

Also part of the object of the invention is a screen printing process for manufacturing a conductive polymer thick film. This process comprises the steps:

1 ) providing a stretchable substrate,

2) providing the stretchable conductive ink composition of the invention, 3) depositing said ink by screen printing onto that substrate to obtain a wet polymer thick film, and

4) drying the deposited wet polymer thick film.

Usually the drying step is performed by heating at a temperature comprised between 100^Q C and 140⁵ C, preferably for 10 to 30 minutes.

Also part of the object of the invention is the use of the conductive polymer thick film of the invention for manufacturing an electric circuit.

Also part of the object of the invention is an electric circuit comprising the conductive polymer thick film. The electric circuit can comprise a pattern of a series of interdigitated conductive material lines, deposited by screen printing of a stretchable substrate using the ink of the invention. The lines consisting of conductive polymer thick film can be dried in an oven to obtain the desired circuit.

Application tests

Conductivity measurements as a function of the elongation may be made in an instrument such as, for example, Zwick/Roell Z005, by methods well known to the skilled in the art.

Application tests of the conductive ink of the invention can be performed, for example, by applying the ink by screen printing on a substrate such as a polyurethane foil and determining the loss of conductivity while elongating the substrate. The loss of conductivity is measured as a variation of resistance (AR/R₀), being R₀ the initial resistance, versus the elongation (L-L₀)/U, being L₀ the initial length and L the length during the elongation test.

It is observed that the stretchable conductive ink of the invention provides good electrical conductivity of the printed pattern.

The invention includes the following embodiments:

1 . - Ink composition characterized in that it comprises:

1 ) a conductive material,

2) an ethylene/vinyl acetate copolymer,

3) a co-solvent selected from C1-C4 alcohols, glycols, and mixtures thereof,

4) a dispersing agent, and

5) water

2. - Ink composition according to embodiment 1 , characterized in that the conductive material is selected from silver, copper, gold, carbon, and mixtures thereof.

3.- Ink composition according to embodiment 2, characterized in that the conductive material is selected from silver flakes, silver nanowires, and mixtures thereof. 4. - Ink composition according to embodiment 3, characterized in that the conductive material consists of silver flakes

5. - Ink composition according to embodiment 3, characterized in that the conductive material is a combination of silver flakes and silver nanowires.

6.- Ink composition according to embodiment 5, characterized in that the conductive material comprises from 95% to 99% by weight of silver flakes and 1 % to 5% by weight of silver nanowires.

7.- Ink composition according to embodiment 1 , characterized in that the ethylene/vinyl acetate copolymer shows a Tg comprised from -18⁵ C and 8⁵ C.

8.- Ink composition according to embodiment 7, characterized in that the ethylene/vinyl acetate copolymer has a solid content comprised from 30% to 80% by weight.

9. - Ink composition according to embodiment 7, characterized in that the ethylene/vinyl acetate copolymer has a particle size comprised from 0.1 to 3 μηι.

10. - Ink composition according to embodiment 1 , characterized in that the content of the ethylene/vinyl acetate copolymer is comprised from 2% to 15% by weight, expressed as dried matter, based on the total weight of the ink composition.

1 1 . - Ink composition according to any one of embodiments 1 to 10, characterized in that the co-solvent is a glycol selected from ethylene glycol, propylene glycol, 1 ,2- butanediol, and 1 ,3-butanediol.

12.- Ink composition according to embodiment 1 1 , characterized in that characterized in that the co-solvent is propylene glycol.

13.- Ink composition according to any one of embodiments 1 to 12, characterized in that the co-solvent is comprised from 5% to 20% by weight based on the total weight of the ink composition.

14.- Ink composition according to any one of embodiments 1 to 13, characterized in that the dispersing agent is a surfactant.

15. - Ink composition according to embodiment 14, characterized in that the dispersing agent is an anionic surfactant.

16. - Ink composition according to embodiment 15, characterized in that the dispersing agent is an anionic surfactant selected from soaps, alkylbenzene sulfonates, a-olefin sulfonates, alcohol sulfates, alcohol ether sulfates, dialkylsulfosuccinates, alkyi and alkyi ethoxy sulfosuccinates, mono sulfosuccinates, alkyi ether carboxylates, alkyi and alkyi ether phosphates, petroleum sulfonates, alkane sulfonates, alkyi phenol and alkyi phenol ether sulfates, a-sulfo methyl esters, fatty acid isethionates, acyl sarcosinates, taurides, and mixtures thereof. 17. - Ink composition according to embodiment 16, characterized in that the dispersing agent is an anionic surfactant selected from the group of CrCi₈ alkyl phosphates and C1 -C18 alkyl ether phosphates.

18. - Ink composition according to any one of embodiments 1 to 15, characterized in that the content of the dispersing agent is comprised from 1 % to 5% by weight based on the total weight of the ink composition.

19. - Ink composition according to any one of embodiments 1 to 18, characterized in that the ink composition further comprises a defoamer.

20. - Ink composition according to embodiment 19, characterized in that the defoamer is selected from polysiloxanes (silicones), mineral oils, vegetable oils, polymers, and mixtures thereof.

21 . - Ink composition according to embodiment 20, characterized in that the defoamer is a polymer, which is CrCi₈ alkyl polyalkyleneglycol ether.

22. - Ink composition according to any one of embodiments 19 to 21 , characterized in that the content of the defoamer is comprised from 0.05% to 0.5% by weight based on the total weight of the composition.

23. - Ink composition according to embodiment 1 , characterized in that it comprises:

1 ) from 30% and 95% by weight of conductive material,

2) from 2% to 15% by weight of EVA copolymer,

3) from 5% to 20% by weight of a co-solvent selected from C1 -C4 alcohols, glycols, and mixtures thereof,

4) from 1 % to 5% by weight of a dispersing agent,

5) from 0.05% to 0.5% by weight of a defoamer, and

6) water to 100%.

24.- Ink composition according to embodiment 1 , characterized in that it comprises:

1 ) from 30% and 95% by weight of conductive material selected from silver flakes, silver nanowires, and mixtures thereof,

2) from 2% to 15% by weight of EVA copolymer having a Tg of from -18⁵ C to 8⁵ C,

3) from 5% to 20% by weight of a glycol as co-solvent,

4) from 1 % to 5% by weight of an anionic surfactant as dispersing agent,

5) from 0.05% to 0.5% by weight of a polymeric defoamer, and

6) water to 100%.

25.- Ink composition according to embodiment 24, characterized in that it comprises:

1 ) from 65% to 75% by weight of conductive material selected from silver flakes, silver nanowires, and mixtures thereof, 2) from 6% to 9% by weight of EVA copolymer having Tg of from -8⁵ C to -4^Q C,

3) from 9% to 12% by weight of a propylene glycol,

4) from 2.5% to 3.5% by weight of a dispersing agent selected from the group of C1-C18 alkyl and C1-C18 alkyl ether phosphates,

5) from 0.09% to 0.1 % by weight of C1-C18 alkyl polyalkyleneglycol ether as defoamer, and

6) water to 100%.

26.- Use of the ink composition of any one of embodiments 1 to 25 for manufacturing a conductive polymer thick film by screen printing.

27.- A conductive polymer thick film, characterized in that it comprises dried ink composition according to any one of embodiments 1 to 25.

28. - Screen printing process for manufacturing a conductive polymer thick film, characterized in that it comprises the steps:

1 ) providing a stretchable substrate,

2) providing the ink composition of any one of embodiments 1 to 25,

3) depositing said ink by screen printing onto that substrate to obtain a wet polymer thick film, and

4) drying the deposited wet polymer thick film.

29. - Use of the conductive polymer thick film of embodiment 27 for manufacturing an electrical circuit.

30. - An electric circuit comprising the conductive polymer thick film of embodiment 27.

Next, several examples of the invention are provided for illustrative but not limitative purposes. Examples

Determination of particle size can be performed using known procedures in an instrument such as, for example, Mastersizer^® 3000 (Malvern), according to the procedure provided by the manufacturer.

Determination of Tg of the EVA copolymer can be determined by differential scanning calorimetry (DSC) in commercially available instruments such as, for example, Seiko EXTAR DSC 6100. The DSC methodology is based on the temperature increasing at a constant rising rate. This means more or less heat energy is put into the sample to make the temperature increase at a constant rate. When a phase change occurs, much higher than normal levels of heat are needed. The point of the maximum heat input is called the melting point. The determination of the Tg can be performed, for example, taking samples weighing 10-20 mg sealed in aluminium pan adding energy to raise the temperature to a constant rate of 10⁵ C/min in air.

Determination of resistance can be carried out by using a commercially available digital multimeter, such as those provided by the company Fluke.

Determination of elongation can be carried out by using a commercially available extensometer such as, for example, Zwick/Roell Z005 (Zwick), according to the procedure provided by the manufacturer.

Example 1 Preparation of a stretchable conductive ink containing silver flakes

A stretchable conductive ink was prepared with the components listed in

Table I:

TABLE I

a Copolymer of vinyl acetate and ethylene containing 55% weight of solids and a Tg of -6⁵ C.

b Anionic dispersing agent based on a polyoxyethylene alkyl ether phosphate. c Non-silicone defoamer based on an alkylpolyalkyleneglycolether.

The process for preparing this ink consisted of the addition of the dispersing agent to the co-solvent under stirring. Afterwards silver flakes were added to the above solution and the resulting suspension was dispersed in a planetary ball mill for about 10 minutes at 300 rpm. The binder was added to the dispersion and further dispersed in the planetary ball mill for about 30 minutes at 300 rpm. The final refining was carried out in a roll mill.

The ink showed a viscosity of 4410 cP at 80 s^~1.

The ink showed long-term storage stability Example 2: Preparation of a stretchable conductive ink containing silver flakes and silver nanowires

A stretchable conductive ink was prepared with the components listed in

Table II:

TABLE II

a Silver nanowires supplied as a suspension in ethanol containing 1 % by weight of solids.

b Copolymer of vinyl acetate and ethylene containing 55% by weight of solids and a Tg of -6⁵ C.

c Anionic dispersing agent based on a polyoxyethylene alkyl ether phosphate. d Non-silicone defoamer based on an alkylpolyalkyleneglycolether.

The process for preparing this ink consisted of the addition of the silver flakes and the dispersing agent (about 3% by weight on the basis of the weight of silver flakes and silver nanowires) to the suspension of silver nanowires. The suspension was dispersed in a planetary ball mill for about 10 minutes at 300 rpm. The resulting dispersion was dried completely at 300⁵ C and the solid was milled. The powdery solid obtained was added to a solution comprising co-solvent, dispersing agent and binder and further dispersed in the planetary ball mill for about 30 minutes at 300 rpm. The final refining was carried out in a roll mill.

The ink showed a viscosity of 4300 cP at 80 s^~1.

Example 4: Application tests The application tests of the inks of the invention were made by applying the ink by screen printing on a stretchable polyurethane substrate to obtain a straight line of 2 mm width and 30 mm length using a 230-325mesh screen.

The substrates printed with the inks of the invention (Example 1 : "A", and Example 2: "B") and with six commercially available inks ("C" to "H") were subjected to elongation tests on a Zwick/Roell Z005 instrument and the electrical resistance was measured with a digital multimeter. "C", "D" and "E" inks are described as flexible inks, "F" and "G" inks are thermoformable inks and "H" ink is sold as a stretchable ink.

Results of the variation of resistance (AR/R₀), being R₀ the initial resistance, versus the elongation (L-L₀)/U, being L₀ the initial length and L the length during the elongation test, for different conductive inks is represented in Figure 1 . The initial electrical resistance R₀ is recovered after the test.

It can be observed that the printed polymer thick film composition prepared using inks according to the invention, "A" and "B", show a substantially constant electrical resistance during the elongation test, in comparison to the tested commercially available inks. Inks according to the invention provide a printed substrate that shows a slight loss of conductivity in the elongation test performing a reliable behaviour.

Claims

1.- Ink composition characterized in that it comprises: 1 ) a conductive material,

2) an ethylene/vinyl acetate copolymer,

3) a co-solvent selected from Ci-C₄ alcohols, glycols, and mixtures thereof,

4) a dispersing agent, and

5) water, characterised in that the conductive material is selected from silver, gold, carbon, and mixtures thereof and the dispersing agent is an anionic surfactant selected from the group of CrCi₈ alkyl phosphates and CrCi₈ alkyl ether phosphates. 2.- Ink composition according to claim 1 , characterized in that the conductive material is selected from silver flakes, silver nanowires, and mixtures thereof.

3. - Ink composition according to claim 2, characterized in that the conductive material comprises from 95% to 99% by weight of silver flakes and 1 % to 5% by weight of silver nanowires.

4. - Ink composition according to claim 1 , characterized in that the ethylene/vinyl acetate copolymer shows a Tg comprised from -18⁵ C and 8⁵ C.

5.- Ink composition according to claim 1 , characterized in that the content of the ethylene/vinyl acetate copolymer is comprised from 2% to 15% by weight, expressed as dried matter, based on the total weight of the ink composition.

6. - Ink composition according to any one of claims 1 to 5, characterized in that the co- solvent is a glycol selected from ethylene glycol, propylene glycol, 1 ,2-butanediol, and

1 ,3-butanediol.

7. - Ink composition according to any one of claims 1 to 6, characterized in that the co- solvent is comprised from 5% to 20% by weight based on the total weight of the ink composition.

8.- Ink composition according to claim 1 , characterized in that it comprises:

1 ) from 30% and 95% by weight of conductive material,

2) from 2% to 15% by weight of EVA copolymer,

3) from 5% to 20% by weight of a co-solvent selected from Ci-C₄ alcohols, glycols, and mixtures thereof,

4) from 1 % to 5% by weight of a dispersing agent,

5) from 0.05% to 0.5% by weight of a defoamer, and

6) water to 100%.

9.- Use of the ink composition of any one of claims 1 to 8 for manufacturing a conductive polymer thick film by screen printing.

10. - A conductive polymer thick film, characterized in that it comprises dried ink composition according to any one of claims 1 to 8.

11. - Screen printing process for manufacturing a conductive polymer thick film, characterized in that it comprises the steps:

1 ) providing a stretchable substrate,

2) providing the ink composition of any one of claims 1 to 8,

4) drying the deposited wet polymer thick film.

12. - Use of the conductive polymer thick film of claim 10 for manufacturing an electrical circuit.

13. - An electric circuit comprising the conductive polymer thick film of claim 10.