Stable Superhydrophobic Coatings Research Articles

Stable superhydrophobic coatings using PVDF–MWCNT nanocomposite

Thermally stable superhydrophobic coatings have been prepared using polyvinylidene fluoride (PVDF)–multiwalled carbon nanotubes (MWCNTs) by spray coating method. The effects of MWCNT (0–66wt.%) and temperature (300–623K) on wettability have been studied. A transformation from hydrophobic to superhydrophoic state has been achieved with increase of CNT content. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) studies reveal that with increase in CNT content, α-phase of PVDF decreases suggesting that MWCNT has strong effect on the phase separation of PVDF. Field emission scanning electron microscopy (FESEM) studies show that the coatings have rough surface with porous structure. With increase in MWCNT content the protrusion like structures decrease that leads to micro/nano scales. The coatings are thermally stable up to 573K exhibiting superhydrophobicity and thereafter transformed to superhydrophilic state at 623K. Energy dispersive X-ray spectroscopy (EDXS) analysis shows the absence of fluorine after annealing at 623K suggesting decomposition of PVDF. X-ray photoelectron spectroscopy (XPS) of C1s and F1s core levels in as-deposited PVDF–MWCNT coating show the presence of CF2 related species. Concentration of fluorine drastically decreases after heat treatment of the coating at 623K. The main advantage of the present method is feasibility for application over large area and the coatings are stable up to 573K.

Highly transparent and thermally stable superhydrophobic coatings from the deposition of silica aerogels

Transparent superhydrophobic coatings, having high water contact angle (>160°) and low sliding angle (<5°), were prepared by using poly(methylhydrosiloxane) (PMHS) and tetraethoxysilane (TEOS) as precursors based on a simple sol–gel process. The influence of different mass ratios of PMHS to TEOS on the transparency and superhydrophobicity of resulting coatings was investigated herein to get the optimum performance coating. The structure, composition and morphology of optimum performance coating were characterized by various technologies including Fourier transform infrared (FT-IR) spectroscopy, thermal analysis, BET, 29Si CP MAS NMR, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The optimum performance coating exhibits superhydrophobicity (CA, 164.7° and SA, 2.7°), a high transparency (transmittance closes to 90%) and a good thermal stability (up to 400°C). Note that the optimum coating directly from sol–gel process exhibits poor moisture resistance. The low surface energy and high volume-fraction porosity structure are responsible for the superhydrophobicity, transparency and thermal stability of the as-prepared coating, while the poor moisture is attributed to the untreated hydroxyl groups on the surface of coating. The moisture resistance of coating can be improved by further treated by cetyltrimethoxylsilane (CTMS), and the coating could switch from superhydrophobic (164.7°) to superhydrophilic (0°) after heat-treating at 600°C.

Stable superhydrophobic coatings from thiol-ligand nanocrystals and their application in oil/water separation

Stable superhydrophobic coatings were successfully achieved from thiol-ligand nanocrystals. Nanocrystals included VIII and IB metals and oxide nanoparticles, such as Fe, Co, Ni, Cu and Ag. We present a simple and available method that facilitates the synthesis of superhydrophobic textiles and sponges, in which the interaction between the nanocrystals and thiol plays a significant role in the formation of a special wetting surface. Meanwhile, the superhydrophobic textiles could also be endowed with new functionalities. For example, textiles with Fe3O4 nanocoatings possess magnetic properties, and Ag nanocrystals provide an antibacterial effect. If perfluoroalkyl thiol was used to replace alkyl thiol, the as-modified surfaces became oleophobic from superoleophilic. The proposed strategy was fit for various nanoparticles from as-established methods, including the preparation in different polar solvents, and the usage of surfactants as capping agents. As-prepared superhydrophobic nanocoatings show good durability towards hot water, surfactant aqueous solutions, and ultrasonic treatment in nonpolar solvents. The superhydrophobic and superoleophilic nanocoatings were effectively used for application in oil/water separation.

Stable superhydrophobic composite coatings made from an aqueous dispersion of carbon nanotubes and a fluoropolymer

An easy method is reported for fabricating a stable superhydrophobic coating made from a composite comprising carbon nanotubes and a fluoroplastic such as polytetrafluoroethylene or perfluoroalkoxy resin (Teflon® PFA). The superhydrophobic coatings with hierarchical structure characteristics are fabricated by a spray coating technique using an environmentally friendly aqueous dispersion containing carbon nanotubes and perfluoroalkoxy resin. Coatings with a contact angle of 153.1 ± 2° and a sliding angle less than 5° are stable under much stress conditions, i.e., either soaking in concentrated hydrochloric acid and concentrated sulfuric acid, or attacking with extreme-pH or ionic-strength solutions. Both the contact angle and sliding angle can be tuned simply by changing the carbon nanotube loading. To further the understanding of the formation of superhydrophobicity, we investigated both the formation of the nano/microscale structures and their effects.

Electrostatic powder spraying process for the fabrication of stable superhydrophobic surfaces

Nano-sized Al2O3 particles were modified by heptadecafluorodecyl trimethoxysilane and 2,3-epoxy propoxy propyl trimethoxysilicane to make it both hydrophobic and reactive. The reactive nano-particles were mixed with polyester resin containing curing agents and electrostatic sprayed on stainless steel substrates to obtain stable superhydrophobic coatings after curing. The water contact angle (WCA) on the hybrid coating is influenced by the content of Al2O3 particles in the coating. As the Al2O3 concentration in the coating was increased from 0% to 8%, WCA increased from 68° to 165°. Surface topography of the coatings was examined using scanning electron microscopy (SEM). Nano-particles covered on the coating surface formed continuous film with greatly enhanced roughness, which was found to be responsible for the superhydrophobicity. The method is simple and cost effective and can be used for preparing self-cleaning superhydrophobic coating on large areas.

Fabrication of Stable Superhydrophobic Coatings with Furfural Acetone Resin/PTFE Composites

We describe a simple and inexpensive technique to fabricate superhydrophobic coatings with furfural acetone resin/poly (tetrafluoroethylene) (FAR/PTFE) composites. An interesting hierarchical structure with dispersed cavities and protuberances in microscale was achieved. The water contact angle (CA) of the superhydrophobic surface is 157° and the sliding angle is about 5°. The coatings can be applied to large substrates by spray coating with ease and possess high cohesional strength with the substrates. Meanwhile, the superhydrophobic coatings show long-term stability in pure water and acidic solutions with higher concentration.

UV and thermally stable superhydrophobic coatings from sol–gel processing

A method for the preparation of inorganic superhydrophobic silica coatings using sol–gel processing with tetramethoxysilane and isobutyltrimethoxysilane as precursors is described. Incorporation of isobutyltrimethoxysilane into silica layers resulted in the existence of hydrophobic isobutyl surface groups, thereby generating surface hydrophobicity. When combined with the surface roughness that resulted from sol–gel processing, a superhydrophobic surface was achieved. This surface showed improved UV and thermal stability compared to superhydrophobic surfaces generated from polybutadiene by plasma etching. Under prolonged UV tests (ASTM D 4329), these surfaces gradually lost superhydrophobic character. However, when the as-prepared superhydrophobic surface was treated at 500 °C to remove the organic moieties and covered with a fluoroalkyl layer by a perfluorooctylsilane treatment, the surface regained superhydrophobicity. The UV and thermal stability of these surfaces was maintained upon exposure to temperatures up to 400 °C and UV testing times of 5500 h. Contact angles remained >160° with contact angle hysteresis ∼2°.

Superhydrophobic CFxCoating via In-Line Atmospheric RF Plasma of He−CF4−H2

Stable superhydrophobic coatings on various substrates are attained with an in-line atmospheric rf plasma process using CF4, H2, and He. The coating layer is composed of CFx nanoparticulates and has an average roughness of approximately 10 nm. This roughness is much smaller than other surfaces reported for superhydrophobicity in the literature. The superhydrophobic coatings are produced on both metallic and insulating substrates without any need of separate microroughening or vacuum lines.

Stable Superhydrophobic Coatings from Polyelectrolyte Multilayers

The present study demonstrates that the superhydrophobic behavior of the lotus leaf structure can be mimicked by creating a honeycomb-like polyelectrolyte multilayer surface overcoated with silica nanoparticles. Superhydrophobicity was achieved by coating this highly textured multilayer surface with a semifluorinated silane. The surface maintains its superhydrophobic character even after extended immersion in water. The key structural elements needed to create stable, superhydrophobic coatings from polyelectrolyte multilayers are discussed.