New Developments In Polyester Film For Flexible Electronics
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New Developments In Polyester Film For Flexible Electronics Bill A MacDonald , Keith Rollins, Robert Eveson, Karl Rakos, Bob A Rustin, Makoto Handa DuPont Teijin Films, PO Box 2002, Wilton, Middlesbrough, UK, TS90 8JF ABSTRACT Polyester films are well-known substrates for a wide range of electronic applications. This contribution will describe new developments in polyester film substrates for OLED displays. INTRODUCTION Polyester films are well-known substrates for a wide range of electronic applications such as membrane touch switches and flexible circuitry [1]. Their use in electronics continues to grow with e.g. ITO-sputtered polyethylene terephthalate (PET) films (Melinex® brand) being successfully used for electroluminescent lamp and touch screen applications due to the versatile properties and high quality of the available base films. The current interest in the general area of “plastic electronics” based on conjugated polymers opens up new opportunities for combining the advantages of a flexible substrate with roll to roll processing. OLED based displays are currently based on glass substrates due to the need for a very smooth surface with good dimensional stability and high barrier properties. A plastic substrate alternative however is being sought that can be processed cost effectively in high volumes using roll to roll techniques. This contribution will describe new developments in polyester film substrates for this application area. In particular the development of polyethylene naphthalate (PEN) films (Teonex® brand) which offer a unique combination of excellent dimensional stability, low moisture pickup, good solvent resistance, high clarity and very good surface smoothness will be discussed. This combination makes PEN a promising substrate for subsequent vacuum and other coating processes, leading to the potential use of this material as a flexible substrate for device manufacture INTRODUCTION TO PEN FILMS
O
O
C
C
PET Tm 255C Tg 78C O
CH2CH2
O
n
O O
C
O
CH2CH2
PEN Tm 263C Tg 120 C
O
C
n
Figure 1 The Structure of PET and PEN
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Biaxially drawn polyester film based on PET (figure 1) was developed by ICI in Europe and DuPont in the USA in the 1950’s with DuPont introducing the first commercial film line in the late 1950’s [2].The first patent covering PEN (fig 1) was filed in 1948 [3], not long after the discovery of PET [4] but it was not until the 1990’s that PEN films were launched commercially. Sales of PEN film are currently of the order of several thousand tonnes and DuPont Teijin Films is the leading producer with its Teonex® brand range of films. Given PET films regular use in the electronics industry it is perhaps interesting to first contrast the advantages that PEN films offers over PET films. The difference in chemical structure between PET and PEN is shown in Figure 1. The substitution of the phenyl ring of PET by the naphthalene double ring of PEN has very little effect on the melting point (Tm), which increases by only a few degrees Centigrade. However, there
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