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Due to their excellent robustness and water-repellence properties, materials with low surface energy such as fluoroalkyl compounds (perfluoroalkyl silanes and fluoroacrylic copolymers) and organosilane-based chemistries are used for superhydrophobic coatings fabrication. However, these materials can cause a severe environmental impact and generally are not biodegradable or recyclable. For this reason, new environmentally friendly methods using natural materials are still being developed to obtain similar features, especially for packaging, textile and medical applications. The use of plant-based materials shows potential for creating superhydrophobic coatings, as many of them are naturally hydrophobic and can produce the desired surface textures. The main challenges to making superhydrophobic coatings from plant-based materials are abrasion resistance, strong adhesion, functionality in certain environments, and durability, but at the same time, they must be biodegradable. This chapter summarizes the recent approaches for superhydrophobic coatings made from environmentally safe materials and their applications.
School of Engineering and Materials Science, Queen Mary University of London, London, United Kingdom
Colin R. Crick
School of Engineering and Materials Science, Queen Mary University of London, London, United Kingdom
*Address all correspondence to: b.resendiz-diaz@qmul.ac.uk
1. Introduction
In nature, superhydrophobicity is observed in various plants, animals, and insects. They possess a unique surface texture that allows them to repel water effortlessly. Researchers are focusing on mimicking such surface structures for real-world applications to generate properties such as surface protection, anticorrosion, anti-icing, self-cleaning, antifouling, and so on [1]. To fabricate an artificial superhydrophobic surface, two main factors must be considered (1) hierarchical micro- and/or nanoscale roughness and (2) surface chemistry [2]. Due to their low surface energy, synthetic materials are typically used for superhydrophobic coatings fabrication [3]. Superhydrophobic coatings that have been traditionally used are made from fluorinated and sulfhydryl compounds, along with silicones. However, these materials are harmful to the environment, toxic, and nondegradable, and they are relatively expensive. Moreover, their fabrication involves the use of organic solvents, which makes them challenging to dispose of and keep human health safe [4, 5]. Despite this, they successfully achieve lower surface free energy and provide a degree of durability [4]. Recently, researchers have been placing importance on substituting these materials as their main priority. New eco-friendly alternatives including bio-based polymers such as cellulose, chitosan, and plant-based waxes are being implemented to produce robust coatings [4, 5, 6, 7, 8]. The advantages of these materials are their biodegradability, non-toxicity, and low-cost, which makes them good prospects for industries like food packaging [4]. However, most plant-based materials are hydrophilic, which limits their application. As a result, chemical modifications and new innovative techniques are necessary to achieve properties such as hydrophobicity, resilience, and longevity [4, 7, 8].
When fabricating superhydrophobic coatings, materials such as cellulose, chitosan, and lignin are commonly used as rough agents, while waxes and fatty acids are utilized as low surface energy materials [8]. The combination between them leads to obtaining contact angles above 150°, which is the value to consider a surface superhydrophobic [9]. This chapter aims to provide a summary of common biopolymers and other plant-derived materials used for fabricating superhydrophobic coatings, along with the modifications needed to achieve hydrophobicity properties considering biosafety and cost-effective scale-up.
Polymers that are derived from renewable sources, carbon neutral, and can biodegrade are known as bio-based polymers. This encompasses all plants, animals, or microorganisms’ mass resources [10, 11]. Different biopolymer classifications depend on the production process, the source material, and the material’s life cycle [11]. They can be classified into three categories based on their origin and production (Figure 1) [14]:
Polymers extracted from biomass, that is, polysaccharides such as starch, cellulose, and proteins.
Polymers made by conventional chemical synthesis using bio-based monomers, that is, polylactic acid.
Polymers produced by microorganisms or genetically modify them through bacteria, that is, polyhydroxyalkanoate.
Figure 1.
Biopolymers classification [12, 13].
Biopolymers as raw materials are gaining importance in various industries. These polymers are preferred by companies due to their environmentally friendly and nontoxic nature [13]. Sectors such as food packaging, textiles, cosmetics, and medicine are working toward achieving similar properties as synthetic polymers [15].
Biopolymers are a potential alternative to film fabrication due to their biodegradability, renewability, and low-cost [10]. Many of these substances exhibit hydrophilic behavior (presence of -OH groups), resulting in reduced resilience, poor cohesion and adhesion, and inferior mechanical properties. To expand their application fields, it is necessary to modify them to make them hydrophobic chemically [10].
Polysaccharides are created from monosaccharides that can be found in plants such as cellulose, alginates, and starch, as well as in animals like chitosan [16, 17]. They are commonly used as a raw material; however, the presence of hydrophilic groups leads them to have poor water repellence, which limited them for certain applications [10]. Researchers are developing new strategies to enhance hydrophobic behavior to fabricate novel superhydrophobic surfaces from polysaccharides that mainly work as roughness agents [4, 7, 8].
3.1.1 Cellulose
Cellulose is considered one of the most eco-friendly, renewable, and biodegradable materials in the world [15]. Primarily, cellulose is extracted from the cell wall of woody plants and is combined with lignin and hemicellulose [18]. It has been established that cellulose is composed of glucose units arranged in a polymer structure (poly-b-(1,4)-D-glucose) and hydroxyl groups in an equatorial position [15, 19, 20]. The material’s strength is attributed to the numerous hydroxyl groups that form strong hydrogen bonds [18].
Chemical modification of cellulose is required to replace hydroxyl groups with hydrophobic groups to achieve superhydrophobic properties [21, 22]. A variety of strategies have been performed to enhance the efficiency of the surface hydrophobicity to improve compatibility and dispersibility in different solvents [23]. Recently, cellulose has been used as a coating ingredient to generate surface roughness by forming nano/micro cellulose fibrils or cellulose nanoparticles [8]. To avoid using hazardous organic solvents, researchers developed lignin-coated cellulose nanoparticles (L-CNC). Lignin which is less hydrophilic than cellulose, was used to build a rough structure which was then combined with a water-soluble nontoxic polymer, polyvinyl alcohol (PVA) which acts as a binder material (Figure 2) [24]. Using the spray coating technique, lignin-cellulose nanoparticles/PVA solutions demonstrated superhydrophobicity with water contact angles around 150° [25].
Figure 2.
Surface morphology of L-CNC coating at higher magnification and its particle surface. The coating presented high roughness. Reproduced (adapted) from Ref. [24] with permission from the Royal Society of Chemistry.
A green and effective nanofibrillated cellulose-based superhydrophobic coating was built by using a coupling reaction of ethyl orthosilicate and cetyltrimethoxysilane. The coating revealed nano roughness, low surface energy, and good thermal stability. Additionally, the coating exhibited a certain degree of antibacterial activity. The wettability of the surface was monitored for 30 days, and it demonstrated good water resistance (contact angle above 160°) [26]. A semitransparent superhydrophobic surface with a rough texture was created using nano- and microstructure cellulose by utilizing the spray coating technique. To make this surface, building blocks were prepared and then molded to mimic the surface of lotus leaves. After substituting the hydroxyl groups of cellulose with stearoyl groups, the static contact angle was significantly increased to over 158°, and the contact angle hysteresis was lowered to 5°. As a result, it exhibited excellent water repellency against a jet of water impinging [27].
3.1.2 Chitin and chitosan
Chitin is the second most abundant biopolymer in nature, which is derived from the exoskeleton of crustaceous as well as from the fungi and insects’ cell walls [28]. It is a polymer with a long chain of β-(1,4)-N-acetyl-D-glucosamine and N-acetyl-D-glucosamine [15, 18]. Chitosan is derived from the N-deacetylation of chitin using alkali [15, 28]. The degree of deacetylation varies from 40 to 98%, which determines the molecular weight of the polymer and its applications [28]. Chitosan is a weak base and insoluble in water, which is commonly used for medical and pharmaceutical applications [15].
Chitosan micro/nanoparticles have the potential to be used in superhydrophobic coatings [8]. The physicochemical properties of these particles, such as crystallinity, molecular weight, and functional groups, can vary depending on the method used to create them. The particles can have either a spherical or an amorphous morphology. Chitosan, due to its amino and hydroxyl functionality, is suitable for chemical modification, such as deacetylation [23].
Chitosan-based nanoparticles offer certain advantages for medical textiles such as antibacterial activity and low toxicity [29]. A proposed superhydrophobic coating using chitosan nanoparticles was made through the covalent bonding of octadecylamine (ODA) to chitosan with the help of a cross-linking agent, glutaraldehyde (GA) (Figure 3). To reduce surface energy, ODA has been used thanks to its long hydrocarbon chain, while particle accumulation (measuring 200 nm in size) was used to enhance roughness. The process involved the extraction of chitin from crab shells, followed by cleaning and grinding, decolorization, deproteinization, demineralization, and deacetylation. Afterward, chitosan underwent chemical modification with GA and ODA before being sprayed onto polyester fabric, medical-grade cotton, and PU sponge using the spray coating technique, obtaining water contact angles for all materials above 158° [30].
Figure 3.
Crab shells are processed to obtain chitosan powder for the fabrication of chitosan-based superhydrophobic material. Reprinted (adapted) with permission from [30] Copyright 2023 American Chemical Society.
A safe and environmentally sustainable fabrication method was used to obtain a chitosan-fluorine-free superhydrophobic coating to be transparent. The process consists of two main steps: (1) chitosan functionality modification and (2) solvent-free deposition. During the first stage, stearoyl chloride reacts with the hydroxyl and amine groups on the chitosan backbone, resulting in esterification and secondary amide formation. In the second stage, the modified chitosan is applied onto a glass slide and heated in an oven to ensure it adheres to the substrate. It was demonstrated that by increasing the number of chitosan, the hydrophobicity enhances, leading to higher contact angles (>150°) [31].
3.1.3 Starch
Plants such as corn, rice, and potato contain abundant biopolymer known as starch, which can be stored in various parts of the plant like roots, tubers, leaves, and seeds [15, 32]. It is insoluble in water, semicrystalline, and dense [32]. These carbohydrates consist of linear polysaccharide amylose and branched polysaccharide, which are formed at the end of photosynthesis [15]. It is composed of glucose units linked to α-1,4 and α-1,6 glycosidic bonds. Starch is a highly adaptable biomaterial that has garnered attention for its widespread availability, lack of toxicity, affordability, and ability to biodegrade [33].
The high performance of nano-based superhydrophobic coatings has been increasing interest in a variety of sectors [34, 35]. Starch nanoparticles show excellent micro/nano structure to form environmentally friendly superhydrophobic coatings. By combining a lower surface tension material (i.e., PDMS) covering and nano starch-based particles, the robustness water repellent coatings can be achieved with a water contact angle >150° [35]. It is important to notice that pH could help to monitor the freshness of the coating when it is in contact with different types of food [34]. A thermostable and colorimetric starch-based superhydrophobic coating (>150°) with edible materials (starch nanoparticles, stearic acid, and anthocyanin) was fabricated. The combination of stearic acid and starch nanoparticles contributes to the micro/nano roughness of the coating, while anthocyanin is used to monitor freshness levels through pH levels [36].
Starch is mainly used to enhance the roughness of the surface. By utilizing starch-based materials, polyethyleneimine (utilized as an interlayer binder), and beeswax, a superhydrophobic coating was created. Beeswax density of 1.1 mg cm−2 was the correct value to maintain superhydrophobicity having contact angles above 150° and a sliding angle of 6°. The coatings indicated resistance to immersion of water for a long time and showed the ability to repel liquid foods [37].
3.1.4 Lignin
Lignin is a biopolymer present in the cell walls of woody plants (Figure 2) [38]. It is formed through the oxidative coupling of p-hydroxy cinnamyl alcohol monomers and other related compounds. The cell wall gains rigidity, strength, and increased resistance through the covalent bonding of lignin and hemicellulose [39, 40]. The structural organization of lignin contributes to the physical and mechanical properties of wood [41]. It is a natural and renewable material that has recently been exploited as raw material for industrial applications [38]. However, chemical modification is necessary to enhance hydrophobicity due to its hydrophilicity and poor dispersibility [42].
Lignin-based superhydrophobic coatings are still developed due to the complex chemical structure and difficult modification of lignin [8]. Recently, lignin nanospheres have been successfully fabricated for electrochemistry utilization to form rough structures, fluorine-free silane coupling to reduce the surface energy, and nanocellulose crystals to act as reinforcement material. The hydrothermal lignin nanospheres without modification reached a contact angle of 139° for just 1 minute after the surface become hydrophilic. However, by combining both methyltrimethoxysilane and hexadecyltrimethoxysilane, the coating reached a contact angle of 164° [43]. A different instance of utilizing lignin for constructing a superhydrophobic coating involved coating cellulose nanocrystals with lignin. Varied particle sizes aid in achieving an appropriate rough surface structure. To create the coating, the lignin particles were first modified to have low surface energy. Then, they were placed on the substrate that had been covered with adhesive. Finally, a wooden bar was used to press the particles into the adhesive, resulting in contact angles above 160°(Figure 4) [25].
Figure 4.
SEM micrographs of surface morphology using L-CNC particles at high magnification and cross section. Reproduced with permission from [25] published by Materials, 2017.
Lately, a green effective approach using lignin-micro-nanospheres (LMNS) chemically modified with γ-Valerolactone (GVL) was proposed. The lignin microspheres were utilized for the fabrication of superhydrophobic coatings on wood surfaces by immersing them in the solution (LMN, ethanol, F13-TMS, and epoxy). It was demonstrated that LMNS were uniformly anchored to the wood surface and showed spherical morphology. The hierarchical micro/nano structure provides the surface with hydrophobicity with contact angles of 164.4° and 162.3° and sliding angles of less than 10°. Additionally, the coatings presented strong resistance to organic solvents and exhibited a significant photothermal effect [44].
Proteins are biological macromolecules whose monomer units are α-amino acids sequence of the polypeptide chain, which play an important role in most of the biochemical functions of the cell. Their functionality depends on their structure and can be chemically, enzymatically, or physically modified due to the higher number of functional groups [45, 46]. Inspired by nature and enhancing interfacial adhesion, researchers are developing the use of proteins (i.e., Lysozyme) to provide nanoscale surface roughness [47, 48].
4.1.1 Zein
Zein is classified as a prolamin protein and is primarily derived from the endosperm of corn, accounting for 80% of its protein composition [49]. Its unique amino acid sequence gives it an amphiphilic nature. It has a high concentration of hydrophobic amino acids (50% hydrophobic residues), making it insoluble in water. However, it can be dissolved in the presence of organic solvents such as ethanol, high concentrations of urea and alkali (at pH 11), or anionic detergents (around 21–26% hydrophilic) [49, 50].
Recently, there has been a growing interest in zein as a renewable and biodegradable material to act as a coating agent for industries such as food packaging, biomedicine, and pharmaceuticals [51]. Researchers have been using the electrospinning technique to create zein fibers for the fabrication of superhydrophobic coatings [8]. These coatings have been found to exhibit contact angles above 150°. The concentration of zein determines the hydrophobicity of the surface. When the concentration is low, the surface becomes rough due to collapsed beads forming. However, when the concentration increases, the surface becomes smoother, forming fibers that reduce the contact angle [52]. Another eco-friendly method using zein is by producing zein nanoparticles that can enhance the antimicrobial activity of textiles [53]. To achieve this, ellagic acid was added as it is known for its effective antibacterial properties against E. coli and S. aureus (Figure 5) [54]. Despite the use of zein nanoparticles, the surface roughness was insufficient to attain superhydrophobicity, which requires contact angles of higher than 150° [53].
Figure 5.
Antimicrobial fabric with zein nanoparticles [53].
A dual-layered superhydrophobic coating can be fabricated through a cost-effective spray coating method by combining waxes’ (beeswax and candelilla wax) low surface energy and roughness while zein/pectin nanoparticles and cellulose nanofibers act as a supporting layer. The coating possesses higher water repellence to different types of food liquids (tea, milk, honey, and coke) with water contact angles greater than 150° and sliding angles less than 20° [55].
4.1.2 Other proteins: Lysozyme and bovine serum albumin
Recent reports have proved that other types of proteins can be applied for superhydrophobic coatings. Lysozyme is a type of glycohydrolase that shows great potential. [48, 56]. It can hydrolyze the β-1,4-linkages found between N-acetylmuramic acid and N-acetyl-d-glucosamine residues in the peptidoglycan of bacterial cell walls. It is extracted from eggs and milk, and it is recognized as safe when in contact with food [57]. By utilizing cysteine as a reducing agent, the phase transition of lysozyme results in the production of micro/nano roughness that can effectively adhere to various surfaces. A durable and water-repellent coating (contact angles above 148° and sliding angles less than 10° depending on the food liquid) that resists adhesion to different types of food, such as yoghurt, milk, honey, and so forth, can be formed by using a combination of phase-transited lysozyme and carnauba wax. Additionally, to this, the coating showed high repellency to blood, and the blood droplet rolled easily without any residue [48].
In addition to this, it is possible to modify lysozyme (microparticles with 500 nm in diameter) to provide surface roughness by transitioning it into a microparticle-aggregated necklace network with internal amyloid stacking structures [8]. Lysozyme particles combined with a hydrophobic treatment turned different types of surface materials (cotton, ceramic, wood, PET, etc.) into superhydrophobic surfaces. It was demonstrated that lysozyme crystallization can be accelerated and facilitate protein-based superhydrophobic surfaces by using the facile protein spotting technique [47].
Bovine serum albumin is another type of protein composed of a single polypeptide that contains 583 amino acid residues [58]. Bovine serum albumin nanoparticles were synthesized by adding dipentaerythritol penta-acrylate to form covalently cross-linked nanoparticles in which bovine serum amine groups react with acrylate groups. The nanoparticles were deposited in cotton fibers achieving contact angles above 150° due to the rough structure and chemical modification, making them resistant to certain physical manipulations, extended UV radiation exposure, and chemical exposure [59].
Lipids are hydrophobic and amphipathic molecules with high molecular weight. They are primarily soluble in organic solvents and not soluble in water. These structures act as energy storage within cells and offer thermal insulation [15]. Plants and algae primarily contain phosphoglycerides and glycosylglycerides as their main lipid components. When in contact with water, lipids exhibit a variety of behaviors such as forming monolayers, micelles, bilayers, membranes, or vesicles. This is due to water’s strong cohesive self-attraction, which repels the hydrocarbon chains present in lipids [60].
In many organisms such as plants, lipids contribute to the surface covering including surface waxes, cutin, and suberin. Commonly, an example of this is the lotus effect exhibited by the leaves of the Nembulo nucifera flower [1]. The nanoscale rough structure combined with a natural hydrophobic wax helps with self-cleaning action. This action causes water droplets to roll into the surface, protecting the leaves from foreign organisms [61].
5.1.1 Waxes
Waxes consist of long-chain compounds that are typically nonpolar, meaning they are pure hydrocarbons or have a small hydrophilic component that does not easily interact with water, and they are highly hydrophobic in nature [60, 62, 63]. They can be categorized into natural and synthetic. Natural waxes are obtained from plants and animals [7]. Waxes such as carnauba wax, rice bran wax, soy wax, and candelilla wax are plant-derived, while beeswax is an example of a wax derived from animals [62, 63]. Additionally, petroleum waxes like paraffin and microcrystalline are also considered sustainable materials [6, 63, 64]. Waxes are soluble in organic solvents, solid at room temperature, and malleable [63, 65]. Their chemical composition makes them one of the most hydrophobic materials in nature. A significant amount of data indicates that waxes’ chemical composition is complex and has not yet been fully established [62, 63]. The chemical composition of waxes varies depending on how the wax was extracted, the stage in which waxes were collected, and the environmental conditions [66]. They are mostly constituted by long-chain aliphatic compounds (10–60 carbon atoms) including fatty acids, hydrocarbons, alcohols, ketones, aldehydes, and esters (Table 1) [63].
Fatty acids (typically C20–C26) and fatty alcohols (C30–C36) [69, 70].
Beeswax
Esters (67%), free acids (12%), free alcohols, hydrocarbons (14%), and other non-identified components (6%) [71].
Soy wax
It is mainly composed of triacyl glyceride with a high portion of stearic acid. The wax came from soy oil, which consists of several fatty such as palmitic acid (10%), stearic acid (4%), oleic acid (18%), linoleic acid (55%), and linolenic acid (13%) [72, 73].
Chemical composition of some biodegradable waxes based on references.
Because waxes are economical, readily available, biodegradable, and can even be consumed, they could be a sustainable solution for industrial applications including the food, textiles, and paper industries [75]. Waxes are often utilized as materials with low surface energy. However, it is also recognized that waxes can cause the formation of rough structures because of the crystallization process, which creates crystals when they are deposited in a substrate [76]. For this reason, waxes can create superhydrophobic surfaces by themselves combining both features [62, 77].
A superhydrophobic coating was made of candelilla wax and rice bran wax via a one-step spray solution. The waxes were dissolved in hot ethanol using temperature above their melting point. The coating presented micro/nanoroughness, which allowed it to repel several types of food liquids (milk, cola, orange juice, etc.) with WCA ~ 155° [78]. Another superhydrophobic coating was fabricated using soy wax dispersed in hot ethanol. The non-wettable coating had excellent repelling properties against different non-Newtonian food liquids with a higher apparent contact angle of 159° [73]. In addition to this, a general all-by-one superhydrophobic coating was made with food-grade waxes (beeswax and paraffin wax). The suspension was created using ethanol (95%) at 80°C and then ultrasonicated to homogenize the components. The superhydrophobic coating was applied to distinct types of food packaging materials obtaining water contact angles of ~160° and sliding angles of ~6.2° [76]. From the combination of candle soot particles and carnauba wax, a low-cost superhydrophobic surface can be fabricated. By depositing the concentrations of candle soot and carnauba wax into the glass substrate, the solvent evaporated completely leaving a rough structure with a contact angle of 172° [79].
A novel superhydrophobic coating with coffee lignin and beeswax (Figure 6). The wax wrapped the coffee lignin surface to increase hydrophobicity. Due to the low melting point of beeswax, the thermal stability was unsatisfactory at higher temperatures (120°C), causing a decrease in contact angle (from 165 to 89°C) [80]. Another approach using waxes as low surface energy material is by combining polylactic acid (PLA) and carnauba wax (CW). PLA/CW coating presented contact angles above 150° when it is in contact with different liquids. However, if the coating is exposed to a higher temperature (above the melting point of the wax), the coating without PLA starts to lose roughness while PLA/CW coating maintains superhydrophobic properties due to PLA rough structure [81]. Based on the results, it appears that using the spray coating technique is more effective than the dip coating technique. This is because it improves the roughness, which is necessary for achieving superhydrophobicity [81].
Figure 6.
Superhydrophobic coating prepared by coffee/lignin and beeswax. The coating has high repellence to different types of liquid foods [80].
5.2 Fatty acids
Fatty acids are part of the lipids and play an important role in metabolism. They are the main constituents of oils, fats, waxes, and lipid-based materials [82]. Fatty acids are carbon chains with a methyl group at the end of the molecule and a carboxyl group at the other end [83]. Most fatty acids are insoluble in water, which makes them hydrophobic in nature. Depending on the degree of saturation and length, the properties of the fatty acid vary [8]. Moreover, it is common for fatty acids to associate with one another, resulting in the creation of monolayers and micelles [83]. Stearic acid is a commonly used fatty acid for creating superhydrophobic surfaces and hydrophobic compounds [8]. Stearic acid (CH3(CH2)16COOH) is a long-chain saturated fatty acid, and due to its non-toxicity and biocompatibility, it is used for cosmetics and pharmaceutical applications (Figure 7) [84, 85].
Figure 7.
Stearic acid structural formulate [84].
Inexpensive solutions for repellent surfaces to avoid water corrosion and another type of liquids for aluminum alloys are becoming attractive for industrial applications [8, 85, 86]. Applying the chemical etching technique, a superhydrophobic coating from stearic acid was made. The morphology of the coating shows micrometer and nanometer structures, which were able to trap enough air, preventing water penetration. The coating with a water contact angle of 156 ± 1° was more stable and resilient to corrosive liquids such as acids and bases compared to the coatings made by traditional techniques [86]. Another approach uses stearic acid and epoxidized soybean oil in cotton fabric etched by deep eutectic solvent. To improve the surface roughness of cotton fabric, the eutectic solvent was used while stearic acid and soybean oil coated the surface to provide low surface energy. A durable and flexible superhydrophobic cotton fabric (>150°) with self-cleaning properties and potential application for water separation was achieved [87]. An environmentally friendly and economical superhydrophobic coating was made from sepiolite nanoparticles functionalized with fatty acids (cinnamic acid and myristic acid) using the dip coating method. Cinnamic acid and myristic acid provide low surface free energy to sepiolite nanoparticles. The coatings showed a static contact angle of 160° with anti-biofouling and antimicrobial characteristics. A decrease of Gram-positive and -negative bacteria from 30% to 2–8% was observed [88].
6. Superhydrophobic coatings from other plant-based materials
Researchers are investigating alternative methods to create biodegradable superhydrophobic coatings. One possibility is using lycopodium spores, a type of plant-based material with a rough surface and naturally hydrophobic. By combining the spores’ natural roughness with a material that has lower surface energy, it is possible to achieve superhydrophobicity [8].
6.1 Lycopodium spores
Lycopodium spores are plant-based materials that come from ground pine. These spores are intrinsically hydrophobic and present spherical shapes. They have a characteristic length scale of 20 μm, which provides surface roughness (Figure 8) [90]. Surfaces made from lycopodium spores have a higher water contact angle of 144° and stronger adhesion to surfaces, making them classified as “sticky” [91]. Lycopodium spores are commonly used as a carrier for oral drugs and vaccines [89]. These spores serve as natural protective packaging that offers high uniformity and large cavities to encase different materials, particularly for medical purposes [92].
Figure 8.
SEM micrographs of lycopodium spores showing the honeycomb-like architecture. Reprinted (adapted) with permission from [89] Copyright 2023 American Chemical Society.
Researchers are investigating new uses for lycopodium spores, particularly in creating superhydrophobic coatings. For example, a superhydrophobic surface was made by lycopodium spores to disperse in a Mater-Bi polymer matrix (a blend of vegetable-derived polyester with thermoplastic starch). Researchers achieved a water contact angle of approximately 150°. However, the coatings were sticky, preventing water droplets from moving freely across the surface, commonly referred to as the “rose petal effect” [93]. Recently, a superhydrophobic coating using lycopodium spores with a combination of two natural waxes (Carnauba wax and Beeswax) has been fabricated. The components were probe sonicated to correctly disperse the particles in water. As a result, they obtained contact angles above 155° and contact angles of hysteresis less than 7° [90].
Numerous types of plant-based materials have been reported in the literature for creating superhydrophobic coatings. Although these materials offer a range of useful functionalities, their commercial development has been limited. This could be attributed to different factors such as low physical resilience, durability, material compatibility, scaling up complications, and so forth [94]. Comparing the coatings fabricated by synthetic materials and superhydrophobic coatings made from environmentally friendly materials, it can be inferred that both have been facing the same issues. Coatings derived from plant-based materials offer various benefits such as their ability to biodegrade, nontoxic nature, and reduced impact on the environment [8]. However, they are still needed chemical modification due to their fragility and hydrophilic behavior. These factors are important to consider for real-world applications to have a potential low-cost manufacturing process [4].
The global market of superhydrophobic coatings is incorporating more fluorine-free coatings. These commercial products are mainly used in nonmedical fields such as construction, automotive, aerospace, and marine [4]. Sectors such as food packaging aim to incorporate biodegradable coatings to minimize plastic pollution. This shift toward sustainability could also be implemented in the medical and textile sectors [3, 8]. Currently, to the best of our knowledge, there are no superhydrophobic coatings available in the market that are made solely from natural materials without any chemical modification.
In this chapter, we have outlined some affordable techniques and eco-friendly materials for fabricating superhydrophobic coatings. The use of biodegradable materials including biopolymers such as polysaccharides, proteins, and lipids as well as other plant-based materials including lycopodium showed tremendous potential for various applications. Based on the literature, it has been shown that many materials can function as rough agents by undergoing modifications that result in the formation of nanoparticles for the creation of rough surface textures. In addition, lipid compounds such as waxes can act as low surface energy materials; by combining both, it is possible to obtain superhydrophobicity.
Several low-cost and simple techniques (e.g., spray coating, dip coating, etc.) were used to fabricate such coatings. It is important to notice that superhydrophobic coatings from bio-based materials are still being developed to achieve better robustness and longevity, while avoiding using organic solvents and other chemical modifications, which involves toxic and hazardous compounds. Adding environmentally friendly materials would enhance the biodegradability and recyclability of the coatings.
BRD would like to declare no conflict of interest.
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Written By
Brenda Resendiz Diaz and Colin R. Crick
Submitted: 14 August 2023Reviewed: 16 August 2023Published: 30 October 2023
Open access peer-reviewed chapter
