Development of PFAS-Free Hydrophobic Coatings Using Plasma-Enhanced Chemical Vapor Deposition Methods

Per- and polyfluoroalkyl substances (PFAS) have recently come under scrutiny due to their resistance to decomposition in the environment and potential adverse effect on humans and animals. PFAS are known as "forever chemicals" and have been the focus of government environmental agencies around the world recommending restrictions in their production and usage. If the proposed ban takes full effect, it would prohibit the manufacture, use, or placement of PFAS chemicals on their own or within other substances. The restriction of PFAS-based chemical substances will impact the performance of a plethora of coatings produced and used by various industries. For example, these coatings provide bio-inertness, and resistance to aggressive sterilization processes applied on implantable medical devices, catheters, and contact lenses. PFAS coatings are also used in aircraft components, fire-resistant materials, while automotive components such as gaskets, seals, and lubricants that require resistance to heat, chemicals, and wear, also benefit from the chemical resistance of perfluorinated compounds. Several other industries such as construction, textile and electronics have a need for water-repellant, stain-resistant, robust barrier coatings that have been traditionally produced using PFAS polymers. The numerous applications requiring hydrophobic coatings have accelerated research and development efforts in the coatings field, both at the academic and industrial level over the past 4 years. Plasma-enhanced chemical vapor deposition is a method widely used to deposit thin functional coatings on a variety of substrate materials such as polymers, ceramics and metals. The method is preferred over other traditional technologies, such as coatings derived from chemical solutions, as it is solvent-free, does not require special post-coating processing such as drying, and utilizes low volumes of precursor solutions. Recently, with the advancement of atmospheric pressure plasma jet (APPJ) technology, thin coatings are produced in an ambient air environment and do not require the use of vacuum chambers. The talk will present the results from recent studies in the development of water-repellant, chemical resistant coatings using plasmas. A siloxane-based plasma chemistry, free of fluorine, was employed to deposit coatings of thickness in the range of 50nm - 2µm on plastic surfaces such as polyimide, acrylonitrile butadiene styrene (ABS), polyethylene terephthalate (PET) and metals, like stainless steel and nickel. Results from water contact angle (WCA) goniometry testing showed that the coatings were very hydrophobic, with WCAs in the range of 90º to 130º. The coatings retained their hydrophobicity after being subjected to high temperature and UV light exposure for 30 days. Detailed materials analysis showed that the key to the enhanced hydrophobicity was the control of the chemical composition of the coatings through the control of the plasma parameters such as the liquid precursor and gas flow rates, the power applied to the plasma, etc. Improved water repellency was achieved through the tuning of the atomic concentration ratios of oxygen and carbon, as confirmed by X-ray photoelectron spectroscopy (XPS). Along with the discussion of the coating analysis results, a few examples from the industrial application of the plasma deposited hydrophobic coatings will be presented.