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(May 2011)
 
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Laboratory of Pulp and Paper Science and Graphic Arts

IV - Printing Processes

IV - 5 - Surface functionalization process engineering

In his PhD (defended in July 2007), Mathieu Fenoll initiated the studies on surface functionalization in the STG group. His work concerned the optimization of the different transfer parameters, regarding the printing process. Inks based on conductive polymers (such as polythiophene) were formulated. The study of the surface and physical properties of different papers and polymers was conducted in order to select two model substrates for printing tests. Finally, for three printing processes (offset, gravure and flexography), the optimization of printing parameters demonstrated their influence on the characteristics of the deposits, especially concerning their electrical conductivity.

In offset printing, the ink formulation is relatively more complex, and the choice of the substrate is of prime importance. A rough substrate will allow a larger amount of ink transfered, but the layer deposited is not homogeneous enough. Consequently, the electrical resistance is higher [Figure 1].

Offset - photographies d’impression réalisées sur PBT 
   avec une pression de 100 N (a), 300 N (b), et 500 N (c)
Figure 1 - Offset - photographies d’impression réalisées sur PBT
avec une pression de 100 N (a), 300 N (b), et 500 N (c).

Gravure printing gave interesting results with inks based on conductive polymers, such as PEDOT. However, the lowest electrical resistances were achieved with inks based on metallic particles. In the latter, the obtained layer is not a regular solid, but it follows the cell contours of the engraved cylinder, which is due to the viscosity of the inks used. Nevertheless, the whole layer is regular enough to make a conductive pattern [Figure 2]. Flexography has the particularity of forming blurring around the printed elements, which is not compatible with fine patterns. On the contrary, this technique does not involve high pressure and seems to be adequate for solid printing (dielectric layers, protective layer,...).

Héliogravure - Photographies réalisées à l’InfiniteFocus 
   des impressions d’encre à pigments métalliques Xink sur le PET (a) 
   et sur le papier (b)
Figure 2 - Héliogravure - Photographies réalisées à l’InfiniteFocus des impressions d’encre à pigments métalliques Xink sur le PET (a) et sur le papier (b).

During his PhD in collaboration with CEA LITEN, Franck Medlege showed that flexography was suitable to print functional inks, aiming manufacturing of autonom energy ressources. Thus, this study was focused on making a lithium-based accumulator, each component implying the formulation of new functional inks. Consequently, new binders and new additives were used. They had to be inert toward the electrochemical performances of the active materials of the accumulator.

More precisely, two main key points had to be removed:

Schéma de principe de l’impression flexographique réalisée
Figure 3 - Schéma de principe de l’impression flexographique réalisée

Highest performances were reached on coated electrodes with the new polymer binder [Figure 4], in comparison with the standard PVDF binder, which is the reference for our study.

Courbes signatures de piles LiCoO2/Li, réalisées 
   avec des électrodes imprimées ou enduites
Figure 4 - Courbes signatures de piles LiCoO2/Li,
réalisées avec des électrodes imprimées ou enduites

Finally, in a recently started project (European contract, 19 partners, PhD thesis to be defended in 2011), Rita Faddoul works on the potentialities of the gravure and screen printing processes, in order to made multilayer systems for micro-electronics. The substrates are ceramics, for example, and functional inks based on conductive particles are developed. The aim of the study is to combine different printing processes, in order to make, at low cost, various elements dedicated to different applications, such as automotive, lighting… This work is focused on the adequation of the new functional inks with the selected printing processes and all the further operations of manufacturing, while guaranteeing the final properties of the object.

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