Superhydrophobic surfaces with silane-treated diatomaceous earth/resin systems Helanka J. Perera, 1 Bal K. Khatiwada, 1,2 Abhijit Paul, 1,3 Hamid Mortazavian, 1 Frank D. Blum 1 1 Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma, 74078 2 Department of Chemistry, University of the Ozarks, Clarksville, Arkansas, 72830 3 Polymer Science and Engineering Department, Conte Center for Polymer Research, 120 Governors Drive, University of Massachusetts, Amherst, Massachusetts, 01003 Correspondence to: F. D. Blum (E - mail: fblum@okstate.edu) ABSTRACT: Superhydrophobic coatings were prepared using fluorosilane-treated diatomaceous earth (DE) with either polyurethane or epoxy binders. The surface wettability and morphology of the films were analyzed using contact angle measurements and scanning electron microscopy (SEM), respectively. The water contact angles were studied as a function of the fluorocarbon fraction on DE and the particle loadings of treated DE in the coating. The contact angles exceeded 1508 for coatings with at least 0.02 fluorocarbon frac- tion (mass of fluorosilane/mass of particle) on the DE and with 0.2 particle loadings (mass of treated particles/mass of coating). The water contact angles of the surfaces were dependent on the nature of the binder below 0.2 particle loadings of the superhydrophobic DE particles, but were independent of the binder type after attaining superhydrophobicity. The results were consistent with the super- hydrophobicity resulting from the migration of the superhydrophobic DE moving to and covering the surfaces completely. It was also shown that the treatment with fluorosilanes restricted the pores in DE and reduces the specific surface area of the material. However, these changes had effectively no effect on the superhydrophobicity of the coatings. The results of this work clearly identify some important considerations relative to producing superhydrophobic coatings from inexpensive diatomaceous earth. V C 2016 Wiley Periodi- cals, Inc. J. Appl. Polym. Sci. 2016, 133, 44072. KEYWORDS: coatings; epoxy; resins; polyurethanes; surfaces and interfaces Received 12 March 2016; accepted 9 June 2016 DOI: 10.1002/app.44072 INTRODUCTION Diatoms are unicellular algae of the class of Bacillariophyceae of Phylum Bacilloriophyta. 1 Diatoms extract silicon from water for the production of their exoskeletons, called frustules or hydrated silica shells. 2,3 When diatoms cells die, their tiny shells sink, and with time, these shells form layers of fossil deposits. These fos- silized deposits are known as diatomaceous earth (DE) or kie- selgur. 3,4 DE particle sizes can vary between 1 mm and several mm in diameter. 5,6 There are more than 100,000 different spe- cies with unique three-dimensional frameworks. 7 Each three- dimensional DE structure contains millions of microscopic, hol- low, perforated cylindrical, and disk shaped shells. The resulting DE is an inert, highly porous, lightweight, and thermally resis- tant material. 5,8 Naturally occurring DE is hydrophilic; conse- quently, it can be used in applications as adsorbents, 4,9 in filtration, 10–14 and in construction materials as a filler. 15 Chemi- cally modified, DE has been used in additional applications, such as materials for superhydrophobic coatings, 6,16–18 metal adsorbents, 4 and drug delivery. 19–22 Surfaces that form static water contact angles greater than 1508 and have sliding angles less than 108 are defined as superhydro- phobic surfaces. 23–27 The superhydrophobicity of a solid surface is determined by two factors: its chemical composition and micro-nano hierarchical texture. 23–25,28 Modifying a surface with low energy chemical groups can effectively increase the water contact angle of a solid surface. Surfaces with CF 2 and CF 3 groups generally have low surface energies with contact angles of about 1208 on a flat surface. 29,30 Roughening the sur- face can result in contact angles as high as 160 to 1758, and the surfaces become non-wettable. 31,32 These superhydrophobic coatings are water-repellent, self-cleaning, and can be used in many applications, such as anti-icing, anti-oxidation, anti- fogging, non-wetting, buoyancy, and flow enhancement. 33–35 There are many ways to fabricate superhydrophobic surfaces; they include plasma etching, 36–38 graft-on-graft polymerization, 39 chemical vapor deposition, 40 lithography, 41 sol–gel processing, 42 and self-assembly of low surface energy materials. However, most of the methods used to fabricate superhydrophobic surfaces are V C 2016 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM J. APPL. POLYM. SCI. 2016, DOI: 10.1002/APP.44072 44072 (1 of 9)