Long-term water-repellent metal surfaces may be one step closer
Superhydrophobic coatings have found widespread application in the maritime sector – from protecting ship hulls from ice and corrosion, to reducing friction, or keeping windscreens clear. But because these coatings owe their water-repellency to micro- or nanoscale features, they can be easily damaged and need to be frequently reapplied. In the past five years, there’s been a growing interest in ‘self-healing’ materials that could act as robust, long-term superhydrophobic coatings for a range of surfaces.
The latest stage in this effort comes from a team at Suzhou University in China, and appears in Materials Letters [DOI: 10.1016/j.matlet.2018.07.040]. They focused on the environmental question, ultimately developing a coating that uses water, rather than volatile solvents, as a dispersion medium. The researchers started with silica nanoparticles, which were then chemically modified with a series of polymers, including polysiloxane latex and POSS-V8 (octavinyloctasilsesquioxane). After being dispersed in varying quantities of in deionized water, the now superhydrophobic emulsions could be sprayed onto substrates of interest, and left to dry at 80°C.
The coating successfully adhered to aluminium, cotton, glass and wood, and once the modified silica nanoparticle content exceeded 20 wt%, the coating was shown to be superhydrophobic. The optimal composition was 40 wt%, at which point, water contact angles of ~155° were achieved. The authors also measured the contact angle of other liquids, including milk, coffee and soy sauce, and found that the surfaces remained dry.
They applied mechanical load to the coated substrates, pressing down on them with their fingers, but the samples retained their superhydrophobicity. The samples also showed good self-cleaning ability, easily shedding dust and abrasive sand from their surfaces, despite repeated applications. Each sample was exposed to UV light for four hours, and had acids and alkali solutions applied to them – in all cases, there were no measurable changes in contact angle. However, oxygen plasma treatment did have an impact, as did immersion in water. In both cases, heating the sample repaired it, and allowed it to fully recover its water-repellent properties.
Though currently restricted to the lab, at lab-scale, the authors claim that their production processes meet the requirements of industrial coating producers, and that their “waterborne polymer dispersions…can be produced in high tonnage.”
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Run Zhao, Yu Chen, Guozhen Liu, Yucheng Jiang, Kunlin Chen. “Fabrication of self-healing waterbased superhydrophobic coatings from POSS modified silica nanoparticles” Materials Letters 229 (2018) 281-285. DOI: 10.1016/j.matlet.2018.07.040