Abstract

Sustainable printing is a necessity today. Traditional inks were formulated to reproduce an optical theme or to transfer a message. Current developments in printing technology place value on functional inks, which need to perform targeted functionalities (thermal or electrical conductivity, semiconductivity etc). At the same time functional inks are called to minimize their environmental burden.

One of the rapidly growing type of conductive inks is graphene inks, where the pigment is either (a) pristine graphene which is produced mainly by the liquid phase exfoliation process or (b) chemically reduced graphene oxide (rGO), that is produced by reducing graphene oxide (GO). The production of pristine graphene nanosheets via liquid exfoliation of graphite is achieved in selected organic solvents or surfactant aqueous solutions. However, despite the high quality of pristine graphene nanosheets and their remarkable properties, their low yield at high costs and up-scaling difficulties are restrictive parameters for their large scale production.

Graphene oxide is a non-conductive hydrophilic material that is produced by oxidizing graphite under highly acidic conditions which causes definitely environmental burden due to the waste generated. The reduction of GO to rGO is necessary in order to obtain conductive properties and is achieved by simple heat treatment or by various reducing agents or electrochemical and / or photothermal. However, the reduction process should lead to soluble derivatives when used for inks development. Under appropriate conditions, the simultaneous reduction and chemical modification of GO by a sulfonated aromatic diamine is possible, thereby producing highly conductive hydrophilic rGO, suitable for the synthesis of conductive water-based inks without the use of dispersing aids.

Although rGO is inferior in electrical conductivity than pristine graphene, it is produced with much greater efficiency, an advantage that has led to its commercial production.

As a carbon-based material, graphene is generally safe. However, the manufacturing processes for graphenic materials can use hazardous chemicals such as acids, oxidizing and reducing agents, solvents. On a small scale, these chemicals are easily handled. However, on commercial production scale the dealing with large quantities of hazardous materials is significant. Concerning graphene, an important challenge is its sustainable production using renewable sources and the manufacturing process modification so as to have the minimum possible environmental impact.

Resins are the binder of the ink pigmented particles and help to pigment dispersing, carry and hold the pigment to the substrate, facilitate solvent release and deliver end use properties, such as flexibility, adhesion, gloss, dry and wet abrasion. The resins commonly used in printing inks formulations are condensation or addition polymers. However, the resins are non-conductive material and therefore their percentage, on the dry film of the ink, must be kept to the minimum possible, so that to contribute as much as possible to the conductivity and simultaneously not to burden the environment.

Depending on the type of solvent, inks may be solvent or water-based. Water based inks are considered by their nature environmentally friendly but they still contain petrochemical-based raw materials that are poorly biodegradable. Water-based inks compared to solvent-based inks have a low evaporation rate, thus maintaining a constant viscosity during printing. In water-based inks the resins must be water soluble, but after printing must be insoluble to provide the necessary strength. This can be achieved by adding alkali to a suitable resin, which is converted from insoluble to soluble form. Acrylic resins are mainly used for this purpose. In their acidic form they are insoluble in water, but once neutralized, and need to be neutralized in order to keep them in the solution.

Then as the amine evaporates, the dried ink film develops the water resistance necessary for most end uses. Water-based inks formulated with biodegradable resins are a challenge.

In order to enhance or modify the final ink properties, additives may be added to the ink formulation. Such additives are waxes, defoamers, surfactants etc. Inks may benefit from

addition of certain additives, but in functional inks the presence of additives is not always desirable, as it can adversely affect functional properties, such as electrical conductivity, while contributing negatively to the environment as well.

In the present work we focus on the primary environmental impact assessment of gravure and flexographic conductive graphenic inks using two different methods of preparation process. The first involves the GO reduction using a sulfonated aromatic diamine and the second is based on the exfoliation of graphite in suitable solvents. Then is following the recording of chemical reagents, energy and time required for production of 1 g graphenic material and the corresponding amount in grammars of graphene ink taking into account that the percentage of the solids in the final suspension was 55% w/w in graphenic material.

For ink's formulation various commercial resins (Druckfarben SA) were used. We tried to correlate the chemical composition of each resin with the printing quality (printability) of the ink and the environmental profile of the resin. The printing tests were done using the printability testers IGT G1-5 with raster patterned printing cylinders or IGT F1. Various types of paper where used as printing substrates. Concerning graphene preparation, the chemical reduction process (rGO) was found to be less environmental impacting method compared to the exfoliation of graphite in suitable solvents.

The as prepared graphenic materials do not require any annealing process due to their high conductivity and this is another enviromental advantage. Also, the excellent dispersibility of these highly conductive pigments does not require any energy consumption to produce stable water based inks. Concerning to conductive ink printability, the ink formulation involving a mixture of carboxylated acrylic and methacrylate polymers exhibited the best printing results.

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Keywords

sustainability, conductive inks, graphene, reduced graphene oxide

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Acknowledgements

This research has been co‐financed by the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship and Innovation, under the call RESEARCH – CREATE – INNOVATE (project code:T1EDK-02093).