HMDSO-Based Thin Plasma Polymers as Corrosion Barrier Against NaOH Solution Montgomery Jaritz , Christian Hopmann, Stefan Wilski, Lara Kleines, Marcel Rudolph, Peter Awakowicz, and Rainer Dahlmann (Submitted November 21, 2019; in revised form March 15, 2020) HMDSO-based films with excellent corrosion barrier properties against strong alkaline solutions were deposited on chemically non-resistant SiO x barrier coatings, which were previously applied on polished gold-coated Si-Wafers and PET films for coating analysis. The plasma process parameters are seen to have a strong influence on the achievable corrosion barrier properties of the plasma polymers. Coatings, which were applied in a pulsed microwave plasma with low mean power input, exhibit a substantially higher resistance against NaOH aqueous solution in electrochemical tests than those applied in higher energy plasmas. An analysis of the coatings revealed that the great difference in chemical resistivity of the investigated coatings can be explained by their chemical composition as well as their nano-porosity and surface topography. XPS measurements indicate that a higher organic content in the films contributes to their chemical resistivity. FTIR measurements showed that an ordered Si-O-Si network with methyl groups, which promote steric shielding, lead to superior corrosion resistance. Furthermore, a correlation of pro- tective performance and nano-porosity was found in cyclic voltammetry measurements. Coatings with good corrosion protection proved to be initially pore free and even after 30 min of exposure to NaOH, an open pore surface of only 2% can be measured. Finally, measurements of the oxygen transmission rate (OTR) of coated PET substrates showed that the barrier of a coating system comprising a non-resistant barrier layer and a protective top coat can withstand up to at least 90 min of exposure to hot NaOH solution without significant loss in barrier performance. After this, the barrier of the system is gradually reduced. To slow down this reduction process, a multilayer approach proved to be effective. Keywords coatings, coatings and paints, Corrosion and wear, hexamethyldisiloxan (HMDSO), organic, plasma-enhanced chemical vapor deposition (PE-CVD), porosity 1. Introduction Brazil, Mexico, Argentina, Uruguay, Panama, Chile, South Africa, Sweden, Denmark, Portugal and Germany use refillable PET bottles. It has to be distinguished between single use PET bottles and refillable PET bottles. Post-consumer single-use PET bottles are shredded into PET flakes and then recycled in a bottle to bottle process. Refillable PET bottles are purified with NaOH after usage and then refilled. Under certain conditions is the washing and refilling of PET bottles after usage the most environmentally friendly solution, causing lower emissions of the greenhouse gas CO 2 in comparison with glass and single- use PET bottles. It is industrial standard to coat single-use PET bottles on the inside with thin plasma polymerized SiO x gas barrier coatings to ensure the necessary protection and shelf-life of oxygen-sensitive beverages. For refillable PET bottles, however, this is to date no applicable solution. The widely used SiO x barrier coatings are not chemically stable against NaOH solution and are washed away within seconds in the refilling washing process of the bottles. Furthermore, the PET itself is corroded by NaOH, which has a limiting effect on the possible number of reusing cycles of the bottles. A protective top coat, which is applied in the same processing step as the barrier coatings, would therefore offer the opportunity to also store oxygen-sensitive products in refillable PET bottles and prolong the bottles lifecycle. The usage of environmentally friendly refillable PET bottles would be made more attractive, which could promote the adaption of this environmentally friendly system in more countries. Plasma-polymerized barrier coatings are most commonly applied using either hexamethyldisilazan (HMDSN), hexam- ethyldisiloxan (HMDSO) or ethin (C 2 H 2 ) as monomer. If the very same base monomers could be used for the application of a top coat to protect the barrier coatings and the PET against NaOH in the same coating process, production time and cost would be kept at a minimum. Several research groups have shown that materials can be protected against corrosion by applying protective coatings with PECVD (Ref 1-7). However, most of these works focus on neutrally or subalkaline corrosive media or metallic substrates. Hydrocarbon coatings have shown a particularly high potential for good corrosion protection properties in several works (Ref 3, 7-11). However, these films are usually brown in color. Coatings produced by HMDSO and HMDSN are transparent, which is favorable for most food and especially beverage bottle application. Furthermore, HMDSO is in contrast to HMDSN a non-hazardous and inexpensive monomer. This paper therefore focuses on the influence of strong alkaline solutions (NaOH, Montgomery Jaritz, Christian Hopmann, Stefan Wilski, Lara Kleines, and Rainer Dahlmann, Institute of Plastics Processing (IKV) at RWTH Aachen University, Seffenter Weg 201, 52074 Aachen, Germany; and Marcel Rudolph and Peter Awakowicz, Electrical Engineering and Plasma Technology, Ruhr-University Bochum, Universita ¨tsstraße 150, 44780 Bochum, Germany. Contact e-mail: montgomery.jaritz@ikv.rwth-aachen.de. JMEPEG ÓASM International https://doi.org/10.1007/s11665-020-04821-x 1059-9495/$19.00 Journal of Materials Engineering and Performance