INNO-KOM-Ost MF 150063
The printing of highly conductive structures has been very expensive in terms of time, materials and costs up to now. Digital printing technology is to be used for the rapid implementation of printed electronics. The highly conductive substances to be printed are based on silver or carbon particles that have to be introduced into a paste or binder system and kept in suspension. The goal of the project, the production of printed conductive structures on textile substrates using ChromoJet spray printing technology, was achieved.The bypass technology specially developed for this purpose is suitable both for printing silver nanowire dispersions and for partial or full-surface printing of substrates with textile finishing aids.
A newly established process for producing conductive prints on textile substrates is the ChromoJet digital spray printing process. It is more flexible and less cost-intensive than the currently used screen printing when it comes to quick design changes. In the previous project ELTRO-Druck (INNO-KOM-Ost MF110132), conductive polymers were used to create conductive structures for sensors and heaters on textiles using the chromojet technique. Using the printed structures, both a touch sensor and a heating structure integrated in a shirt have been built as demonstrators. Further research work with SMEs after completion of the project shows that there is a need for further research when it comes to the long-term stability of the electrical characteristics and the adhesion of the conductive layers.The aim of the research project presented here is therefore the development of a technology for the production of highly conductive printed structures on textiles using chromojet technology, which feature long-term stability of the conductivity.
The highly conductive substances to be printed are based on silver or carbon particles that have to be introduced into a paste or binder system and kept in suspension. The latter is achieved with the bypass technology for chromojet technology developed together with J. Zimmer Maschinenbau GmbH (Klagenfurt, Austria) (Figure 1).The bypass keeps the printing paste in constant motion. The ink flows continuously past the print head, alternating between two supply vessels. Sedimentation and segregation of inks with functional pigments are thus prevented.
Results and Applications
With the newly developed bypass technology, sedimentation phenomena and segregation of inks with functional pigments are prevented. Thus, for the first time, sedimentation in dispersions with metallic pigments can be suppressed for hours, but metallic pigments > 1 µm cannot pass through the protective filters. The simulations for calculating the sheet resistances of printed dispersions with metallic conductive pigments show that with printable solid contents of less than 10 %, sheet resistances of less than 10 W/□ can only be expected for particle sizes of < 50 nm (Figure 2). Print tests with diluted dispersions of silver nanowires confirm the simulation. Silver nanowires with a diameter of approx. 5 nm and a length of 50 nm can be printed without problems in concentrations of 0.1-0.25 % with the ChromoJet technology. For concentrations > 0.25 %, the bypass technology must be used for reproducible homogeneous printing.For sheet resistances < 10 W/□, diluted (0.1-0.25 %) inks up to 10-fold or inks with concentrations of 1-5 % must be printed with the bypass technology. Experiments with the bypass technology generally show that only metallic nanomaterials lead to the desired conductivity, as shown by the simulation and SEM images (Figure 3) of diluted and printed silver nanowires. By using silver nanowires, resistances of 8 Ω/□ were measured and thus conductive prints were realised on textiles. The bypass technology developed and attached for the ChromoJet system is suitable both for printing silver nanowire dispersions and for partial or full-surface printing of substrates with avivages and finishes in textile finishing. With the digital printing process and the correspondingly adapted silver nanowire-based inks, printed highly conductive structures can be produced in various layouts. Sheet resistances of less than 10 Ω/□ have been realised. Using bypass technology and when using a 3-4% silver nanowire dispersion, even sheet resistances of less than 1 Ω/□ down to 0.2 Ω/□, as required for electronic circuits, can be achieved. In the project, exemplary interdigital structures and heating structures have been realised in the form of full-area prints with inks based on conductive polymers and carbon-containing additives and implemented in functional samples. However, these have surface resistances in the kiloohm range and are unsuitable for electronic circuits. With conductive polymer prints, sheet resistances in the range of 100-1000 Ω/□ can be realised, but these are neither stable in the long term nor capable of carrying current.Sheet resistances below 10 Ω/□ can only be realised with silver nanowire dispersions. Electrical currents, as required for textile heaters, can be realised via the printed surfaces. Long-term stable resistances are achieved under current flow (Figure 4).
Dr. Yvonne Zimmermann