Roll-to-roll thin film coating on fluoropolymer webs - Status, challenges and applications
Thin film deposition on fluoropolymer webs - both in vacuum and at atmospheric pressure - faces several critical challenges such as 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® layers are deposited on ETFE, PVDF and ECTFE webs to form single- and multi-layer permeation barrier systems. The influence of relevant process parameters on the layer adhesion and permeation barrier performance is reviewed. Permeation barrier coatings have been selected as main application for this study, because they are very sensitive to substrate surface irregularities and mechanical damage due to low adhesion, high strain or roll-to-roll processing issues and therefore are a good measure for the quality of the coating on the substrate. Both the wet coated ORMOCER® layers and reactively sputtered zinc-tin oxide (ZTO) layers show surprisingly good adhesion on both ETFE and PVDF surfaces. Both ZTO and - with adhesion promotion treatment - also aluminum oxide (Al2O3) layers have the potential for low water transmission rates (WVTR) below 5·10− 2 g/(m2 d) at 38 °C/90% r.h. on these substrates. Further reduction of the WVTR is demonstrated with a combination of wet coated ORMOCER® layers and sputtered ZTO layers yielding a WVTR of 1·10− 3 g/(m2 d) at 38 °C/90% r.h. These values were achieved using adapted process parameters and layer stack designs such as reduced web tension and lower layer thicknesses. The results shown in this paper demonstrate the potential of coated fluoropolymer webs for functionalization of membrane roofs and façades with flexible thin film solar cells; thermal insulation and solar control functionality; flexible thin film electroluminescent and OLED lighting panels as well as electrochromic devices.