Wax-based artificial superhydrophobic surfaces and coatings

https://doi.org/10.1016/j.colsurfa.2020.125132Get rights and content

Highlights

  • Man-made superhydrophobic surfaces and coatings using hydrocarbon waxes comprehensively reviewed.

  • Wax-based composite coatings incorporating polymers, ceramic nanomaterials and carbon nanostructures described.

  • Reports on anti-corrosion, anti-fouling, paper and textile, food, oil separation, and biomedical applications debated.

  • Wax-based superoleophobic surfaces discussed.

Abstract

Inspired by the superhydrophobicity of the waxy layer on lotus leaves, researchers are in constant pursuit in fabricating advanced wax-based superhydrophobic surfaces. This article comprehensively reviews reported studies on human-made superhydrophobic surfaces and coatings where natural or synthetic hydrocarbon waxes are employed. Followed by a brief introduction on superhydrophobicity and waxes, available information on wax-incorporated superhydrophobic surfaces are systematically presented based on their application area, viz. anti-corrosion and anti-fouling, paper and textile, wood and plastic, food industry, oil separation, and others. Wax-based superoleophobic surfaces are also described. Both wax-only cases and wax-incorporated formulations along with polymers, ceramic nanomaterials and carbon nanostructures are covered. Future scopes of R & D presented.

Introduction

Biomimetic superhydrophobic (SHPC) surfaces have attracted significant recent research attention owing to their excellent surface properties. The universe is gifted with several SHPC plant surfaces that include lotus leaf, rice leaf, grasses, ginkgo trees, and aroids. Examples from the animal world comprise shark skin, water strider leg, gecko foot, butterfly wing, and mosquito compound eye. The self-cleaning ‘lotus effect’ is celebrated, and that is attributed to the hierarchical surface structure, which consists of nanostructured hydrophobic epicuticular wax crystals [[1], [2], [3], [4], [5], [6]]. The superhydrophobicity (SHPY) in nature are well described in recent reviews [3,7,8].

The surface wettability could be defined by contact angle (θCA, characterizes static wettability), sliding angle or tilt angle (θSA, describes dynamic wettability) and contact angle hysteresis (θCH) (Fig. 1). Typically, for a surface to be SHPC, the apparent θCA should be higher (> 150°) with a low θCH (< 10°) and θSA (< 5°). The low θSA and θCH render the surface with the self-cleaning property. θCH provides essential information on contact line pinning [4,8]. The surface chemistry (low surface energy) and roughness (micro/nano hierarchical) are the two critical factors determining SHPY. When in contact with water, such surfaces can maintain an air layer at the solid-liquid interface [3,[9], [10], [11], [12]].

Early models based on Young [13], Wenzel [14], and Cassie–Baxter [15] equations have well explained the concept of wettability. Young's equation gives contact angle on a smooth surface with respect to interfacial tensions (Fig. 1), whereas Wenzel equation describes the contribution of roughness. Cassie and Baxter revealed that the solid/air interface could affect the SHPY. More recently, these theories were successfully used to explain SHPC surfaces found in nature [1,2]. Detailed information on different models of wettability can be found elsewhere [5,16,17].

SHPY can be rendered to a surface by suitably modifying the hierarchical surface roughness and topography of low energy surfaces by chemical/physical modification and/or by coating the surface with a SHPC component. It can be achieved by modifying the surface geometry followed by a change of surface energy, or vice versa; the two steps can also be performed together [18,19]. Both top-down (lithography, template-based, micromachining, plasma treatment, etching etc.) and bottom-up (chemical vapour deposition (CVD), layer-by-layer (LBL), sol-gel, colloidal assemblies, electrochemical etc.) approaches or combinational approaches (casting, phase separation, electrospinning etc.) can be used for making SHPC surfaces [10,20,21]. Wang et al. classified the fabrication methods into physical, chemical and physicochemical processes where physical practices described include plasma treatment, laser treatment, phase separation, template methods, spin/spray-coating, electrohydrodynamics/electrospinning, and ion-assisted deposition and the chemical processes include sol-gel, electrochemical, LBL, self-assembly, and solvothermal. Physicochemical techniques debated are vapor deposition and etching. Unconventional methods such as nanocasting, supercritical CO2 solution method, electron irradiation etc. are also mentioned [4]. Two or more of the stated methods can be employed simultaneously to achieve the SHPY. More details on the fabrication methods are described somewhere else [4]. SHPC surfaces have successfully fabricated on a variety of substrates such as metal, paper, textile, wood, and various polymers, ceramics and composites. In general, for intrinsically hydrophobic materials, creation of the required surface roughness is the key aspect whereas, for hydrophilic materials, an additional hydrophobic surface treatment is necessary. SHPC surfaces need to have excellent durability against aggressive environments and conditions [3,5,18,19].

SHPY and the associated self-cleaning property have been widely studied for numerous applications ranging from anti-corrosion, anti-fouling, liquid-repellent papers and textiles, oil/water separation, anti-icing, anti-fogging, drag reduction, smart membranes, sensors, batteries, photovoltaic devices, microfluidics, cell adhesion and drug delivery [4,22]. Excellent reviews are available where readers can find precise fundamentals of SHPC materials [4,5,10,20,[23], [24], [25], [26], [27], [28], [29], [30], [31], [32]]. A few recent reviews explicitly addressed the self-cleaning property [22,33]. Several topical reviews specific to the application areas are also available that includes anti-corrosion [[34], [35], [36], [37]], paper [38], textiles [39,40], anti-icing [41,42], biomedical [43], drag reduction [6], energy [17], and seawater application [11].

Developing SHPC surfaces with desirable characteristics can be challenging, particularly when constrained by environmental concerns. Several fabrication processes are typically complicated and involve the use of harmful solvents. The mainly used chemistries for lowering the surface energy, the fluorine and silane chemistries, however, are not nature or consumer-friendly [4,21]. Several research efforts are currently going on in finding eco-friendly alternatives. In this direction, fabrication of advanced wax-based SHPC surfaces has high significance. Waxes are suitable to uphold the required chemistry and morphology for SHPY, and they are capable of reorganizing on heating to regenerate the surface structure. There are several important recent reports where waxes alone or wax-incorporated formulations are employed to attain SHPY. The next section provides a brief introduction on hydrocarbon waxes.

Waxes are one of the most hydrophobic substances available in nature [44]. The SHPY in plants, as discussed above, is correlated to the nanostructured surface epicuticular wax layer and the associated hierarchical micro/nano-structure [1,45]. These natural waxes occur in different 3D morphologies, such as tubules, filaments, rodlets, prisms and flakes (Fig. 2) [8,46]. The wax crystals on the waxy bumps on lotus leaves (θCA, 161 ± 2.7° and θCH, 2°) are also shown in the figure (Fig. 2e,f). The nonacosanol wax tubules on lotus leaves have average diameter and length at the range of 100–200 nm and 0.3–2 μm, respectively [46]. The self-assembly of the waxes is allied to their inherent crystallinity [8].

Typically, waxes can be categorized into natural or synthetic. The natural waxes can be either renewable (plant and animal waxes) or non-renewable (mineral waxes). Examples of plant waxes include carnauba wax, candelilla wax, soy wax, sunflower wax, berry wax, rice bran wax and castor wax. Examples of animal waxes includes beeswax, and wool wax. Montan and petrolatum are examples of mineral waxes. Petroleum waxes constitute the most considerable fraction of the current market and are often considered as a separate classification entity along with natural and synthetic waxes. The two important petroleum waxes are paraffin and microcrystalline waxes [[48], [49], [50], [51]]. H. Bennett has provided the conventional classification where natural waxes are categorized into paraffin wax, microcrystalline waxes, other mineral waxes (montan, lignite, ozocerite, ceresin, utah, peat), vegetable waxes (bayberry, candelilla, carnauba, cotton, esparto, fir, Japan, ouricury, palm, rice-oil, sugar cane, ucuhuba, cocoa butter) and animal waxes (beeswax, Chinese, shellac, spermaceti, wool wax) [50]. The author classified the synthetic waxes into (i) fatty alcohols and acids, (cetyl alcohol, lanette wax, stearyl alcohol, stearic acid, palmitic acid, myristic acid), (ii) fatty acid esters and glycerides (glyceryl stearates, glycol fatty-acid stearates, sorbitol stearates, polyethylene glycol stearates), (iii) hydrogenated oils, (iv) ketones, amines, amides, (stearone, laurone, aliphatic amines, aliphatic amides), (v) chloronaphthalenes, (vi) synthetic mineral waxes (Fisher-Tropsch, duroxon), (vii) synthetic animal waxes, and (viii) miscellaneous synthetic waxes [50]. The unique production processes often provide synthetic waxes with distinct properties such as high purity, high heat resistance, higher set times and more extended protection [[50], [51], [52]]. There are several commercial synthetic waxes available in the market. For example, a list of commercial amide waxes can be found in [49]. The magnitude of annual worldwide wax consumption is revealed to be: paraffin (3), polyolefin (0.2), Fischer-Tropsch (0.1), montan (0.05), carnauba (0.015), beeswax (0.01), candelilla (0.001) and other vegetable waxes (0.002 million tonnes/year) [53].

Waxes are constituted by long-chain (10–60 carbon atoms) aliphatic compounds such as fatty acids, hydrocarbons, alcohols, ketones, aldehydes, and esters [8,46,48,50,51,54,55]. The nature of the constituents varies with the type and source of the wax. Carnauba wax with melting point (Tm) at the range of 82–85.5 °C is the most commercially important plant wax. It is widely used for car and furniture polishes, medical products, cosmetics, food products, wood industry and semiconductors devices [48]. Its constituents are shown to be esters (aliphatic, v-hydroxy, p-methoxycinnamic, p-hydroxycinnamic), free acids and alcohols, lactides, hydrocarbons, and resins [50,56]. Candelilla wax (Tm = 68.5–72.5 °C) has major use as fruit coatings. Its chief components include hydrocarbons, free acids and alcohols, wax esters, sterols, and natural resins [50]. The major constituents of rice bran wax are aliphatic acids and higher alcohol esters [48]. The most common type of animal wax is beeswax, and they are mainly used in candle making and cosmetics. The main constituents of yellow beeswax are hydrocarbons, esters, free acids and alcohols, and lactones [50,57]. The primary source of the montan wax is lignite deposits [48]. Paraffin is the most commonly used petroleum wax having applications in candle making, paper coatings and crayons. Microcrystalline waxes are mainly used in the tire and rubber industry, and cosmetics. Table S1 (Supporting Information) shows structure and composition of representative waxes. There is a current trend towards natural renewable waxes due to environmental concerns and the cost factor. More details of different types of waxes can be found elsewhere [48,50,51,[58], [59], [60], [61], [62]]. Several fundamental works on waxes are reported before the 1960s [[63], [64], [65], [66], [67], [68]].

Although the concept of SHPY owes to the waxes, no review articles are available focusing on human-made SHPC surfaces based on hydrocarbon waxes. On the other hand, there are several important recent works reported in this area (see Section 2). Hence, we have made a systematic approach to comprehensively present the reported information on SHPC surfaces and coatings where natural or synthetic waxes are employed. Works reported on wax-based superoleophobic (SOPC) surfaces are also presented.

Section snippets

Wax-based artificial superhydrophobic surfaces

The easy availability, economic feasibility and natural water-repelling properties of wax have attracted significant research attention in fabricating advanced SHPC surfaces in numerous applications. Several works utilized wax-only SHPC coatings, whereas many reports employed wax as a major component in the formulation. The following section describes them according to the application area, even though such a classification is not impeccable. A few important works where the wax used as a minor

Superoleophobic surfaces

Superoleophobic (SOPC) surfaces are characterized by their unique anti-wetting properties for low surface tension liquids. They are characterized by an oil θCA > 150° with low θCH. Oil molecules are nonpolar, and the surface tension of oils and organic liquids are typically low when compared to that of water. So to make a SOPC surface, a surface coating with a material having very low surface tension less than 6 mN⋅ m−1 is required, and that can be satisfied by compounds with –CF3 groups.

Conclusions and perspectives

Waxes are primary materials of interest when conferring SHPC surfaces and coatings. The eco-friendly wax modifications are advantageous due to the economic factors, simple fabrication methods and scope for large-scale production. Excellent reviews are available on wax-based natural SHPC materials in plant and animal kingdom. However, review article focusing on wax-based artificial SHPC surfaces are lacking. Here, we analysed the topic and presented all the information under one roof according

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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