EP4551658A1

EP4551658A1 - Water-based conductive printing ink with a pigment produced by one-step reaction

Info

Publication number: EP4551658A1
Application number: EP23747996.9A
Authority: EP
Inventors: Theodoros Steriotis; Georgia CHARALAMBOPOULOU; Dimitra Giasafaki; Christina Mitzithra; Vasiliki PANAGIOTOPOULOU; Vasiliki Belesi; Vasilios GEORGAKILAS
Original assignee: Druckfarben Hellas Ae; Ethniko Kentro Erevnas Fysikon Epistimon National Centre For Scientific Research "demokritos"; Panepistimio Dytikis Attikis University Of West Attika; Panepistimio Patron University Of Patras
Current assignee: Druckfarben Hellas Ae; Ethniko Kentro Erevnas Fysikon Epistimon National Centre For Scientific Research "demokritos"; Panepistimio Dytikis Attikis University Of West Attika; Panepistimio Patron University Of Patras
Priority date: 2022-07-07
Filing date: 2023-07-05
Publication date: 2025-05-14
Also published as: GR1010492B; WO2024008813A1

Abstract

A method is provided for the development of a conductive water-based printing ink for intaglio/flexography and for screen-printing. The method provides an ink that can be industrially produced and used by using resins that are already used in industry for the production of intaglio/flexography printing inks. The development of conductive materials/pigments is based on the simultaneous reduction of silver salts and graphene oxide in the presence of aminophenylsulfonic derivatives. The presence of aminophenylsulfonic derivatives is essential for the reduction procedure and in parallel leads to the enrichment of the graphene surface with aminophenylsulfonic groups that provide a high hydrophilicity to the final materials/pigment.

Description

TITLE

WATER-BASED CONDUCTIVE PRINTING INK WITH A PIGMENT PRODUCED BY ONE-STEP REACTION

DESCRIPTION

FIELD OF THE INVENTION

The present invention relates to a process for preparing a water-based conductive printing ink for intaglio/flexography and screen-printing, by using graphene oxide, a reducing agent and metal salts as starting materials.

The procedure comprises the development of conductive pigments and is based on the use of aminophenylsulfonic acid derivatives for the suitable chemical modification of the starting materials in one step. Thereafter, the conductive pigments are combined with the optimal mixtures of acrylate resins and acrylate/styrene resins used in the printing ink industry to provide the final conductive inks for intaglio/flexography and screen-printing.

BACKGROUND

Printing inks consist of dye or pigment and the carrier that consists of resins, dispersing agents, solvents and additives (e.g., anti-foaming agents). The co-presence of all these components simultaneously in an ink is necessary in order to achieve the optimal printing result (Leach, 2007).

The dye or pigment provides the colour, whereas in the functional inks it provides the desirable functional property (e.g., conductivity). Resins aid in the binding and wetting of the pigment particles, the transport, coating and anchoring of the ink on the printing substrate by forming a film, the strength, elasticity and gloss after drying. For an optimal printing result, a combination of resins is usually required, whereas additives also contribute in that, e.g., waxes (reduction of the friction coefficient), anti-foaming agents (for water-based inks), dispersing agents etc.

Finally, depending on the nature of the solvent, inks are classified into water based and solventbased inks. Water-based inks, compared to organic solvent-based inks, have a low volatility, and thus they keep a constant viscosity during printing when the technical features of the ink (e.g., pH) or printing conditions (e.g., temperature) remain constant. In this case, the resins must be water-soluble, however after printing they must be insoluble to ensure the required strength. The currently available conductive inks are mainly prepared by dispersing metal nanoparticles (NPs) (Ag, Cu), conductive polymers, amorphous carbon (carbon black), graphite, or mixtures thereof in solvents, which may contain specific binders, additives or stabilizers (Alieva and Pignataro 2014, Huang and Zhu 2019). The metal nanoparticle-based systems are advantageous as regards conductivity (10⁷ S/m), however they require either post-printing thermal treatment steps (preventing the use of temperature-sensitive substrates), or alternatively the use of pulsed light (laser and intense pulsed light sintering), in order that the conductive particles are joined between them and allow the flow of electrons (Huang and Zhu 2019, Ko 2016, Bhat 2016). Carbon materials are advantageous as regards the cost (average cost of graphite l$/kg in 2016), however they result to relatively low conductivity values (2 to 4 10³ S/m, a range also observed in conductive polymers), whereas problems regarding the adhesion of carbon particles on specific substrates or even printing failures due to reduced flexibility and reduced resistance to friction are frequently observed.

Graphene is a highly conductive material (10⁴-10⁵ S/m) with a remarkable stability (thermal and chemical) and thus it is an ideal component of inks for printing on flexible and heat-sensitive substrates (paper, plastics), particularly when considering that high conductivity can be achieved without strong heat treatment thereof. Graphene inks are expected to have a much lower cost (up to 25 times lower) and negligible toxicity compared to respective silver inks which dominate in the market today (Karagiannidis, 2017). Also, printed circuits made of graphene can be bent without significant deterioration of their excellent conductivity (Georgakikas, 2015). However, the successful dispersion of graphene in industrial ink resins is highly challenging. Simultaneously, the hydrophobic character of pure graphene (Georgakikas, 2015) prevents its direct application in water-based inks, which acquire a continually increasing commercial interest compared to conventional solvent-based systems, due to the continually increasing demand for environmentally friendlier printing procedures.

Due to these dispersion problems, and despite the significant advantages of graphene, no related products have been developed industrially. Novalia has proceeded to a limited production of graphene inks for screen-printing and flexography, and confirmed the significant challenges faced in the development of these specialized products. Various methods have been presented (Karagiannidis 2017, Secor 2013, 2015a, b), which may lead to systems having good electrical properties, however they are mainly based on the use of surfactants, hydrophilic polymers or a combination thereof, and also require specialized techniques (microfluidization, shear exfoliation) to prevent agglomeration phenomena. However, these additives negatively affect conductivity since they either behave as insulators or destroy the extended aromatic system in graphene, which is responsible for its conductivity. Thus, the final printing product usually requires further treatment to ensure the desirable conductivity, e.g. by photonic annealing (Arapov 2016, Secor 2015a) or treatment at high temperatures (250-450 ⁰ C) (Huang and Zhu 2019, Overgaard 2017), which not only makes the production procedure burdensome and affects it energetically and financially but also is unsuitable for heat-sensitive substrates.

Recent research articles mention hybrid inks which are based on metal nanoparticle-containing graphene derivatives, as an alternative means for improving the electrical properties of the conductive inks (Deng 2017, Kholmanov 2012, Tran 2015, Zou 2019, Htwe 2019, Zhang 2016). However, these systems frequently present silver inhomogeneity issues (Zu 2011, Singh 2012) or time-consuming and complex methods (He et al, 2018, Wang, et al., 2012).

In situ reduction of silver ions and deposition of nanoparticles on graphene is the simplest and most efficient method (Tian 2012, Wang 2012, Qin 2012, Shen 2011). It is performed with various reducing agents such as sodium borohydride (He 2018, Tan 2011, Shen 2011, Zhang 2011, Li 2012, Guo 2012), however it also requires two steps, the reduction of silver to form nanoparticles and the reduction of graphene oxide (GO) to reduced oxide (rGO).

Thus, it is important to achieve a conductive graphene/silver nanoparticles hybrid in one step (He 2018). For example, this has been achieved by using formaldehyde, ascorbic acid or tannic acid, however the reducing action is relatively weak (He 2018, Long 2014). In some other cases, simultaneous application of microwaves or of another energy source was necessary (Long 2014). The electrical conductivities of these hybrid materials are in the range of 0.3-2.0 x 10 ³ S/cm, however these are achieved by heat treatment at temperatures in the range of 100-400 ⁰ C (Saidina 2019, Deng 2017, Yang 2017, Zhang 2016). A disadvantage of these materials is the high temperature required for their preparation.

Therefore, there is still a need, in the technological field of the present invention, for improved synthesis of appropriate graphene hybrid-derivatives and for efficient incorporation thereof in the industrial production of conductive inks, and particularly of water-based conductive inks. SUMMARY

More particularly, in one aspect the invention relates to a method for the preparation of a conductive ink comprising the steps:

- addition of GO in H₂O at a GO concentration between 1 mg/ml and 10 mg/ml

- exfoliation of the GO of the said dispersion by ultrasound and stirring

- addition of a metal salt and an organic reducing agent which is an aminophenylsulfonic acid derivative

- heating of the mixture at a temperature higher than 50° C

- filtration, collection, and purification of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles

- mixing of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles with one or more resins, wherein the organic reducing agent is one or more organic aminophenylsulfonic acid derivatives.

In one embodiment of said method, said organic reducing aminophenylsulfonic acid derivative is 3,4-diamino benzenesulfonic acid and/or 2,5-diamino benzenesulfonic acid and/or a combination thereof at any ratio.

In one embodiment of said method, the GO mass to the reducing agent mass is in the range from 1 to 2 to 1 to 4.

In one embodiment of said method, said metal salt is a silver salt or a copper salt or a combination thereof.

In one embodiment of said method, the content by weight in metal is lower than 50% w/w, preferably higher than 8.5% w/w.

In one embodiment of said method, the exfoliation of GO is effected by keeping it in an ultrasound system of 100 W and 40 kHz for a time period more than 20 minutes, preferably, less than 60 minutes.

In one embodiment of said method, said filtration and collection of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticlesis performed by using suitable Nylon filters preferably having a pore diameter of 0.45 pm or by multiple centrifugations, preferably performed at 9000 rpm. In one embodiment of said method, the purification of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticlesis effected by washing, preferably with de-ionized water and/or ethanol and/or acetone.

In one embodiment of said method, the mass ratio (dry mass) of resin to conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles is in the range 30/70 to 70/30, preferably 55/45.

In one embodiment of said method, heating is performed under reflux, preferably at a temperature between 50° and 100° C.

In one embodiment of said method, the resins used are based on (a) an acrylate emulsion, or (b) a styrene acrylate ester emulsion or (c) polyurethane or (d) polyurethane-polyacrylate or (e) salts thereof or systems derived from a combination thereof or with other supplementary resins.

In one embodiment of said method, the mass of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticlesto the total mass of the ink is 5-25% and the solvent quantity added is suitable so that the ink viscosity under high shear rates is in the range of 0.05-0.3 Pas in order to be suitable for intaglio/flexography.

In one embodiment of said method, the mass of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticlesto the total mass of the ink is 10-35% and the solvent quantity added is suitable so that the ink viscosity under high shear rates is in the range of 0.1-0.5 Pas in order to be suitable for screen-printing.

BRIEF DESCRIPTION OF THE DRAWINGS

Description of the figures Silver nanoparticles (AgNPs) deposited on functionalized rGO (f-rGO) as a result of one pot reduction of GO and silver ions.

Stability test of the dispersion of f-rGO/Ag in water using the transparency of a dispersion during one week measurements

UV-vis absorption spectrum of the Ag/f-rGO hybrid. diagrams of GO and f-rGO/AgNPs

Raman spectraof f-rGO and f-rGO/Ag NPs with 5 and 15 %w/w Ag. The spectra were obtained after 25-point processing (average signal from a mapping at 25 points)

Typical printed item on paper by flexography DETAILED DESCRIPTION

The present invention relates to an industrially applicable method of producing a graphenebased conductive ink, comprising the steps of (a) producing conductive graphene hybrid materials and (b) developing the formulation of the ink produced by the use thereof.

The innovation of the current invention lies in the application of such new methods for the simultaneous (i) conversion of graphene sheets to hydrophilic ones without affecting the aromaticity and thus their conductivity (Georgakilas 2015), (ii) reduction of silver ions in a simple, quick and industrially applicable manner and (iii) their integration in the production procedure for the development of the ink formulation.

The present method is directed to the use of organic compounds with aminophenylsulfonic groups, which can simultaneously (in one step) reduce GO and silver ions, and at the same time they are covalently bound to the graphene product, thereby increasing its hydrophilicity, an effect that has not been mentioned in the state of the art. The aminophenylsulfonic group- containing reducing agents can provide the modified, arylsulfonate-containing graphene materials with a high hydrophilicity which contributes toan increased conductivity due to the formation of a continuous conductive network through the pigment sheets. Another advantage of the invention compared to the state of the art is that in all process steps for the development of the ink, water was used as unique solvent, instead of solvent systems.

2,4-Diamino-benzenesulfonic acid (isomer) has already been used and results in a conductive and hydrophilic rGO modified with aryl sulfonate groups (Belessi 2019), however in the absence of silver. The specific application presents serious technical problems such as time-consuming filtration and purification/washing of the product, which make it unsuitable for further use on an industrial scale.

The method described herein leads to a significant modification of the process and materials such that the new method is inventive and at the same time new systems of conductive graphene inks were developed by using conventional resins already used in industry. Thus, the ink production procedure may be industrially used without modification of its already established infrastructure.

Advantages of the invention over the state of the art

The invention relates to a method of producing graphene/silver nanoparticle hybrids which is performed in one step but in less time than the already available methods of more than one step. Furthermore, this method leads to a) due to the use of a single reagent as reducing agent and surface modifier of graphene, the production of an exceptionally conductive and water-soluble pigment (graphene hybrid) in a single production step without adding surfactants or further hydrophilic polymers etc., for the stabilization of graphene. b) full reduction of both the silver ions and the GO sheets, by using a single reagent in a remarkable reaction yield ("'80%) c) the synthesis of a conductive ink that does not require any special treatment (e.g., heat treatment or the use of pulsed light) for the improvement of its conductivity, d) the production of a hybrid graphene material, which is scalable without altering its qualitative and functional features, and also it is suitable for further application on a larger scale, i.e. in industrial scale without being energy- and time-consuming.

It should be noted that the said reducing agent is used for the first time as common in-situ reducing agent for silver ions.

As regards the second step of the production of the water-based conductive hybrid ink, the innovation lies in a) the direct dispersibility of the conductive graphene pigment in a varnish system during the ink production, without requiring the time-consuming basic three steps (Premix-Grinding-Let-down) applied in industry for the incorporation-dispersion of the pigments in the resin system to form the inks. b) the development of new systems of conductive graphene printing inks by using conventional resins already used in industry. This allows the prepared conductive inks to be used and applied as conventional printing inks. c) the development of a conductive ink with increased solid content and thus a high load in conductive material without agglomeration d) the possibility to provide the components of the ink separately packaged, since mixing of the system in standard printing stations with common equipment is possible — avoiding solidification commonly observed in conventional inks over time. e) avoidance of any post-printing treatment (e.g. heat treatment, plasma treatment, photonic treatment, microwave treatment or treatment with chemical reagents) in order to make the ink functional. f) its usability in three printing methods (intaglio, flexography, screen-printing). g) the preservation of the final ink for a larger time period than the conventional inks due its antimicrobial properties.

The conventional water-based inks present bacterial growth during their storage and require pH adjustment to suitable values as a preventive measure.

In one aspect the invention relates to the development of a conductive ink by a procedure comprising the following steps:

- preparation of an aqueous GO dispersion with GO concentration between 1 mg/ml and 10 mg/ml

- exfoliation of the GO of the said dispersion by ultrasound and stirring

- heating of the mixture at a temperature higher than 50 °C

- filtration and collection of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles (or otherwise solid conductive pigment)

- mixing of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles with one or more resins.

The organic reducing agent is one or more (mixture) aminophenylsulfonic acid derivatives in any possible ratio, whereas excellent results are obtained by 3,4-diamino benzenesulfonic acid or 2,5- diamino benzenesulfonic acid, the mass ratio of GO to reducing agent is in the range of 1 to 2 to 1 to 4. In an embodiment, the metal salt is a silver salt or a copper salt or a combination thereof, whereas the resins used are based on (a) a polymeric acrylate emulsion, or (b) a styrene acrylate ester polymer emulsion or (c) polyurethane or (d) polyurethane-polyacrylate or (e) salts thereof or systems derived from a combination thereof or with other supplementary resins.

Also, the reaction is performed in an aqueous environment and the reaction mixture, in an embodiment, is refluxed under magnetic stirring. The final product is obtained by filtration, or by using suitable filters (Nylon membrane filters, 0.45 pm pore size), or by centrifugation (9000 rpm, 2h) and washing with de-ionized water, ethanol and acetone. Thereafter, the final product is mixed and homogenized with resins for the preparation of the conductive graphene ink systems. Conventional industrial resins, which are already used in industry, were employed. In an embodiment, the mixture was mixed in a special laboratory or industrial homogenizer for 5 min with excellent results. For the development of the inks, various resins of the type (a) in polymeric acrylate emulsion, or (b) in styrene acrylate ester polymer emulsion or (c) in polyurethane or (d) in polyurethane-polyacrylate or (e) in salts thereof or systems derived from the combination thereof or with other supplementary resins were tested and some exemplary results from these types of commercial resins are presented in Table 1 .

In the tests performed, the inks produced for intaglio-flexography are also suitable for screenprinting, which is a very advantageous result since in the prior state of the art no conductive inks with good properties for all the three methods (intaglio flexography, screen-printing) have been prepared.

Conditions used in various embodiments described below.

The above-described reaction is performed under a preferred rnco/mreducing agent ratio during reduction 1/3. This reaction is performed very satisfactorily also under mco/m reducing agent = 1/2 to 1/4 (where the reducing agents are the sulfonate derivatives of phenyl amines). By using these initial conditions, parametric optimization of the reaction was performed by studying the influence of the following parameters:

Optimal dispersion time for GO is 20-60 min under ultrasound (HOW, 40 kHz). Magnetic stirring at room temperature for 24 hours is then performed. The unique solvent during the reaction is water.

The preferred GO concentration in the aqueous system (dispersion) used for the reduction is 1 mg/ml, without limitation thereto. Also GO dispersions at 2 mg/ml to 10 mg/ml may be used.

Excellent results as regards the conductivity of the hybrid f-rGO-Ag materials are obtained when the following phenylamine sulfonate derivatives (S-ArNHj) are used: 2,5-diamino benzenesulfonic acid (2,5-DBSA) and 3,4-diamino benzenesulfonic acid (3,4-DBSA). From a cost perspective, use of 2,5-DBSA is preferred. Also, combinations of these reducing agents in any ratio from 0.1% (2,5- DBSA) and 99.9% (3,4-DBSA) to 99.9% (2,5-DBSA) and 0.1% (3,4-DBSA) are satisfactory. Also, all the respective combinations for all the phenyl diamine sulfonate derivatives may be used.

Optimal results during reduction of metal salts were obtained for the Ag nanoparticles.

Advantageously, the % w/w content in metal nanoparticles was up to 50%. Considering the energetic, operational and environmental parameters, i.e. the production energy cost, the ink conductivities, the discharge of metal particle waste into the environment, respectively, a maximum content of 8.5% Ag is preferred. The reduction time may range from 0.5 to 5 hours however the optimal reduction time is2 hours. The purification of the material is a time-consuming procedure. Filtration in 0.45 pm Nylon filters is preferred than centrifugation.

For the development of inks suitable for intaglio/flexography, resins based on a polymeric acrylate emulsion or on a styrene acrylate ester polymer emulsion or on salts thereof or on a combination with a styrene-acrylate or acrylate resin mixture at an optimal ratio of 80/20 were used as optimal resin system.

For the development of the inks, the mass ratio of pigment solids to the carrier solids (resin/pigment mass ratio) is preferably 55/45 (without being limited to it).

Depending on the printing method in which the ink is to be used, its content in solvent (water) and solids is adjusted, which essentially adjusts the viscosity of the final system, For intaglio, flexography and screen-printing inks, the viscosity range (Pas) is according to the literature in the range 0.1- 1, 1-2 and 1-10, respectively. Practically, the flow times by using cups (DIN 4) is 14-15 and 16-20 sec for intaglio and flexography, respectively, whereas no flow is observed for the screen-printing ink.

The excellent dispersibility of the hybrid materials allows preparation of conductive inks of high viscosity (40-50 Pas for intaglio and flexography, 100-150 for screen-printing) under low shear rates 10^-2 and 10° s’¹, which are much higher compared to other conductive inks. However, in high shear rate ranges (operation conditions of the printing machines) the ink viscosity is in the usual range of 0.05-0.3 Pas and thus the inks are efficient during printing.

Specifically, when the mass of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticlesto the total mass of the ink is 5-25% then the ink viscosity, under high shear rates, is in the range of 0.05-0.3 Pas so that it is suitable for intaglio/flexography. Whereas, when the mass of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles to the total mass of the ink is 10-35%, then the ink viscosity, under high shear rates, is in the range of 0.1-0.5 Pas and the ink is suitable for screen-printing.

The present invention provides a method for the industrial application of a conductive ink by a simple procedure, which starts with the production of the conductive inks and ends in the final step of mixing the inks with resins that are already used in industry in the production of conventional printing inks, without addition of other additives.

Also, the development of the conductive ink does not require any modification of the already installed industrial ink production line and does not cause any wear thereon. Detailed description of the preparation of materials

The reaction will be described in detail by the following examples.

Example 1: Preparation of an 8.5% Ag product

GO ion: GO was prepared according to the modified Staudenmaier method (Poh 2012.).

In 2 g graphite (powder < 20 pm; synthetic, Aldrich) and a mixture of concentrated H2SO4 (80 mL,

95-97%, Merck) and HNO₃ (40 mL, 65 % Riedel de Haen) at 0 °c, KCIO3 (40 g) was added under stirring. The mixture was kept under stirring for 18 hours and then diluted and washed. GO was collected by centrifugation, washed with distilled water until pH ~7 and left to dry at room temperature. Note: Alternatively, a respective commercial product may be used as GO. ion: 75 mg GO is added in 75 ml HjOto form a 1 mg/ml aqueous GO suspension.

The suspension is kept for 20-30 min in an ultrasound bath (HOW, 40 kHz) and then under magnetic stirring at room temperature for 24 hours. Subsequently, 1 1 mg AgNOs is added in the aqueous GO dispersion. The system is kept under stirring. Finally, 150 mg 2,5-DBSA is added, and the mixture is refluxed for 2 hours, from the initiation of the boiling, under magnetic stirring to reduce GO and Ag ions simultaneously in one step. After completion of the reaction, the mixture is left to cool at room temperature under stirring. The final product is obtained by filtration either by using suitable Nylon filters of 0.45 pm pore diameter (Whatman), or by multiple centrifugations (9000 rpm/60 min). Washing is effected by de-ionized water (3 times with 50 ml each), ethanol (2 times with 10 ml each) and acetone (1 time with 10 ml).

Preparation of a water-based conductive ink

After obtaining the final conductive hybrid, this is mixed in commercially available resin systems. The commercially available resins or resin mixtures that after numerous experiments provided satisfactory stability results are shown in Table 1.

For the development of inks suitable for intaglio/flexography, either a single one

(a) in polymeric acrylate emulsion or (b) in styrene acrylate ester polymer emulsion or (c) in polyurethane or (d) in polyurethane-polyacrylate or (e) in salts thereof or systems derived from the combination thereof or with other supplementary resins are required. The mixture is stirred in a special industrial or laboratory homogenizer for 5 min. Table 1 The present invention is described in detail by the following non-limiting examples.

Example 1

1 g hybrid material is dispersed in 1.615 g JONCRYL 1685 resin, 0.285 g JONCRYL 90 resin and 6.7 g de-ionized water. The system is mixed in the special homogenizer.

The same course may also be followed using the following quantities:

Table 2

Example 2

1 g hybrid material is dispersed in 1.53 g JONCRYL 8052 resin, 0.27 g JONCRYL 90 resin and

6.7 g de-ionized water. The system is mixed in the special homogenizer.

The same course may also be followed using the following quantities:

Table 3

Example 3

1 g hybrid material is dispersed in 0.1 g JONCRYL FLX-5000 resin, 1.9 g JONCRYL FLX-5020 resin and 6.7 g de-ionized water. The system is mixed in the special homogenizer.

The same routemay also be followed using the following quantities:

Table 4

Experimental results

Figure 1 (A-C) shows the TEM (transmission electron microscopy) images for the f-rGO/AgNPs hybrid obtained by using 2,5-diamino benzenesulfonic acid as reducing agent, the typical morphology of the rGO nanosheets with characteristic folds as well the good dispersibility of the spherical silver nanoparticles (-12 nm) in the hybrid, respectively.

The dispersion of the materials was examined in 21 different solvents at 0.5 mg/ml concentration and is excellent in the case of polar solvents. The hybrid does not disperse in non-polar solvents, whereas the ethanolic and aqueous dispersions remain stable for more than 4 months.

The stability of the aqueous f-rGO/AgNPs hybrid is shown in Figure 2, as examined by the measurement of transmittance (%T), at 550 nm, as a function of time. As it was observed, the measured transmittance value remained essentially constant for a week.

Figure 3 shows a characteristic UV-Vis absorption spectrum of the f-rGO/AgNPs material, with a characteristic absorption at 279 nm due to the n,n* of the aromatic C=C bonds C=C bonds in the graphene system. The broad peak at 500 nm is attributed to the presence of Ag nanoparticles on the rGO surface.

Characteristic XRD diagrams of GO, f-rGO/AgNPsmaterials are shown in Figure 4. The XRD diagram of GO shows a sharp peak at 11 .9°, indicating a well-stratified sheetlike material and corresponds to an interlayer distance of approximately.743 nm due to water molecules entrapped between the hydrophilic GO sheets (Belessi 2019). For the f-rGO/AgNPs material, as also for the Ag-free f-rGO material (not shown in Figure 4), this sharp peak at 11 .9 ⁰ is completely absent, indicating that the f-rGO surface has extensively changed from hydrophilic to hydrophobic, and thus water molecules are removed from the interlayer space after drying. Simultaneously, a broad peak at ca. 25.8 °, corresponding to a d-spacing ofO.345 nm and being close to s the d002 spacing of graphite (0.335 nm), is observed. This peak is attributed to the partially graphitic character of the reduced materials, which now have a structure intermediate between the purely graphitic one and the amorphous one. Collectively, it is apparent that during reaction, GO sheets were reduced and partially organized into a small number of layers, whereas in a second step the functional groups from 2,5-DBSA were inserted in the nanosheets, providing these with their desirable hydrophilic character in aqueous environment.

Finally, the XRD pattern of the product contains all the characteristic peaks of Ag° that corresponds to the several crystallographic planes of phase centered cubic silver crystals.

The analysis of the Raman spectra (Figure 5) showed the main characteristics of the graphitic materials, i.e. the D and G bands at 1352 and 1598 cm ¹, respectively, as well as the 2D, D+D' and 2D bands at 2702, 2943 and 3217 cm'¹ , respectively at higher frequencies. The D band is attributed to the symmetry breakage of the lattice of the sixfold aromatic structure of the graphitic rings due to the high concentration of defects associated with the reduction. This leads to a sp ³ C-C vibrations corresponding to the Alg breathing mode at the Brillouin zone boundary The G band is attributed to the in-plane bond-stretching motion of pairs of sp2 carbon atoms which assigned to E2g mode at the Brillouin zone center. The 2D, D+D' and 2D bands at higher frequencies have been attributed to second order harmonics as well as to combinations of fundamental modes. The D+D' band requires the presence of defects for its activation, whereas no defects are required for the activation of the 2D and 2D' bands (Ferrari, 2013). Apart from the above, the Raman spectra contained several bands of lower intensity, which can be attributed either to graphitic structural defects or to various impurities. If the /ID/IG ratio cannot be reliably calculated, it is clear that the percentage Ag concentration has not any observable effecton the average graphitic structure. The marginal increase in signal intensity associated with the increase in Ag concentration is likely a statistical coincidence and can not be attributed to plasmon resonance. In conclusion, the spectra of the f-rGO/Ag samples derived by the simultaneous reduction of GO and Ag ions are similar to those of f-rGO, indicating that the specific preparation method does not influence the structure of the final rGO.

Exemplarily, the surface resistance of the hybrid f-rGO/Ag (8.5%) material is 4.3 Ohm/sq. It is established that chemical modification influences the electrical properties of graphene materials (Sreeprasad 2013). The reduction of GO with aminophenylsulfonic groups leads to graphene derivatives with a remarkably low sheet resistance (Rs) and thus a high conductivity, whereas further enrichment of these materials with silver nanoparticles further decreases surface sheet resistance.

Evaluation of the resins

The selected resins (Table 1) were evaluated, after dissolving solid copper phthalocyanine (11% w/w) in the test resins, as regards their adhesion on a paper substrate (ASTM-D3359-09e2, Standard Test Methods for Measuring Adhesion by Tape Test, ASTM International), heat resistance (80 °C, 780 N, 0.1 sec, Brugger HSGCC Heat sealing machine), printability. Also, qualitative evaluation as regards the stability and dispersibility of each resin system was performed.

Printing was performed by using a K Hand Coater/Kbar 0 (deposition thickness of a liquid ink film 4 pm) and the qualitative evaluation of the sample is based on visual inspection. The evaluation of the resins (Table 1) was also performed with an aqueous carbon black dispersion (11 °/ow/w) in the test resins for the same technical parameters.

Printing results

Before the printing tests, checks were performed by using a K Hand Coater/Kbar 0, 1, 2 and 4 as mentioned above.

Thereafter, special devices for the quality control of intaglio inks (IGT Printability Tester Gl -5) and flexography inks (IGT Printability Tester Fl) were used (Figure 6).

During intaglio printing, the inks were printed at a rate of 0.4 m/s and a roller pressure of 300 N on paper substrates (smart-paper type-2, IGT2846). The printing roller used has the following IGT 402.226 features (60; 80; 100; 140 lines cm ^-1, screen angle 53, stylus angle 130, cell volume 16; 11; 9; and 7 ml m^-2).

In flexography printing, the inks were printed at a rate of 0.5 m/s, roller pressure 50 N and an anilox roller 16 ml nr² on paper substrates (smart-paper type-2, IGT2846) (Figure 6) Also, screen-printing with a manual system and a 90 T gauze was successfully performed.

Definitions

Unless otherwise defined, scientific and technical terms used herein have the meanings that are commonly understood by those of ordinary skill in the art. In the event of any latent ambiguity, definitions provided herein take precedent over any dictionary or extrinsic definition.

The term "about" or "approximately" means the mentioned value +/-10%, for example about 10 shall mean 9 to 11.

The terms "intaglio" and "gravure" are used in the present disclosure interchangeably.

Bibliography

• Aleeva Y, Pignataro B (2014) Recent advances in upscalable wet methods and ink formulation for printed electronics. J Mater Chem C 2:6436-6453. https://doi.org/10.1039/C4TC00618F

• Arapov K., Bex G., Hendriks R., Rubingh E., Abbel R., Gijsbertus de With and Friedrich H., Adv.Engin. Mater. 2016, DOI: 10.1002/adem.201500646

• Belessi V., Petridis D., Steriotis T., Spyrou K., Manolis G., Psycharis V., Georgakilas V. "Simultaneous reduction and surface functionalization of graphene oxide for highly conductive and water dispersible graphene derivatives" SN Applied Sciences 1 (1), 1-142019. • Bhat K. S., Ahmad R., Wang Y., Hahn Y.-B. "Low-temperature sintering of highly conductive silver ink for flexible electronics" J. Mater.Chem. C,2016 4,8522, DOI: 10.1039/c6tc02751b

• Deng D., Feng S., Shi M., Huang C. In situ preparation of silver nanoparticles decorated grapheme conductive ink for inkjet printing J Mater Sci: Mater Electron (2017) 28:15411-15417. DOI 10.1007/S10854-017-7427-Z

• Ferrari, A. C.; Basko, D. M., Raman Spectroscopy as a Versatile Tool for Studying the Properties of Graphene. Nat Nano 2013, 8, 235-246.

• Georgakilas V., Demeslis A., Ntararas E., Kouloumpis A., Dimos K., Gournis D. et.al. "Hydrophilic nanotube supported graphene-water dispersible carbon superstructure with excellent conductivity" Adv. Funct. Mater. 25 (2015) 1481-1487. doi:10.1002/adfm.201403801.

• Guo Y., Sun X., Liu Y., Wang W., Qiu H., Gao J. "One pot preparation of reduced graphene oxide (RGO) or Au (Ag) nanoparticle-RGO hybrids using chitosan as a reducing and stabilizing agent and their use in methanol electrooxidation" Carbon 50(7) 2513-2523 (2012). doi:10.1016/j. carbon.2012.01.074

• He K., Zeng Z., Chen A., Zeng G., Xiao R., Xu P., Huang Z., Shi J., Hu L., and Chen G. "Advancement of Ag-Graphene Based Nanocomposites: An Overview of Synthesis and Its Applications" Small 2018, 14, 1800871. DOI: 10.1002/smll.201800871

• Htwe Y.Z.N., Chow W.S., Suriati G., Thant A.A., Mariatti M. "Properties enhancement of graphene and chemical reduction silver nanoparticles conductive inks printed on polyvinyl alcohol (PVA) substrate" Synthetic Metals 256 (2019) 116120. https://doi.Org/10.1016/i.synthmet.2019.116120

• Huang Q. and Zhu Y., Printing Conductive Nanomaterials for Flexible and Stretchable Electronics: A Review of Materials, Processes, and Applications. Adv. Mater. Technol. 2019, 4, 1800546. DOI: 10.1002/admt.201800546

• Karagiannidis P. G., Hodge S. A., Lombardi L., Tomarchio F., Decorde N., Milana S. et.al. Microfluidization of Graphite and Formulation of Graphene-Based Conductive Inks ACS Nano, 2017, 11, 3, 2742-2755* DOI: 10.1021/acsnano.6b07735

• Kholmanov L.N., Magnuson C.W., Aliev A.E., Li H., Zhang B., Suk J.W., Zhang L.L., Peng E., Mousavi S.H., Khanikaev A.B., Piner R., Shvets G., Ruoff R.S., Improved electrical conductivity of graphene films integrated with metal nanowires. Nano Lett. 12, 5679-5683 (2012)

• Ko S. H. Low temperature thermal engineering of nanoparticle ink for flexible electronics applications Semicond. Sci. Technol.31 073003, 2016. • Leach R.H., R. J. Pierce (Editors), The Printing Ink Manual, 5^th Ed. Springer, 2007.

• Li Q., Qin X., Luo Y., Lu W., Chang G., Asiri A.M., Al-Youbi A. O., Sun X. , One-pot synthesis of Ag nanoparticles/reduced graphene oxide nanocomposites and their application for nonenzymatic H2O2 detection, Electrochimica Acta 83 (2012) 283- 287.

• T. Long, L. Hu, H. Dai, Y. Tang, Facile synthesis of Ag-reduced graphene oxide hybrids and their application in electromagnetic interference shielding, Appl. Phys. A 2014, 116, 25. DOI 10.1007/s00339-014-8517-x

• Overgaard M. H., Kuhnel M., Hvidsten R., Petersen S. V., Vosch T., Nprgaard K., Laursen B. W., Highly Conductive Semitransparent Graphene Circuits Screen-Printed from Water-Based Graphene Oxide Ink, Adv. Mater. Technol. 2017, 1700011

• Poh, H.L.; Sanek, F.; Ambrosi, A.; Zhao, G.; Sofer, Z.; Pumera, M. Graphenes Prepared by

Staudenmaier, Hofmann and Hummers Methods with Consequent Thermal Exfoliation Exhibit Very Different Electrochemical Properties. Nanoscale 2012, 4, 3515-3522, doi:10.1039/C2NR30490B.

• Qin X., Luo Y., Lu W., Chang G., Asiri A. M., Al-Youbi A. O., Sun X., One-step synthesis of Ag nanoparticles-decorated reduced graphene oxide and their application for H2O2 detection Electrochim. Acta 2012, 79, 46-51.

• D.S. Saidina, N. Eawwiboonthanakit, M. Mariatti, S. Fontana and C. Herold "Recent Development of Graphene-Based Ink and Other Conductive Material-Based Inks for Flexible Electronics" Journal of Electr. Mater., Vol. 48, No. 6, 2019. https://doi.org/10.1007/sll664-019-07183-w

• Secor E.B., Prabhumirashi P.L., Puntambekar K., Geier M.L., Hersam M.C., Inkjet printing of high conductivity, flexible graphene patterns, J. Phys. Chem. Lett. (2013) 1347-1351. doi:10.1021/jz400644c.

• Secor E.B., Ahn B.Y., Gao T.Z., Lewis J. A., Hersam M.C., Rapid and versatile photonic annealing of graphene inks for flexible printed electronics, Adv. Mater. 27 (2015a) 6683-6688. doi:10.1002/adma.201502866.

• Secor E.B., Hersam M.C., Emerging carbon and post-carbon nanomaterial inks for printed electronics, J. Phys. Chem. Lett. 6 (2015b) 620-626. doi:10.1021/jz502431r.

• Shen J., Shi M., Yan B., Ma H., Li N. and Ye M., "One-pot hydrothermal synthesis of Ag-reduced graphene oxide composite with ionic liquid" J. Mater. Chem., 2011, 21, 7795. DOI: 10.1039/cljml0671f • Singh, M. K., Titus, E., Krishna, R., Hawaldar, R. R., Goncalves, G., Marques, P. A. A. P., & Gracio, J. (2012). Direct nucleation of silver nanoparticles on graphene sheet. Journal of nanoscience and nanotechnology, 12(8), 6731-6736.

• Sreeprasad T.S., Berry V., How do the electrical properties of graphene change with its functionalization? Small. 9 (2013) 341-350. doi:10.1002/smll.201202196.

• X. Z. Tang, Z. Cao, H. B. Zhang, J. Liu, Z. Z. Yu, Growth of silver nanocrystals on graphene by simultaneous reduction of graphene oxide and silver ions with a rapid and efficient one-step approach Chem. Commun. 2011, 47, 3084

• J. Tian, S. Liu, Y. Zhang, H. Li, L. Wang, Y. Luo, A. M. Asiri, A. O. Al-Youbi, X. Sun, Environmentally Friendly, One-Pot Synthesis of Ag Nanoparticle-Decorated Reduced Graphene Oxide Composites and Their Application to Photocurrent Generation Inorg. Chem. 2012, 51, 4742.

• Minh-Hai Tran, Hae Kyung Jeong Synthesis and characterization of silver nanoparticles doped reduced graphene oxide Chemical Physics Letters 630 (2015) 80-85. http://dx.doi.Org/10.1016/j.cplett.2015.04.042

• Wang X., Huang P., Feng L., He M., Guo S., Shen G., Cui D., "Green controllable synthesis of silver nanomaterials on graphene oxide sheets via spontaneous reduction" RSC Adv. 2012, 2, 3816.

• Xu, Zhengxia, Hanyang Gao, and Hu Guoxin. "Solution-based synthesis and characterization of a silver nanoparticle-graphene hybrid film." Carbon 49.14 (2011): 4731-4738.

• W. Yang, C. Wang,-V. Arrighi,-F. Vilela "One step synthesis of a hybrid Ag/rGO conductive ink using a complexation-covalent bonding based approach" J Mater Sci: Mater Electron (2017) 28:8218-8230. DOI 10.1007/sl0854-017-6533-2

• Z. Zhang, F. Xu, W. Yang, M. Guo, X. Wang, B. Zhang, J. Tang A facile one-pot method to high- quality Ag-graphene composite nanosheets for efficient surface-enhanced Raman scattering Chem. Commun., 2011, 47, 6440-6442. DOI: 10.1039/clcclll25f

• Zhang W., Bi E., Li M., Gao L., "Synthesis of Ag/RGO composite as effective conductive ink filler for flexible inkjet printing electronics" Colloids and Surfaces A: Physicochem. Eng. Aspects 490 (2016) 232-240.

• Zou Q., Cao C., Zhu H., and Hou C. "Preparation of Low Temperature Sintered Graphene/Silver Nanocomposite-Based Conductive Ink" P. Zhao et al. (eds.), Advances in Graphic Communication, Printing and Packaging, Lecture Notes in Electrical Engineering 543, Springer Nature Singapore Pte Ltd. 2019. https://doi.org/10.1007/978-981-13-3663-8 101

Claims

1. Method for the preparation of a conductive ink comprising the steps:

-addition of GO in H2O at a GO concentration between 1 mg/ml and 10 mg/ml

-exfoliation of the GO of the said dispersion by ultrasound and stirring

-addition of a metal salt and an organic reducing agent

-heating of the mixture at a temperature higher than 50 °C

-filtration, collection and purification of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles

-mixing of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles with one or more resins, wherein the organic reducing agent is one or more organic aminophenylsulfonic derivatives.

2. Method according to claim 1, wherein the organic reducing aminophenylsulfonic derivative is 3,4-diamino benzenesulfonic acid and/or 2,5-diamino benzenesulfonic acid and/or a combination thereof at any ratio.

3. Method according to anyone of claims 1 or 2, wherein the GO mass to the reducing agent mass is in the range from 1 to 2 to 1 to 4.

4. Method according to anyone of claims 1 to 3, wherein the metal salt is a silver salt or a copper salt or a combination thereof.

5. Method according to anyone of claims 1 to 4, wherein the content by weight in metal is lower than 50% w/w and preferably higher than 8.5% w/w.

6. Method according to anyone of claims 1 to 5, wherein the exfoliation of GO is effected by keeping it in an ultrasound system of HOW and 40 kHz for a time period more than 20 minutes, preferably between 20 to 60 minutes.

7. Method according to anyone of claims 1 to 6, wherein filtration and collection of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles is performed by using suitable Nylon filters, preferably having a pore diameter of 0.45 pm, or by multiple centrifugations, preferably performed at 9000 rpm.

8. Method according to anyone of claims 1 to 7, wherein the purification of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles is effected by washing, preferably de-ionized water and/or ethanol and/or acetone.

9. Method according to anyone of claims 1 to 8, wherein the mass ratio (dry mass) of resin to conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles is in the range 30/70 to 70/30, preferably 55/45.

10. Method according to anyone of claims 1 to 9, wherein heating is performed under reflux, preferably at a temperature between 50 and 100 °C.

11. Method according to anyone of claims 1 to 10, wherein the resins used are based on (a) an acrylate emulsion, or (b) a styrene acrylate ester emulsion or (c) polyurethane or (d) polyurethane-polyacrylate or (e) salts thereof or systems derived from a combination thereof or with other supplementary resins.

12. Method according to anyone of claims 1 to 11, wherein the mass of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles to the total mass of the ink is 5-25% and the solvent quantity added is suitable so that the ink viscosity under high shear rates is in the range of 0.05-0.3 Pas in order to be suitable for intaglio/flexography.

13. Method according to anyone of claims 1 to 11 wherein the mass of the conductive hybrid of functionalized reduced graphene oxide doped with metal nanoparticles to the total mass of the ink is 10-35% and the solvent quantity added is suitable so that the ink viscosity under high shear rates is in the range of 0.1-0.5 Pas in order to be suitable for screen-printing.