On the path to flexible displays

Transcription

1 Dr. Sabine Philipp Tel.: +49 (6 21) Fax: +49 (6 21) Trade press conference K 2004 on June 22 and 23, 2004, in Ludwigshafen On the path to flexible displays Ultrason E high-temp for high-tech Report by Dr. Christian Maletzko, Marketing Ultrason BASF Aktiengesellschaft Ludwigshafen Phone: Germany Communication plastics

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3 There is scarcely another market with shorter product life-cycles and stiffer competition than in information technology and consumer electronics. Innovation-driven industries rely on high-quality, highperformance materials. One such material is polyethersulfone (PES), a high-temperature polymer marketed by BASF under the Ultrason E tradename. In 2002, thanks to continuously increasing demand, BASF expanded production capacity significantly to around five thousand tonnes per year. One reason for the higher demand is the ability of Ultrason to penetrate areas that hitherto appeared to be the exclusive domain of metals, ceramics and glass. A typical case is the use of Ultrason E film as a substrate for Liquid crystal displays (LCD) instead of glass (figure 1). Materials used for LCD substrates - including polymers - must meet a tough set of requirements: a very high glass transition temperature (at least 220 C), high transparency, low internal stress (to prevent optical birefringence), high purity and homogeneity. This homogeneity enables films with a surface roughness of less than 10 nanometres to be produced (figure 2). 2

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5 Thinner, lighter and more flexible There are considerable advantages in using a polymer substrate instead of a glass one: a polymer film is not only tough and non-brittle, but also 50% thinner and 30%lighter. And because of its durability, the film is also cheaper to produce and easier to process (using roll-to-roll technology for example). In addition, displays built from a flexible substrate could be rolled or folded up when not in use (figure 3). With Ultrason E 2010 Q28 it has been possible to develop a material that fulfils the requirements to a great extent. For instance, the material exhibits a glass transition temperature of 225 C, a property that might not be so critical for the display's final application but which is essential for its manufacture. Furthermore, the film is highly transparent, with excellent light transmission over the whole of the visible spectrum (figure 4). Broad range of operating temperatures The amorphous nature of polyethersulfone, in combination with its lack of inherent colour, is what gives the film its high transparency. Amorphous materials in general are characterized by the constancy of their mechanical properties right up to the glass transition temperature. This is particularly true for PES, thanks to the stiffness of its chemical building blocks. Thus Ultrason E 2010 Q28 has a very low thermal expansion coefficient, which remains practically constant over a wide temperature range. In addition, the polymer has excellent heat stability and can withstand sustained temperatures ranging from 50 to +200 C without loss in performance. 4

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7 Semi-crystalline materials on the other hand often exhibit significant temperature dependency, with abrupt changes in morphology occurring within the desired temperature range. Such phase changes often cause the material to warp - something that is unacceptable for a precision item like a display substrate (figure 5). High purity prevents optical birefringence Apart from dimensional stability, the surface quality of the substrate film - as measured by its roughness - is of major importance. The film, which is made by a special process, can only meet the required roughness of less than 10 nanometres if it is extruded from a resin of sufficient purity, free of gel particles and other contaminants. BASF's special Ultrason E grades meet such purity requirements. Extrusion trials have shown that the factor limiting the achievable film roughness happens to be the roughness of the chill rolls. Due to the resin's high purity, the roll pressure can be made small enough so that no mechanical stress will be introduced into the film, thus minimizing optical birefringence (figure 6). 6

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9 Commercialization by BASF's South-Korean partner BASF's South-Korean project partner i-components Co. Ltd., the company that manufactures and markets the substrate film, has overcome the remaining two hurdles to the use of PES film in flexible LCDs by developing a coating that gives the substrate the necessary scratch resistance and impermeability to water vapour and oxygen (figure 7). i-components is currently working to improve the film's barrier properties further to enable its use in the fast-growing market for organic light emitting diodes (OLED). 8