Fabrication of a bulk superhydrophobic conductive material by mechanical abrasion
Introduction
It has been centuries since people found that lotus leaves have self-cleaning properties, but it was not until the 1990s that Neinhuis and Barthlott revealed that epicuticular wax crystalloids with microscale and nanoscale roughness cause those properties [1,2]. Superhydrophobic materials have potential applications in anti-icing [3], self-cleaning [4,5], water-oil separation, etc. [[6], [7], [8], [9]], but their weak resistance to mechanical abrasion is one of the barriers that prevents their daily use. To achieve a high degree of superhydrophobicity requires thin microscale “pillars” and a large space between these pillars; however, good mechanical abrasion resistance requires just the opposite [10]. Recently, researchers have developed many methods to improve the mechanical stability of superhydrophobic materials. An effective method is to enhance the adhesion strength between micro-nano dual-sized particles and their substrate with adhesives [[11], [12], [13]]. The durability of these superhydrophobic coatings depends on how powerful the adhesives are, and the diversity of adhesives makes it possible to apply superhydrophobic coatings on many substrates [14,15]. However, the superhydrophobicity is permanently lost when the mechanical abrasion exceeds the adhesion strength of the adhesives. Self-healing is another strategy to extend the life of superhydrophobic coatings. Usually, low-surface-energy materials are one of the constituents of these superhydrophobic coatings [[16], [17], [18]]. If damaged, low-surface-energy materials migrate to surfaces to restore superhydrophobicity through heating [19], ultraviolet radiation [20], or other process [21,22]. For example, Chen et al. prepared a multifunctional self-healing superhydrophobic coating by simply spraying a mixture of TiO2 particles, graphite, poly[methyl(2,2,3,3,4,4,4-heptafluorobutyl)siloxane] (PMSF) and methyltris(methylethylketoxime)silane on substrates. PMSF was the healing agent here. Upon heating the damaged surface, some long chains that contained -CF2- and -CF3 groups migrated to the surface to repair the surface. This process was driven by the minimization of surface free energy. The coating could also restore superhydrophobicity after organic contaminants. TiO2 could photocatalytically degrade organics under UV irradiation, but fluorinated groups were stable [23]. Superhydrophobic coatings have an advantage and can be conveniently applied to various substrates. However, when the top layer of the coatings wears off, superhydrophobicity is lost because most exposed substrates are hydrophilic [24,25]. Superhydrophobic coatings are also not very environmentally friendly because they require the use of volatile solvents or fluorochemicals such as perfluorooctyltriethoxysilane, perfluorodecyltriethoxysilane, or 1H,1H,2H,2H-perfluorodecyltrichlorosilane [16,[26], [27], [28], [29]]. Bulk superhydrophobic materials could resist mechanical abrasion. Compared with coatings, bulk superhydrophobic materials can maintain superhydrophobicity even if the surface layer is damaged. Usually, a new superhydrophobic surface can be generated by some method to maintain its water repellence. A superhydrophobic CNTs–PTFE bulk material was fabricated by pressing a mixture of CNTs and PTFE in a mould at 390 °C under a pressure of 256 kPa for 30 min. The bulk material retained its superhydrophobicity after cutting or 20 abrasion cycles [6]. Another bulk superhydrophobic material, prepared by pressing SiO2/TiO2/PP composite powder in a mould, had high water repellence throughout its whole volume [30]. For bulk hydrophobic material, the primary problem to be solved is how to obtain and keep surface roughness. If mechanical abrasion could create appropriate roughness on the surface of hydrophobic materials, then mechanical abrasion would no longer reduce their superhydrophobicity. Abrasive paper often plays a role as a mechanical abrasion durability tester [[31], [32], [33]] its use could also be a simple approach to induce roughness to material surfaces. Simply sanding PTFE could produce superhydrophobic surfaces with WCA of 151° [34]. PVDF/PTFE or PTFE/RTVSR (room temperature vulcanized silicone rubber) composites obtained superhydrophobicity after abrading with sand paper, which created irregular, micrometre-scale structures on the surface [35,36]. Sanding is not a fine method; abrading a single hydrophobic polymer matrix or a mixture of two hydrophobic polymer matrices usually induces only micro-scale roughness rather than hierarchical structure, leading to relatively lower water repellence. The addition of hard nano-size particles may help to induce nano-scale roughness.
Herein, we report a facile approach to fabricate superhydrophobic, conductive Ketjen black-vapour-grown carbon fibre/polypropylene (KB-VGCF/PP) bulk materials by pressing mixed KB, VGCF, and PP in a mould and then processing the surface with abrasive paper. Because of the different mechanical resistances of KB, VGCF and PP, both micro-scale and nano-scale surface roughness was created during the process. The water contact angle (WCA) of materials was 167.5 ± 0.9°, and the sliding angle (SA) was approximately 1°. The influence of filler content and the ratio of fillers is investigated. The materials were also highly conductive, with a volume resistivity lower than 1 Ω cm, which could be applied to anti-icing.
Section snippets
Materials
Ketjen black EC-300J (KB) was obtained from Akzo Nobel. Vapour-grown carbon fibre (VGCF, 15 μm in length) was supplied by Rijiang Company, Taiwan, China. Polypropylene (PP) 1320X was supplied by BASF SE, Germany. KB and VGCF were used as received, and PP was dried at 100 °C in an oven for 2 h before use. Abrasive paper P80-P2000 was purchased from Starcke GmbH & Co. KG, Germany. Copper mesh, conducting wire, and batteries were commercial products.
Preparation
Superhydrophobic material: PP and fillers (Table
Results and discussion
Fig. 2a shows water repellence of the superhydrophobic bulk material surface. A water droplet maintained a spheroidal shape on the surface, and a silver interface could be observed due to trapped air between water and the bulk material surface. This mirror-like phenomenon was more obvious when putting the material into water (Fig. 2b); the reflection of the tweezers was clearly presented on the interface. The WCA of the bulk material surface (KB12-VGCF3) was 167.5 ± 0.9° (Fig. 2c), and water
Conclusions
We have demonstrated a bulk superhydrophobic material fabricated by a simple melting-mixture method. Mechanical abrasion created microscale and nanoscale roughness on the surface of hydrophobic PP because of the different resistances to mechanical abrasion of fillers and PP, and therefore, the material maintained stable superhydrophobicity under various amounts of mechanical abrasion. KB and VGCF exhibited a synergistic effect in creating nanoscale roughness. The best WCA was 167.5°, and the
Acknowledgements
This work is supported by the Lishui Science and Technology Program of China (grant number 2016ZDYF17).
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