DESC:TITLE OF THE INVENTION: ELECTRICALLY CONDUCTIVE GRAPHENE BASED INK FOR FLEXIBLE ELECTRONICS AND PRINTED ELECTRONICS

FIELD OF THE INVENTION
The present invention is related to a highly conductive ink comprising of graphene additives, conductive filler, and a thermoplastic polymer, said ink having a high electrical conductivity, washability, and viscosity that can be tuned as per requirement.
BACKGROUND OF THE INVENTION
The rapid research and development of flexible electronics has increased the focus and demand for efficient conductive and flexible circuits. Gold and silver nano particles were explored for this purpose, but the prohibitively high costs hindered their development and integration into flexible electronics. Because of this, copper was subsequently explored for the same purpose, but it’s proneness to agglomerating during the synthesis process and rapid oxidation on exposure to environment hindered its integration as well into flexible electronics, since it led to increased annealing temperatures and lower values of electrical conductivity. Metal nanoparticle inks are prone to corrosion when exposed to moisture and they tend to break off when the wearable is washed.
There is, therefore, a need in the art for a highly conductive ink for flexible electronics and printed electronics, which overcomes the aforementioned drawbacks and shortcomings.
SUMMARY OF THE INVENTION
A conductive ink for flexible electronics and printed electronics is disclosed. The conductive ink comprises thermoplastic urethane as a binder; Graphene Nanoplatelets or Graphene Sheets; N-Methyl-2-pyrrolidone, m-Cresol, Dimethylformamide, or a non-organic solvent as a solvent, said solvent facilitating the tuning of the viscosity of the ink; and a surfactant. The concentration of the binder ranges between 10% w/v and 30% w/v; the concentration of the Graphene Nanoplatelets or Graphene Sheets ranges between 70% w/v and 90% w/v; the concentration of the solvent is 0.01% w/v; and the concentration of the surfactant ranges between 5% w/v and 10% w/v. The process of preparing the conductive ink is also disclosed.
The disclosed ink withstands approximately 100 washing cycles, is stable over bending and twisting, has a viscosity and conductivity that can be tuned as per requirement, is anti-corrosive, and is bio-compatible.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates a schematic of the four-probe technique, in accordance with the present disclosure
DETAILED DESCRIPTION OF THE INVENTION
Throughout this specification, the use of the word "comprise" and “include”, and variations such as "comprises" "comprising", “includes”, and “including” may imply the inclusion of an element or elements not specifically recited.
Throughout this specification, the disclosure of any range is to be construed as being inclusive of the lower limit of the range and the upper limit of the range.
A highly conductive ink for flexible electronics and printed electronics is disclosed. The conductive ink comprises a binder in a concentration that ranges between 10% w/v and 30% w/v; Graphene Nanoplatelets or Graphene Sheets in a concentration that ranges between 70% w/v and 90% w/v; a solvent in a concentration of 0.01% w/v, said solvent facilitating the tuning of the viscosity of the ink; and a surfactant in a concentration that ranges between 5% w/v and 10% w/v.
In an embodiment of the present disclosure, the binder is thermoplastic urethane.
In another embodiment of the present disclosure, the solvent is N-Methyl-2-pyrrolidone.
In yet another embodiment of the present disclosure, the solvent is m-Cresol.
In yet another embodiment of the present disclosure, the solvent is Dimethylformamide.
In yet another embodiment of the present disclosure, the solvent is a non-organic solvent.
In yet another embodiment of the present disclosure, the conductive ink is binder free.
In yet another embodiment of the present disclosure, the surfactant is sodium dodecyl sulphate.
In yet another embodiment of the present disclosure, the surfactant is Hexadecyltrimethylammonium bromide.
The process of preparing the conductive ink comprises the following steps:
mixing Graphene Nanoplatelets or Graphene Sheets with the solvent that facilitates the tuning of the viscosity of the ink to obtain a concentration of 0.5% w/v to 1% w/v of Graphene in the solvent, with the concentration of the Graphene Nanoplatelets or Graphene Sheets being between 70% w/v and 90% w/v, and the concentration of the solvent being 0.01% w/v;
adding the surfactant to the Graphene-solvent mixture, with the concentration of surfactant ranging between 5% w/v to 10% w/v;
sonicating the Graphene-solvent-surfactant mixture for four hours; and
adding the binder in a concentration that ranges between 10% w/v and 30% w/v, and stirring the mixture at a temperature that ranges between 70 degrees Celsius and 80 degrees Celsius to obtain the conductive ink.
In yet another embodiment of the present disclosure, the process is binder free.
The conductive ink was tested for viscosity and conductivity, with thermoplastic urethane as the binder and Dimethylformamide as the solvent, as explained below:
Rheology test for Viscosity
Viscosity of the ink was measured using a viscometer. Generally, the viscometers are stress or strain controlled. The distance is inversely proportional to the shear stress.
The important parameters that were considered were: flow generation, flow of liquid/solution, and the velocity on a rotating member/pressure. The main components were: narrow gap concentric cylinder with small-angled cone and plate and parallel capillary tube/plate slit.
The torque was measured with the effect of rotation rate of liquid in the concentric cylinders. Due to this torque, pressure was applied on the capillary tube. When the pressure dropped down, the flow rate was observed.
Probe Technique for Conductivity
Surface resistivity or sheet resistivity is measuring the resistance of a thin film which is uniform in thickness. This is generally used in the characterization of resistive paste printing, doping of semiconductor materials, and various coatings.
This measurement was done by Four-Probe technique. As illustrated in Figure 1, four equally-spaced and co-linear probes (looking like sharp needles; 1, 2, 3, and 4) were used as an electrical contact and the sheet resistance (electrical information) was measured.
Two outer probes (1 and 4) were connected with each other and two inner probes (2 and 3) were connected with each other. When DC current was applied in between the two outer probes (1 and 4), the voltage drop was observed in between the two inner probes (2 and 3).
The sheet resistance of the material (thin film) was calculated by the following formula:
R_s=p/(ln?(2)) ?V/I=4.53236 ?V/I

Here, Rs – is the sheet resistance, I – is the current applied between the two outer probes and ?V – is the change in voltage measured between the two inner probes. The unit of the sheet resistance is O/square.
The sheet resistivity was measured from sheet resistance using the following formula: ?=Rs.t, Where, ? – is the sheet resistivity, Rs - is the sheet resistance and t – is the thickness of the material.
The parameters of the disclosed ink are shown in Table 1.
Parameter Method Value
Sheet Resistance 4-Probe Technique 70-80 O/sq.
Cured Thickness High Magnification Microscope 100 Microns
Coverage Screen Printing 1 gm of ink will cover approximately 100 square centimeters
Viscosity @ 200 sec-1 at 25 °C 30 mPas

The disclosed ink withstands approximately 100 washing cycles, is stable over bending and twisting, has a viscosity and conductivity that can be tuned as per requirement, is anti-corrosive, and is bio-compatible.
It will be apparent to a person skilled in the art that the above description is for illustrative purposes only and should not be considered as limiting. Various modifications, additions, alterations and improvements without deviating from the spirit and the scope of the disclosure may be made by a person skilled in the art. Such modifications, additions, alterations and improvements should be construed as being within the scope of this disclosure. ,CLAIMS:1. A conductive ink for flexible electronics and printed electronics, comprising:

thermoplastic urethane as a binder in a concentration that ranges between 10% w/v and 30% w/v;
Graphene Nanoplatelets or Graphene Sheets in a concentration that ranges between 70% w/v and 90% w/v;
a solvent in a concentration of 0.01% w/v, said solvent facilitating the tuning of the viscosity of the ink, said solvent being N-Methyl-2-pyrrolidone, m-Cresol, Dimethylformamide, or a non-organic solvent; and
a surfactant in a concentration that ranges between 5% w/v and 10% w/v.

2. A conductive ink for flexible electronics and printed electronics as claimed in claim 1, wherein the conductive ink is binder free.

3. A process of preparing a conductive ink for flexible electronics and printed electronics, comprising the steps of:

mixing Graphene Nanoplatelets or Graphene Sheets with a solvent that facilitates the tuning of the viscosity of the ink, to obtain a concentration of 0.5% w/v to 1% w/v of Graphene in the solvent, with the concentration of the Graphene Nanoplatelets or Graphene Sheets being between 70% w/v and 90% w/v, and the concentration of the solvent being 0.01% w/v, said solvent being N-Methyl-2-pyrrolidone, m-Cresol, Dimethylformamide, or a non-organic solvent;
adding a surfactant to the Graphene-solvent mixture, with the concentration of surfactant ranging between 5% w/v and 10% w/v;
sonicating the Graphene-solvent-surfactant mixture for four hours; and
adding a binder in a concentration that ranges between 10% w/v and 30% w/v, and stirring the mixture at a temperature that ranges between 70 degrees Celsius and 80 degrees Celsius to obtain the conductive ink.

4. A process of preparing a conductive ink for flexible electronics and printed electronics as claimed in claim 3, wherein the process is binder free.