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June 2014 • The PCB Magazine 19 comes an internal layer, which is only exposed to the environment when the sacrificial film layer is removed, ideally at the point of device manufacture. The sacrificial layer has the add- ed benefit of protecting the surface of the film from contact damage during film handling and transport. The impact of the sacrificial coextruded lay- er on subsequent processing has been studied by the TNO-Holst Centre within a European funded FP7 program CLEAN4YIELD. A version of the calcium test was used to study and com- pare the decay of the calcium on a barrier film utilizing a regular film surface, and on a film sur- face after the sacrificial layer has been removed. Although multilayers are normally employed, for demonstration purposes a proprietary single layer inorganic barrier material was deposited on the surface of the film. Qualitatively, Figure 4 show that relative to the Regular Melinex ® control, the calcium decays (test squares be- come lighter) at a much slower rate with the On Demand Clean Melinex ® film indicating the impact of the sacrificial protect film in preserv- ing the polyester film's surface quality. A further advantage is that the sacrificial co- extruded layer protects the PET surface from de- fects that may arise by contamination build up at the die lip edge and/or when the film passes over rollers during the manufacturing or slitting processes. Molten polymer flowing over any de- bris build up at the die lip edge can lead to lines of scratches with high-aspect ratio. These are absorbed within the sacrificial coextruded layer leaving the PET film surface undamaged. 2. Low-Bloom Film PET film contains ca 1.4 weight % cyclic oligomer and a portion of these oligomers can migrate to the surface if the film is held at el- evated temperatures for tens of minutes, as can be seen in Figure 5. This "blooming" effect gives rise to haze. PEN with 0.3% cyclic content has significantly lower cyclic oligomer content compared with PET, and there is significantly less "bloom" associated with PEN film. Planarizing coatings or hard coats act as a barrier to cyclic oligomer migration and of- fer one strategy to reduce the haze associated with this blooming effect. Careful control of the filming process offers another strategy to a "low bloom" film. These low bloom films typi- cally have a haze less than or equal to 1% af- ter aging at 150 ° C for 30 minutes. As a further refinement on the low bloom films, refractive index (RI) matching coatings are being devel- oped, which reduce optical iridescence. The undesirable "rainbow" effect can be observed following the deposition of hard coat layers commonly used to manufacture transparent conductor films for touch panel and appliance applications. Therefore, end users are seeking clear film with both a RI match and a low sur - face haze after processing. 3. Light Outcoupling Films As an alternative to the familiar microlens array technology, a range of volume scattering films have been developed which will outcou- ple light trapped in the glass structure when ap- plied as an overlaminate to a rigid OLED. This SUBSTRATES: PoLyESTER FILM FoR THE FLExIBLE ELECTRoNICS INDUSTRy continues Figure 4: calcium test pieces aged at 20 ° c and 50%rH.