Preparation and characterization of halloysite nanotube-based superhydrophobic colored coatings

In this study, the preparation of superhydrophobic colored surfaces, which are required for various industrial applications, with halloysite nanotube-based coating formulations was investigated. For this purpose, different coating formulations were formed using activated halloysite nanotube loaded with Methylene Blue (A-HNT/MM) and various silane compounds such as tetraethoxysilane (TEOS) and polydimethylsiloxane (PDMS), 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES), methyltriethoxysilane (MTES), octyltriethoxysilane (OTES), and hexadecyltrimethoxysilane (HDTMS) through the sol-gel reaction and these formulations were coated onto glass surfaces using the spray coating technique. For the glass surface coated with A-HNT/MM/PFOTES material, optimum TEOS:PFOTES (mol:mol) amount, optimum sol-gel reaction time and optimum curing time were determined to be 5:1, 60 min and 120 min, respectively. Characterization studies were carried out for the fabricated colored glass surfaces having water repellent property, chemical and environmental stability and self-cleaning property and for the prepared coating formulations. The effect of the functional alkyl chain length of the silane compounds used in the coating formulation and also the effect of fluorine-containing and fluorine-free silane compounds with the same chain length on the surface properties of colored surfaces were ascertained. Characterization studies of A-HNT/MM/silane composites or coated glass surfaces were performed by transmission electron microscope (TEM), scanning electron microscope (SEM), mapping, energy dispersive X-ray spectroscopy (EDS), atomic force microscope (AFM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) measurements. The highest water contact angle of the glass surface colored with MM was measured as 163.0±2.1° for the surface coated with A-HNT/MM/PFOTES. It was observed that the water repellency of surface improved as the functional alkyl chain length of silane compounds with different chain lengths used in coating formulation was increased from methyl- group to octyl- and hexadecyl- groups, but the mechanical strength of colored surface weakened. The results of stability studies showed that the polydimethylsiloxane (PDMS) layer applied to colored surfaces improved the mechanical, thermal, chemical and environmental durability of the coating, but reduced the water repellency of coated colored glass surfaces. High water contact angle (>150°) values were measured on various substrates (glass, filter paper, wood and fabric) surfaces coated with A-HNT/MM/HDTMS, A-HNT/MY/HDTMS and A-HNT/BF/HDTMS composite materials having different colors prepared using Methylene Blue, Malachite Green (MY) and Basic Fuchsin (BF) and modified with HDTMS and this result displayed that A-HNT/MM/silane composite coating materials were easily applicable for the different substrates to produce superhydrophobic materials in different colors. It is thought that the A-HNT/dye/silane composite coating materials prepared here will have a great potential in the design of functional surface coatings used in different industries.