Roll-to-roll thin film coating on fluoropolymer webs - status, challenges and applications

Fluoropolymer webs and membranes commonly exhibit superior optical properties such as high transmittance over a broad wavelength range and very good outdoor stability. Therefore, fluoropolymer films are used in architecture, for example, in membrane roofs and facades in stadiums, shopping malls, and airports or as front-side encapsulation for solar cells. However, thin film deposition on fluoropolymer webs – both in vacuum and at atmospheric pressure – face several critical challenges including poor mechanical and thermo-mechanical properties, especially low dimensional stability and low elastic modulus, high and textured surface roughness, and low adhesion of thin films. This paper discusses critical process parameters in roll-to-roll processing for both vacuum processes and wet coating processes with respect to the unique properties of fluoropolymer webs. Reactively sputtered oxide layers as well as wet coated ORMOCER® (a trademark of the Fraunhofer Institute) layers were deposited on ethylene tetrafluoroethylene (ETFE), polyvinylidene difluoride (PVDF), and ethylene chlorotrifluoroethylene (ECTFE) webs form single- and multilayer permeation barrier systems. Layer adhesion and permeation barrier performance was reviewed in relation to the relevant process parameters. Permeation barrier coatings were selected as lead application for this study. Barrier coatings are sensitive to substrate surface irregularities and mechanical damage due to low adhesion, high strain, or roll-to-roll processing issues and therefore were a Germanygood measure for the quality of the coating on the substrate. Reactively sputtered zinc-tin oxide (ZTO) layers as well as wet coated ORMOCER® layers showed surprisingly good adhesion on both ETFE and PVDF surfaces. Both ZTO and – with adhesion promotion treatment – aluminum oxide (Al2O3) layers have the potential for low water transmission rates (WVTR) below 5 × 10-2 g/(m²d) at 38°C/90 % RH. on these substrates. Further reduction of the WVTR using multilayer stacks was demonstrated with a combination of wet coated ORMOCER layers and sputtered ZTO layers yielding a WVTR of 1 × 10-3 g/(m²d) at 38°C/90 % RH. To achieve that, adapted process parameters and layer stack designs such as reduced web tension and lower layer thicknesses were needed. The potential of the coatings for specific outdoor application is discussed as well as possible ways to improve the polymer web itself to gain a larger process window for roll-to-roll coating on fluoropolymer webs. Demonstrating successful coating on ETFE and PVDF, the results reviewed in this paper potentially enable the use of coated fluoropolymer webs for functionalization of membrane roofs and facades with flexible thin film solar cells, thermal insulation and solar control functionality, flexible thin film electroluminescence, and organic light-emitting diode (OLED) lighting panels as well as electrochromic devices