Durable Low-Energy Fluorinated Silicate Surfaces: Influence of Molecular Structure

A.J. Guenthner, P.H. Boyd, T.W. Rost, S. Inceoglu, T.S. Haddad, M.D. Ford, J.T. Reams, J.R. Aston, J.R. Lince, J.M. Mabry
Nano Hydrophobics, Inc.,
United States

Keywords: fluoro-POSS, silicates, heat exchangers

Summary:

Recent work on surface coatings containing fluorinated silicates has demonstrated exceptionally useful liquid repellency, achievable through low-cost processing. For industrial applications, these surfaces must be durable for extended periods in harsh conditions. Factors that affect the durability of these surfaces in harsh environments (for example, elevated temperatures for extended periods) have not received significant attention. We demonstrate that the fugacity (or escaping tendency), which manifests as evaporation in air, or leaching into a liquid, is an important consideration for the durability of these compounds at elevated temperatures, despite molecular weights in excess of 1000 g/mol. For comparison, two chemically similar fluorinated silicates, Bis(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyldimethyl)ether disiloxane, or (F-decyl-M2), and octakis(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-hetpadecafluorodecyl) polyhedral oligomeric silsesquioxane (F-decyl-T8, or F-decyl-POSS), were synthesized at the Air Force Research Laboratory. F-decyl-M2 is a highly soluble liquid with a surface tension of 18-20 mJ/m2, whereas F-decyl-T8 is a highly crystalline solid, with a surface energy of 8-10 mJ/m2. These were dissolved along with poly(methyl methacrylate) (PMMA) in a partially fluorinated hydrocarbon solvent (typically Asahiklin AK225G), at ratios of 1 part silicate : 1 - 4 parts PMMA by weight, with solids content 10-30 mg/mL. Spin cast films a few hundred nm in thickness were annealed at 80-110 °C up to 4 hours, then examined by X-ray photoelectron spectroscopy, Auger spectroscopy, electron microscopy, optical profilometry, thermogravimetric analysis, and contact angle measurement. Interestingly, the F-decyl-M2 and the F-decyl-T8 showed dramatically different behavior. Wheres on annealing for 4 hrs, the F-decyl-T8 crystallized into regions rich in fluorine (as evidenced by Auger element mapping), the F-decyl-M2 surface showed no fluorine content, either by Auger or SEM/EDX mapping, indicating that fluorine had disappeared from the bulk of the film, at 80 °C and 110 °C. High-magnification SEM confirmed that F-decyl-M2 did phase separate from the bulk PMMA into small domains, which subsequently became devoid of material. Thermogravimetric analysis confirmed that the F-decyl-M2 exhibited an evaporation rate at least two orders of magnitude larger than the F-decyl-T8 over 80 – 150 °C, despite a molecular weight difference of only a factor of four. The non-crystalline, weakly interacting nature of the F-decyl-M2 facilitates evaporation, whereas the highly crystalline nature of the F-decyl-T8 likely suppresses evaporation. These results indicate a fundamental difference in the fugacity of these compounds at elevated temperatures. As described in a recently issued patent, fluorinated POSS coatings can reduce mineral scale deposits on heat exchanger plates that operate at elevated temperatures. A recent 30 day test with Lawrence Berkeley Laboratory cooling water at temperatures as high as 55 °C showed that plates coated with F-decyl-T8 in PMMA showed considerably less scaling compared to uncoated plates tested side-by-side, demonstrating that F-decyl-T8 will survive industrial conditions long enough to perform a useful function.