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SPECIAL Conductive, Transparent, Flexible Films. Transparent, conductive films based on flexible plastic sheets open up completely new designs for control and touch-keys. These “invisible” touch-keys can be integrated into transparent user interfaces or into curved (3-D) surfaces. This allows decoration and functionality to be combined with maximum design flexibility, e. g. in cars and smartphones. Transparent conductive films with structured conductors (photo: PolyIC) MARTIN SCHEWE HENNING ROST lexible, optically transparent films are required for applications such as displays, ultra-thin heating elements, and operating controls in automobiles. They must have a coating that possesses the maximum-possible electrical conductivity. Nowadays, polyester-based plastic films coated with indium tin oxide (ITO) are often used, but these are technically difficult and expensive to structure prior to use [1]. The limited conductivity and relative fragility of ITO renders it less suitable for applications involving large and, especially, curvaceous surfaces. F Translated from Kunststoffe 10/2012, pp. 38–42. Article as PDF-File at www.kunststoffe-international.com; Document Number: PE111174 20 Alternatives to ITO technology that do not have these disadvantages are now available in the marketplace. Thus, PolyIC GmbH & Co. KG, Fürth (a member of the Kurz Group) has succeeded in creating a low-cost, high-volume roll-to-roll production process (Fig. 1) for printed electronics and thus also in developing transparent, conductive films. The overall process of manufacturing printed electronics relies heavily on the production of electrodes and conductors – i. e. of conductive structures. These structures are so fine that they cannot be detected by the human eye unaided.Aside from high conductivity, the creation of high-resolution, micron-size structures on thin, flexible polyester films (the typical resolution is 10 microns) can lead to highly transparent layers. Consequently, these are very appealing interesting for applications which were previously the preserve of ITO-coated films, but which suffered from the familiar disadvantages. The degree of transparency and conductivity can be customized by varying the coverage of the surface (width and density of the conductive structures per m2 polyester film) with conductive material. The latter are metals which, in line with the application, are applied over the full length and breadth of the film or as an individual layout in the form of a pattern (see [4] for technical details). i Contact PolyIC GmbH & Co. KG D-90763 Fürth Germany TEL +49 911 20249 8145 > www.polyic.com © Carl Hanser Verlag, Munich Kunststoffe international 10/2012 Electronics SPECIAL Fig. 1. Flexible, conductive and transparent films on the machine The most popular carrier material is polyethylene terephthalate (PET), because it can be obtained in thin, flexible, highly transparent grades and in bulk. Added to which, in comparison to other plastics, it is relatively inexpensive. Flexible, conductive and transparent films are offered by PolyIC under the name PolyTC. They serve as a widespread replacement for ITO in various sensor elements, such as touch-screens and touch-keys (Title picture). Other application areas for PolyTC films are: transparent EMI shielding films, transparent heating films, transparent electrodes for organic light emitting diodes (OLEDs) and organic solar cells (organic photovoltaics or OPVs), as well as electrostatic discharging (ESD). (figs. 1 to 3: PolyIC) Transparent Films for Touch-keys Touch-keys are electronic switches which contain no moving mechanical parts and are operated by the touch of a finger. They are currently the main application for transparent conductive films. Touch-keys are composed of conductive surfaces which are connected to controller chips. These devices detect changes which occur in electrical capacitance when, for example, a finger touches the film. Such a change triggers a switching operation. Eliminating mechanical parts renders the touch-keys very rugged. Furthermore, they are easy to integrate into all kinds of applications because they are flat and have hardly any bulk. Their transparency allows them to be used as discreet switches which become visible only when they are > back-lit. Fig. 2. Transparent conductive films for touch-keys Kunststoffe international 10/2012 www.kunststoffe-international.com 21 SPECIAL Touch-keys made with PolyTC films have obvious advantages. They are transparent, can be back-lit, are thin and flexible and so can be incorporated into curved surfaces, have no mechanical components at all and are subject to little wear, could be used wherever easy-clean surfaces or switches are needed, e.g. in the medical field. Touch-keys open up new, innovative design concepts that have clear technical advantages to offer. Aside from vehicle cabins, conceivable applications are button panels (Fig. 2) for white goods (large electronic domestic appliances such as stoves, refrigerators and washing machines), brown goods (electrical or electronic consumer goods such as televisions, radios, and DVD players), and medical devices. Their flexibility is an advantage which should not be underestimated. Until recently, conventional touch-keys could be integrated into flat, even surfaces only. Now, those based on PolyTC films can be Fig. 3. Flexible, conductive and transparent films in swept or curved surfaces (e.g. dashboards) a single step. This process can slash the cost of production because the plastic parts are already fully decorated at the injection stage. This means that there is no need for a decoration step after injection molding, or for interim storage and additional steps. This process also facilitates to use the PolyTC film in the fabrication of decorative plastic parts with integrated touch functionality by means of an injection molding process and a combination of IMD and IML. The visible surface of the component is not blocked by mechanical switches as a result, because the switching elements are located behind the decoration. Non-conductive metallic IMD films can be used to produce metallic or even brushed-metal surfaces with active buttons. Electrodes for EMI Shielding Electromagnetic interference (EMI) refers to electronic emissions which interfere with an electronic device or which the device itself produces. The growing complexity of electronic circuits, e.g., in the automotive sector and in mobile telephony, means that interference by electromagnetic waves is becoming more common. The use of components that fall into the EMI shielding category can contain this risk. PolyIC can produce highly transparent, conductive PolyTC films for shielding electromagnetic interference in displays (Fig. 4). Such films offer the following benefits for EMI shielding: customized layouts in the form of rhombic, rectangular or hexagonal designs individually controlled surface resistance with appropriate shielding effect and desired transparency, and ! Fig. 4. Possible applications for flexible, conductive and transparent films incorporated anywhere on the dashboard or center console – even swept and curved areas, opening up scope for novel, creative and elegant designs that have no mechanical switches or buttons (Fig. 3). Active Buttons Furthermore, combining decorative film technology and functional film, i.e. PolyTC, leads to radically new design possibilities. Recently, in-mold decoration (IMD) has become the established method of decorating plastic parts (injection of individual films or labels is called in-mold labeling (IML)). IMD combines injection molding and hot embossing in 22 (photo: iStock) Electric Heating Elements The conductors on the PolyTC films can also be made to convert electrical power into heat so that these can serve as heating films. Although high resolution of the PolyTC structures leads to high transparency values, it also lowers the conductivity of the film, and so heat output is limited. Consequently, when a PolyTC heating film is being designed, a good compromise between adequate transparency and sufficiently high conductivity needs to be found. Nevertheless, the films described here offer potential for use in local heating elements, e.g. preventing fogging of windshields and car mirrors. Printed Electronics Printed electronics is a set of printing processes for wholly or partly creating electronic devices. Unlike conventional printing inks, which have no electrical function whatsoever, these processes print electronically functional materials, often in the form of specialty polymers. These conducting, semi-conducting and insulating polymers are dissolved in common organic solvents and printed as a kind of electronic ink [2]. Various layers of the polymers are deposited on each other with great precision to create a structure. Electronic devices such as transistors, diodes and capacitors can be fabricated in this way [3]. The use of modern, high-volume printing methods significantly lowers the manufacturing costs. The ability to print large-area and flexible substrates and the use of novel functional materials open up electronics applications, to which conventional siliconbased electronics had little or no access. New developments made possible by printed electronics include such applications as radio frequency identification tags (RFID tags), flexible displays and solar cells. © Carl Hanser Verlag, Munich Kunststoffe international 10/2012 Electronics SPECIAL cations on the mass market. The first product of this type, namely conductive and transparent films based on PolyTCbrand plastics, is of course for practical, high-volume applications. This novel film offers clear advantages both in terms of product design and integration into the target product. REFERENCES 1 Wörle, J.; Rost, H.: Roll-to-roll production of transparent conductive films using metallic grids. MRS Bulletin 36 (10) (2011) p. 789 2 Rost, H.: From Polymer Transistor to Printed Electronics. Kunststoffe international 10 (2005) pp. 209–214, PE103333 3 Rost, H.; Mildner, W.: On the Way to Printed Electronics: Latest Advances. Kunststoffe international 6 (2008) pp. 60–64, PE104275 4 Wörle, J.; Rost, H.: Invisible Helpers: Conductive Films. Kunststoffe international 4 (2010) pp. 58–61, PE110369 5 www.occhio.de THE AUTHORS MARTIN SCHEWE, born in 1981, is Project Manager at PolyIC GmbH & Co. KG, Fürth, Germany; [email protected] DR. HENNING ROST, born in 1966, is Senior Project Manager at PolyIC GmbH & Co. KG. Fig. 5. Gesture-controlled lamp featuring a flexible, conductive and transparent film incorporated into the lens of the lamp head (photo: Occhio) customer-specific geometry for the desired EMI cut-off frequency. Light Elements Controlled by Gestures At the 2012 Light+Building Show in Frankfurt am Main, Germany, Occhio [5], the a German lighting manufacturer, presented an innovative new lighting series called ìo 3d. The extraordinary thing about this lighting system is not just the design, but the new and innovative operating concept. Instead of conventional switches and buttons, it is operated by simple hand gestures: contactless and intuitively. Moving a hand toward the top of the light makes it brighter or dimmer. A quick flick of the hand at the top switches it on and off. Inside the Occhio lamp, the PolyTC film is integrated into the lens of the lamp head where it acts as the sensor for contactless gesture control. It utilizes the so-called proximity effect to detect an approaching hand, and then translates the gesture into the appropriate command (Fig. 5). Conclusion Printed electronics – that is, the production of electronic components and products by roll-to-roll processes – has left the laboratory behind and notched up its first successful appliKunststoffe international 10/2012 www.kunststoffe-international.com 23