Smart Coatings

C H A P T E R

17 Smart Coatings Sarah B. Ulaeto, Jerin K. Pancrecious, T.P.D. Rajan and B.C. Pai CSIR-National Institute for Interdisciplinary Science and Technology, Trivandrum, Kerala, India

17.1 INTRODUCTION Smart coatings are innovative coatings that provide a spontaneous response to changes in the microenvironment such as heat, light irradiation, mechanical induction, wettability, temperature, pressure, ion exchange, pH variation, aggressive corrosive ions, etc. Smart coatings are also referred to as “stimuli-responsive”, “intelligent” or “environmentally sensitive” coatings [1 4]. Smart coatings are designed to maintain their passive feature, provide superior performance, and recover the functional performance of coatings by exhibiting stimuli-responsive behavior when the need arises. The stimuli-responsive characteristic of smart coatings enhances the efficiency of the coated system. These categories of coatings with phenomenal innovations are unique for its accelerating developments. The driving force in developing smart coating technologies is the continuous demand for higher performance, extending product lifetimes and significantly reducing maintenance cost. Enhancing energy efficiency for both cost and environmental reasons, as well as coping with the change in substrates towards new and lightweight materials, like composites, and lightweight metals, such as aluminum and its alloys, etc. Examples of smart coatings include but are not limited to anti-inflammatory, antimicrobial, and antifouling coatings; corrosion, degradation, and defect sensing coatings; self-healing and pressure-sensing paints; reversible thermochromic; piezoelectric paint; hydrophilic/hydrophobic switching; self-cleaning, pH-responsive, light sensing coatings; self-erasing inks; smart window coatings; photochromic and electrochromiccoatings; radio frequency identification coatings; antibacterial, anti-icing, intumescent coatings, etc. [3 7]. Shifting demand towards nano-based coatings instead of conventional polymer coatings and other microparticle-based coatings due to superior properties and low Volatile

Noble Metal-Metal Oxide Hybrid Nanoparticles DOI: https://doi.org/10.1016/B978-0-12-814134-2.00017-6

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Organic Contents (VOC) emissions are expected to be the significant mainspring for the nanocoatings market. But with the growing preference for advanced materials in sectors such as medical and automotive industry, there is a surge in nanotechnology research and development, expected to boost market growth. With nanotechnology development, coatings nowadays become smarter, stronger, and more durable. The nanocoatings market embraces nanotechnology-based coatings due to its superior properties such as abrasion resistance, ductility, hardness, lubricity, and transparency to mention a few as compared to other conventional coatings. A boost in market demand is expected from the application of antimicrobial and self-cleaning coatings in mechanical and aerospace applications for extending the durability of parts. Nanostructured coatings, however, provide essential functions such as protection from ice, pollutant, UV, fire, heat, bacteria, marine fouling, touch, and corrosion. Properties such as antimicrobial action, product longevity, thermal insulation, antigraffiti, self-cleaning, moisture absorbing, gloss retention, dirt and water repellency, hardness, corrosion resistance, flame retardancy, ultraviolet radiation stability, improved energy efficiency, chemical and mechanical properties to mention a few are significantly enhanced with the use of nanostructured materials [8]. Smart coatings are increasingly used in a variety of applications in the medical, textile, transport, construction, military, electronics, aviation, and several other industries for providing different functionalities alongside protecting against corrosion. They are considered pragmatic candidates without manual intervention. The goal of smart coatings is directed at improving a system’s efficiency by reducing inspection times, significantly reducing the maintenance costs and equipment downtime in many industrial applications. It is a class of coatings capable of impacting on the society in a significant way [1,3 5,9]. The synergy between modern engineering science and nanotechnology has resulted in rapid developments of high-performance multifunctional corrosion-resistant coatings to cater for a broader range of hostile environments. These innovative coating systems are expected to accelerate gainful transformation in the corrosion world [10]. In the case of organic coatings, the incorporation of nanoparticles might enhance their barrier property and decrease the trend for coatings to blister or delaminate. Whereas, by introducing the hard nanocrystalline phases within a metallic matrix, the high hardness could be obtained for metallic coatings [11]. In the following reviews [1,6] studies are presented on corrosion protection with smart coatings. In the present contribution, an update on smart anticorrosive coatings with relevance in corrosion sensing, self-healing, antifouling and self-cleaning actions is considered with a few notable studies. With this brief introduction, this chapter aims to provide a concise perspective on smart coatings and its developments as their utilization cuts across various aspects of science. It is a vibrant topic, quite expansive, and cannot be entirely covered in a chapter. An attempt to highlight some selected smart coatings based on their function is taken into consideration including the update on smart anticorrosive coatings. Applications and commercial viability of smart coatings are explored. A brief conclusion and sources of further information are provided.

17.2 CLASSIFICATION OF SMART COATINGS Smart coatings may be classified based on application, function, responsiveness, material types, level of complexity [12], functional constituent and fabrication methods, etc. [13].

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The functional component within these intelligent coatings can be the resin or from its variety of additives such as pigments, bioactive species, enzymes, microencapsulated agents, nanomaterials, inhibitors, radiofrequency identification devices, micro-electromechanical devices, etc. [14]. Triggers for the required responses come from both internal and external stimuli. When applied to the surfaces of the desired substrates these smart coatings contribute to decoration in their functionality. These chemically active coatings exhibit various functionalities either on film substrate interfaces, air film interfaces or in the bulk of the film. In the different sectors of application, multifunctionality of smart coatings is an advantage [1,3,4,6,9]. To streamline the discussion, selected smart coatings discussed herein are categorized based on the functions exhibited by the coatings. Self-healing coatings can also be referred to as smart-repair coatings. Active sensing coatings include corrosion-sensing and pressure-sensing coatings. Flame-retardant coatings are intumescent and nonintumescent coatings. Antifouling and antibacterial coatings are referred to as bioactive coatings. Easy-to-clean coatings include self-cleaning and antigraffiti coatings. Smart window coatings are optically active coatings. Others are antifingerprint, antireflective, anti-icing, and antifogging coatings.

17.2.1 Self-Healing Coatings The concept of self-healing is a significant property of biological materials and has drawn considerable research interest in the medical sector and pharmaceutical industries and is now very relevant in the development of corrosion-resistant coatings [15]. The self-healing effect is modeled after the natural wound-healing process, and the functional nature of the self-healing coating depends on its chemical composition and structure [1]. The self-healing concept was first proposed in 1979 by Jud and Kausch through molecular interdiffusion across crack interfaces and later advanced by White and coworkers in 2001 by embedding microcapsules containing healing liquid and catalyst particles into the matrix material [16,17]. Self-healing materials are known to provide some advantages over the conventional coating. These include (i) automatic repair process upon damage; (ii) autopreservation of esthetics of surface appearance of coatings, plastic, and films; and (iii) restoration of the mechanical integrity of load-bearing materials as in composites, etc. [15]. Self-healing materials are classified into nonautonomous and autonomous systems. Nonautonomous self-healing (or stimuli-assisted) can be induced and controlled by heat, light, mechanical forces, chemical reactions, pH, etc. while autonomous self-healing does not require any external intervention, the damage itself triggers repair processes. Autonomous systems, therefore, behave as smart, adaptive materials and the autonomous self-healing process can either be intrinsic or extrinsic. Intrinsic involves mending mechanical failures since it is based on the formation of either covalent or noncovalent chemical bonds between cracked interfaces. Although engineering intrinsic processes is quite hard for most of the materials. Extrinsic requires the presence of some externally loaded healing agents, responding when triggered by mechanical damage [18]. Amongst the different approaches for self-healing coatings, the use of nanocontainers has been the most widely accepted and employed due to the versatility of nanocontainer fabrication and the variety

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of possible applicable healing agents [19]. Nanocontainers, such as halloysite clay, mesoporous silica [2,20], cyclodextrins [21], layered double hydroxide, polyelectrolyte multilayers, and mesoporous zirconia, exist and are being utilized [2,6]. Various self-healing mechanisms in polymeric materials are described in Fig. 17.1 [22]. The illustration explains self-healing based on the actions of healing agents in enclosed micro- or nanocapsule embedded in polymer matrices. The self-healing ability of coatings has become a treasured and desirable effect of protective coatings, due to the considerable delay in corrosion when they are in use and a subsequent reduction upon actual corrosion occurrence. Smart self-healing coatings provide a protective film build up via adsorption on the exposed area of the metal or alloy. Physisorption and strong covalent bonds are involved in the attachment of the nanoparticles to the polymer molecules as well as with other functional molecules of micron sizes [1]. It is best to design and establish such a stimuli-responsive release system (self-healing system) that entraps corrosion inhibitors in containers with a high uptake capacity and releases it only in response to specific stimuli. Hence, nanocontainer-based coatings serve as the solution for the challenge of self-healing for corrosion inhibition processes. The nanoparticles, also, occupy small defects and holes formed during curing of the coating and also act as a bridge interconnecting more molecules. The incorporation of

FIGURE 17.1 (A E) Self-healing of different enclosed micro- or nanocapsule healing agents in polymeric matrices. Source: Reproduced with permission Y. Yang, M.W. Urban, Self-healing polymeric materials, Chem. Soc. Rev. 42 (2013) 7446 7467. Copyright 2013, Royal Society of Chemistry.

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nanocontainers loaded with inhibitors into the coating, suppresses further corrosion occurrence by the long-term self-healing effect derived from the high inhibitor uptake and release capacity and response to local changes resulting from corrosion [19,20,23]. Recent developments in self-healing coatings are discussed with emphasis on corrosion protection. Qian et al. synthesized a novel superhydrophobic coating that exhibited good self-healing properties in their anticorrosion studies of Q235 carbon steel in 3.5 wt.% NaCl. The fabricated coating contained benzotriazole (BTA) as the corrosion inhibitor. The selfhealing induced by the shape memory effect of the damaged epoxy coating was investigated under two conditions: (i) heating in an oven at 60 C for 20 min; and (ii) exposure to sunlight for different durations. Healing through thermally triggered shape memory effect of the epoxy polymers is an entropically driven process. The inspiring practicality presented for use in actual outdoor environments was the self-healing achieved under sunlight (Fig. 17.2), revealing a potential for long-term corrosion protection from external

FIGURE 17.2 (A) Confocal laser scanning microscopy (CLSM) images of the scratched coating surface containing BTA-5% after (a1) 0 h, (a2) 1 h, and (a3) 7 days of exposure to sunlight under an outdoor environment. (B) Temperature evolution of the superhydrophobic coating surfaces with time during the outdoor exposure. (C) Bode plots of the healed BTA-5% coatings after 0 h, 1 h, and 7 days of exposure to sunlight in the outdoor environment. Source: Reproduced with permission H. Qian, D. Xu, C. Du, D. Zhang, X. Li, L. Huang, et al. Dual-action smart coatings with a self-healing superhydrophobic surface and anti-corrosion properties, J. Mater. Chem. A 5 (5) (2017) 2355 2364. Copyright 2017, Royal Society of Chemistry.

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damage. The 45 straight line in the Bode plot of the impedance investigation indicated that the anticorrosion performance was already fully repaired after 1 day and the recovered surface maintained after 7 days of exposure [24]. Garnweitner et al. fabricated the thermoresponsive polymer/silica nanocomposite thin film [25]. The authors signaled that the larger the temperature range changes, the higher the magnitude of the swelling/deswelling response is. In another significant study, light-responsive and self-healing anticorrosive coatings were investigated by Chen et al. The incorporation of hollow nanocontainers loaded with benzotriazole having smart molecular switches (photoresponsive azobenzene molecular switches) were used in the fabrication of the intelligent coating to protect the aluminum alloy AA2024. Exposure of the modified water-borne alkyd coating to visible light illumination had a reverse effect. The cis-isomer of the azobenzene molecules grafted in the mesopores of hollow nanocontainers transformed into the trans-isomer closing the pores (Fig. 17.3A). The residual corrosion inhibitors were not sequentially released/wasted but remained encapsulated within the nanocontainers to prevent the next corrosion occurrence. Thus, the reversible light-responsive release system achieved in the self-healing coating was aimed at providing flexibility and avoiding the excess release of the corrosion inhibitors after repairing the initial corrosion affected area. The nanocontainers possessed a high loading capacity and could also control the entrance and release of trapped active molecules by the dynamic motion of azobenzene molecules. With the scanning vibration electrode technique (SVET), local current density measurements around the artificial scratches were performed to verify the active anticorrosion performance of the lightsensitive nanocontainers in 0.1 M NaCl aqueous solution represented in Fig. 17.3B [26].

17.2.2 Active Sensing Coatings 17.2.2.1 Corrosion-Sensing Coatings Corrosion sensing coatings are used to monitor the corrosion risk of the material at the early stages of deterioration. It is well-known that when the corrosion happens, the anodic region possesses an acidic pH and the cathodic region has an alkaline pH. The pHtriggered release of corrosion indicators and healing agents from the microcapsules or nanocapsules is one of the active research areas. The corrosion sensing indicators can be of color dyes or fluorescent compounds. The size and pH sensitivity of the microcapsules or nanocapsules can be controlled by adjusting the preparation method and time. A corrosion-sensing fluorescent coating made of phenylfluorone (PF) into the acrylic paint was prepared for monitoring corrosion of aluminum alloys. This system was sensitive to underlying corrosion processes by reacting with the Al31 produced by the anodic reaction. The corroded areas under the fluorescence quenching spots were identified using an optical microscope. The anodic reaction sensitivity associated with corrosion was determined by applying constant charge current and measuring the charge, where the fluorescence quenching was detected in the coating [27]. A carbon nanotube (CNT) polyelectrolyte composite multilayer thin film has been made for measuring both strain and corrosion processes. The layer-by-layer fabrication exhibited changes in the electrical properties to strain and pH [28]. The corrosion-sensing compounds with color-change or

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FIGURE 17.3 (A) Schematic of reversible release system by utilizing trans-cisphotoisomerization of azobenzene molecules grafted in the mesopores of HMSs. UV irradiation at 365 nm converts azobenzene to the cis form, resulting in pore opening. The cis isomer of the azobenzene molecules transforms into the trans isomer under visible light irradiation (450 nm), leading to pore closing. (B) SVET current density maps of aluminum coated with the alkyd coating without (a) and with (b) the BTA@Azo-HMSs nanocontainers obtained after 1 h (left column) and 10 h (middle column) immersion in 0.1 M NaCl. The passive protection performance of the pure coating and the active self-healing performance of the coating containing BTA@Azo-HMSs was compared when the scratches were exposed to the UV light (right column). Source: Reproduced with permission from T. Chen, R. Chen, Z. Jin, J. Liu, Engineering hollow mesoporous silica nanocontainers with molecular switches for continuous self-healing anticorrosion coating, J. Mater. Chem. A 3 (18) (2015) 9510 9516. Copyright 2015, Royal Society of Chemistry.

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fluorescing to the pH increase associated with the cathodic reaction were also used by applying constant cathodic current and measuring the charge at which color change or fluorescence occurred. The color change of modified acrylic coating systems was controlled by the sensitivity of the coating to pH increase [29]. 17.2.2.2 Pressure-Sensing Coatings Pressure-sensing coatings have been used on wind tunnel models and flight vehicles to measure the surface pressures for aerodynamic and acoustic investigations. It is an optical technique for determining surface pressure distributions by measuring changes in the intensity of emitted light. Mainly, luminescent coatings are painted on the surface which will be excited by light of appropriate wavelength and imaged with digital cameras [30]. Porous pressure-sensitive paint (PSP) containing porous material as the binder with the large surface area can hold luminophores directly. Hence, the response time of porous PSPs is of the order of 1 μs owing to the feasibility of oxygen molecules in a test gas instantly quenching luminescence without having to permeate into a binder layer, as shown in Fig. 17.4. Anodized aluminum, anodized titanium, polymer/ceramic, thin-layer chromatography (TLC) plate, etc. have been used as porous materials [32]. A pressure-sensitive luminescent coating containing tris(4,7-diphenylphenanthroline) ruthenium(II) ([Ru(dpp)3]21) on porous anodized aluminum was made to measure nonperiodic unsteady pressure distribution. The pressure distribution on a delta wing at a high angle of attack in transonic flow is unstable due to the interaction between shock waves and leading-edge vortices [33]. Fast pressure-sensitive paint is an extension applicable to unsteady flow and acoustics. Most fast PSP formulations are associated with porous binders that allow rapid oxygen diffusion and interaction with the chemical sensor. Calibrations of the dynamic response of them show a flat frequency response to at least 6 kHz, with some paint formulations exceeding a response of 1 MHz [32]. Besides the luminescent coatings, functional graphene sensors, such as Raman-based strain sensors, are also prepared by different methods. The strain in graphene/poly

FIGURE 17.4 Schematic illustration of porous pressure-sensitive paint (PSP). Source: Reproduced with permission H. Sakaue, T. Tabei, M. Kameda, Hydrophobic monolayer coating on anodized aluminum pressure-sensitive paint, Sens. Actuators B: Chem. 119 (2) (2006) 504 511 [31]. Copyright 2006, Elsevier.

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(methyl methacrylate) sensors made of mechanically-exfoliated single-crystal graphene flake, as well as scalable chemical vapor deposition (CVD) graphene film, was investigated. The former was more sensitive to strain than the CVD graphene due to exfoliated graphene being a large single-crystal domain with high stiffness. CVD graphene films had an absolute accuracy of  6 0.01% and resolution of  27 με [34]. A highly sensitive, mechanically flexible pressure sensor was made by sandwiching ultrathin gold nanowireimpregnated tissue paper between two thin polydimethylsiloxane sheets for wearable optoelectronic devices. The sensors could be operated at a battery voltage of 1.5 V with low energy consumption. They were able to detect very low pressure with fast response time, high sensitivity, and high stability. This type of material with mechanical flexibility and robustness enabled real-time monitoring of blood pulses as well as detection of small vibration forces from music [35].

17.2.3 Optically-Active Coatings Smart window coatings are thin films with spectrally selective properties on the surface of the glass. They are characterized by their ability to regulate the amount of radiant energy between broad limits. These coatings are commonly referred to as Energy Efficient Window Coatings [36 38]. Smart window coatings enable blocking of inbound infrared radiation from the sun during warm weather conditions and provide heat retention inside a room in the cold seasons. Since the active materials in these coatings block both near infrared (NIR) light and visible light, this allows customization of the window settings to maximize energy efficiency. Thus, energy consuming processes are significantly eliminated. Smart window coatings can be fabricated from any of the following: photochromic coatings, thermochromic coatings, suspended particle devices (SPD), polymer dispersed liquid crystal devices (PDLC), electrochromic coatings, etc. [36,39,40]. SPD or electrochromic windows seem encouraging for dynamic daylight and solar energy applications in buildings based on the achieved transmittance modulation ranges. The electrochromic windows appear to be the most promising in comparison to the suspended-particle type because, with the suspended particle windows, electric field should be maintained whenever the transparent mode of the glass is required, resulting in higher energy consumption [40]. A notable study by Li et al. described an approach where a single nanoparticle structure (VO2@TiO2 core shell nanorods) with both thermochromic and photocatalytic properties offered significant potential for creating a multifunctional energy-saving smart coating. With the VO2 nanorod core, a remarkable modulation ability for solar infrared light was achieved, while the anatase TiO2 shell displayed significant photocatalytic degradation of organic dye [37]. Studies by Liu et al. on the combined effect of KxWO3 and F-TiO2, to achieve FT KWO nanocomposite films for windows exhibited strong near-infrared, ultraviolet light-shielding ability, well visible light transmittance, high photocatalytic activity, and excellent hydrophilic capacity. The proposed multifunctional NIR shielding-photocatalytic nanocomposite window film is meant to solve the energy crisis and deteriorating environmental issues. The working model is described in Fig. 17.5 [41].

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FIGURE 17.5 Schematic of the FT KWO and F-TiO2-coated window applied to dissimilar conditions. Source: Reproduced from T. Liu, B. Liu, J. Wang, L. Yang, X. Ma, H. Li, et al. Smart window coating based on F-TiO2 KxWO3 nanocomposites with heat shielding, ultraviolet isolating, hydrophilic and photocatalytic performance., Sci. Rep. 6 (2016). licensed under a Creative Commons Attribution 4.0 International License.

17.2.4 Easy-to-Clean Coatings 17.2.4.1 Self-Cleaning Coatings Self-cleaning coatings inspired by nature and centered on surface contact angles ensure an independent action of dirt removed from a surface by either a hydrophilic or hydrophobic technique. Self-cleaned surfaces can be elucidated using two distinct approaches. The first approach involves the application of a photocatalytic coating to the substrate surface, where the effect of the sun’s ultraviolet rays catalytically breaks down organic dirt. Simultaneously, the surface becomes superhydrophilic spreading the water evenly over the surface, and less drying traces are formed by dripping. In the other approach, a selfcleaning surface is achieved in congruence with the lotus effect phenomenon in which the surface becomes superhydrophobic. The extent of hydrophobicity is determined by the contact angle of water. Contact angles greater than 90o are regarded as hydrophobic while the surface with contact angles of 150o, or higher are% superhydrophobic and repel water % droplets completely [1,8]. Superhydrophobic surfaces are obtained from roughened hydrophobic surfaces. The effects of both the contact and the sliding angle are essential in defining a superhydrophobic surface with self-cleaning effect. This is because, for self-cleaning in addition to the high contact angle, the water drop should slide off the surface [42]. Fig. 17.6 illustrates self-cleaning related actions such as (i) natural self-cleaning surfaces and corresponding SEM micrographs; (ii) the hydrophilic action of self-cleaning coating via photocatalysis reaction of titanium dioxide coating on a pre-painted aluminum surface; (iii) lotus effect (a) self-cleaning and (b) wetting based on contact angle differences; and (iv) hydrophobic/superhydrophobic action of self-cleaning coatings made of organosilanecoated alumina particles deposited via electrospraying. Anticorrosive self-cleaning coatings with superhydrophobic surfaces are particularly attractive candidates for achieving enhanced anticorrosive performances. In the study led

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FIGURE 17.6 (A) Natural self-cleaning surfaces and corresponding SEM micrographs. (B) Hydrophilic action of a self-cleaning coating. Schematic shows photocatalysis reaction of titanium dioxide coating on a pre-painted aluminum surface. (a) UV light energizes the electrons in TiO2 which transfer energy to oxygen and water in the air forming free radicals like .OH. (b) The free radicals attack organic matter by oxidation as hydroxyls accumulate on the surface. (c) The hydroxyls collapse water molecules on the surface in the cleansing action [1]. (C) Lotus effect (i) self-cleaning and (ii) wetting based on contact angle differences. (D) Hydrophobic/superhydrophobic action of self-cleaning coatings made of organosilane-coated alumina particles deposited via electrospraying. Source: (A and C) Reproduced with permission J. Zhang, X. Suo, J. Zhang, B. Han, P. Li, Y. Xue, et al. One-pot synthesis of Au/TiO2 heteronanostructure composites with SPR effect and its antibacterial activity, Mater. Lett. 162 (2016) 235 237 [43]. Copyright 2016, Elsevier; (D) Reproduced with permission H. Yoon, H. Kim, S.S. Latthe, M. Kim, S. AlDeyab, S.S. Yoon, A highly transparent self-cleaning superhydrophobic surface by organosilane-coated alumina particles deposited via electrospraying., J. Mater. Chem. A 3 (21) (2015) 11403 11410 [44]. Copyright 2015, Royal Society of Chemistry.

by Li et al. a hierarchical structured superhydrophobic coating with nanoflakes was formulated via electrodeposition and solution-immersion processes. The water contact angle was about 157 and sliding angle around 3 after fluorination modification. The prepared self-cleaning superhydrophobic coating was investigated for anticorrosive properties. The fabricated coating had higher Ecorr (20.346 V) and lower Icorr (4.128 3 1026 A/cm2) values compared to the bare steel substrate (Ecorr 5 20.684 V, Icorr 5 7.270 3 1026 A/cm2). The superhydrophobic coating was also investigated for the antiscaling property, and this was confirmed when CaCO3 crystals on the superhydrophobic coating had needle-like features compared to the rhombohedral CaCO3 crystals on the surface of the bare steel substrate.

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FIGURE 17.7 (A) Photos of water droplets on the superhydrophobic coating surface; (B) the superhydrophobic coating can sustain the water droplet due to intrusion pressure (Δp) . 0; (C) sliding process of the water droplet on the superhydrophobic coating; (D) Tafel plots of the bare pipeline steel and the superhydrophobic coated pipeline steel; (E) schematic mechanism of the anticorrosion process; (F and G) SEM micrographs of the superhydrophobic coating before and after the potentiodynamic polarization test. Source: Reproduced with permission H. Li, S. Yu, X. Han, Y. Zhao, A stable hierarchical superhydrophobic coating on pipeline steel surface with self-cleaning, anticorrosion, and anti-scaling properties, Colloids Surf. A: Physicochem. Eng. Asp. 503 (2016) 43 52. Copyright 2016, Elsevier.

The superhydrophobic coating maintained good long-term stability in air, mechanical and thermal stability under certain environment. After the polarization test, the aggressive NaCl solution did not destroy the robust superhydrophobic coating which had dandelionlike hierarchical structures. An illustration of the coating properties is presented in Fig. 17.7 [45]. A self-cleaning superhydrophobic coating with a contact angle of 155.2 6 0.5 and a sliding angle of 3.5 6 1.3 was fabricated by Zheng et al. using a facile and low-cost method and investigated for anticorrosive protection on an aluminum surface. Different sample surfaces were examined during the study. Myristic acid modified samples were denoted as MA-x, and the reference substrate was anodized Al samples without surface modification denoted as AAO-x, where x represents the value of anodization voltage. In Fig. 17.8 samples A, B, C representing hydrophilic aluminum (Al), superhydrophilic (AAO-20), and

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FIGURE 17.8

The self-cleaning process of (A) Al; (B) AAO-20; (C) MA-20 pristine samples before the test; (D) Al; (E) AAO-20; (F) MA-20 samples after immersion in a dirty solution for 1 min and (G) Al; (H) AAO-20; (I) MA20 cleaned samples after water spray and drying; (J) potentiodynamic polarization curves of Al, MA-0, and MA20. Source: Reproduced with permission S. Zheng, C. Li, Q. Fu, W. Hu, T. Xiang, Q. Wang, et al. Development of stable superhydrophobic coatings on aluminum surface for corrosion-resistant, self-cleaning, and anti-icing applications, Mater. Des. 93 (2016) 261 270. Copyright 2016, Elsevier.

superhydrophobic coating (MA-20) were immersed in a dirty solution and dried. Dirt accumulated on Al (D) and AAO-20 (E) surfaces, but a little dirt was observed on MA-20 coating (F). After spraying water on the studied surfaces, the MA-20 coating (I) was as clean as before, in comparison to both Al (G) and AAO-20 (H) surfaces which were still covered with a significant amount of dirt. The findings revealed that it is difficult for dirt to attach to a superhydrophobic surface and if any, can be easily cleaned by spraying water. This is due to the joint action of high capillary forces induced by water droplets and weak adhesion of dirt to the superhydrophobic surface. The anticorrosive performance of the coatings was investigated using potentiodynamic polarization technique in comparison with the uncoated Al substrate (Fig. 17.8J). The Ecorr of the superhydrophobic MA-20 coating was 59 mV more positive than the bare Al substrate and 39 mV higher than the hydrophobic MA-0 coating. The corresponding Jcorr (1.527 3 1029 A/cm2) was reduced by two orders of magnitude compared to the bare Al substrate. Meanwhile, the MA-20 superhydrophobic coating showed higher polarization resistance (Rp), 408 times that of the bare Al substrate and 147 times that of the hydrophobic MA-0 coating. In comparison with other previously reported superhydrophobic anticorrosive coatings, the results showed a remarkable corrosion resistance on Al and its alloy [46]. A recent study by Li et al. on superhydrophobic multiwalled carbon nanotubes dispersed in a thermoplastic elastomer (MWCNT/TPE) smart coating revealed the development of a high-performance coating with sensing ability toward stretching, bending, and torsion. The intelligent coating can be easily fabricated under ambient conditions with no special requirements for cleaning or activation of the substrate. The superhydrophobicity of the coating is maintained irrespective of the substrate (glass, plastic, cloth, and metals) for self-cleaning, drag reduction, or other related applications. The coating showed

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excellent stability to UV radiation, acid/alkali stress, repetitive bending and kneading, and extreme repellency to acidic/alkaline droplets. The smart coating can be exploited as a flexible, high-performance, and multifunctional wearable sensor in human healthcare and human machine interface applications, and being superhydrophobic can be used under wet and corrosive conditions [47]. Regarding the polymer/oxide nanocomposite coatings, Ding et al. fabricated the superhydrophobic self-cleaning fluorinated polysiloxane/TiO2 nanocomposite coatings with longterm durability [48]. Manca et al. synthesized the self-cleaning and antiglare coating on glass plates using modified nanosilica particles. Their nanocoatings exhibited an enormous water repellency (contact angle 5 168 ) and stable self-cleaning property during 2000 hours of outdoor exposure [49]. Bayer’s group developed several superhydrophobic coatings for various applications, by using nano-silica particles and flourinated polymers [50 52]. 17.2.4.2 Antigraffiti Coatings Graffiti is a significant challenge for architectural heritage materials. Graffiti affects a wide variety of surfaces, and the cleaning is costly and quite often, the penetration into the pores contained in the substrate material induces an irreversible effect onto the painted surface. Antigraffiti coatings have been developed by functionalizing nanoparticles and polymers to form a coating repellent to both water and oil simultaneously. As a result, the coated surface can be a non-stick surface, an easy to clean surface, and also able to withstand repeated graffiti attacks [8,53]. To enable hassle-free cleaning of the substrates fluorinating agents have been used to lower surface energy of the coating by migrating onto the coating surface. This offers good hydrophobicity reducing the adherence of staining agent to substrates. Also, outstanding thermal and oxidative stability, low coefficient of friction, and good chemical resistance are achieved with its use [54]. Fluoropolymers exhibit a great deal of oleophobicity since the extent of electronegativity and low polarizability of fluorine atoms in the backbone or side chains induce hydrophobic and oleophobic properties in the coatings synthesized from them [55]. Furthermore, most commercially available antigraffiti paints are siloxane/silicone-based formulations. They repel most of the waterbased paints and markers [56].

17.2.5 Bioactive Coatings 17.2.5.1 Antifouling Coatings Biofouling occurs when microorganisms in water adhere to wet surfaces, multiply, and gradually cover the surface with biofilm. The effects of biofouling costs marine, shipping, and other global industries billions of dollars every year [57]. Fouling occurs in two categories: micro- and macrofouling. Antifouling coatings are meant to reduce biofouling and its attendant effects and are categorized into biocide-free coatings and biocide-containing coatings [1]. Specifically, antifouling coatings can be further classified into chemically active self-polishing coatings (SPC) with booster biocides, or silicone- and fluorine-based fouling release coatings (FRC), based on physicochemical and mechanical effects. The efficacy of SPC is connected to biocidal effects, while for the biocide-free FRC, the adhesion between foulant and the surface is minimized due to the low surface energy and elastic

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FIGURE 17.9 (A D) Marine antifouling coating approaches. Source: Reproduced from A.G. Nurioglu, A.C.C. Esteves, G. de With, Non-toxic, non-biocide-release antifouling coatings based on molecular structure design for marine applications, J. Mater. Chem. B 3 (32) (2015) 6547 6570 [61]. Licensed under a Creative Commons Attribution 3.0 Unported Licence.

modulus. This ensures that biofouling can be removed by hydrodynamic stress during movement or mechanical cleaning. FRC fails to prevent colonization of biofilm but inhibits the adhesion of most macrofouling under dynamic conditions. The biofilms predominantly consist of diatoms which adhere tenaciously to hydrophobic surfaces and are not released from FRC, even on vessels operating at high speeds such as .30 knots [58 60]. Fig. 17.9 illustrates the marine antifouling coating approaches. Marine biofouling is of great concern to the maritime sector due to its related substantial economic losses since substrates are rapidly colonized by both micro- and macroorganisms when immersed in seawater. This process is accountable for important economic and ecological effects related to shipping hulls, which increase fuel consumption and drydocking operations. This further leads to significant increases in pollution and financial burden [58,62]. Characteristics of an effective antifouling coating should include: antifouling properties, durability, good adhesion, corrosion inhibition, smoothness, easy to apply, fast drying, economical, and readily available, etc. [63]. Over time, tributyltin (TBT) has been the most widely used chemical antifoulant compound. To avoid the known adverse effects of TBT and its derivatives on marine environments, rigorous studies have been ongoing towards obtaining eco-friendly antifouling coatings able to withstand the rigours of practical applications such as shipping. As a significant advancement towards the production of eco-friendly antifouling paint, natural, biodegradable compounds have been utilized, such as bacteria isolated from living surfaces in the marine environment to produce chemicals that are potential antifoulants [62].

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Self-cleaning foul release coatings have been developed by Selim et al. in a recent study and the anticorrosive performance was investigated through salt spray test. Low-surface free energy, low microroughness, and ultrasmooth topology of self-cleaning surfaces prevent fouling settlements. Amongst the concentrations studied, the addition of welldispersed 0.5% ZnO SiO2 nanospheres minimized the surface tension inside the polymer matrix, leading to enhanced surface inertness against fouling adhesion alongside excellent mechanical and anticorrosive properties in aqueous salt fog environments. Foulant investigated for biofilm formation were Gram-positive (Micrococcus sp.) and Gram-negative (Pseudomonas putida) strains of bacteria and the fungi (Aspergillusniger), which are widely used in the evaluation of the antifouling performance of marine paints. In a nutshell, the nanocomposite coating demonstrated inert and superhydrophobic properties with a contact angle of 165 6 2 . Superior physical characteristics, lotus effect, thermal stability, long-term durability under UV radiation, and resistance against a wide range of pH solutions was observed making them promising as efficient, eco-friendly fouling release selfcleaning coatings for ship hulls without biocides [64]. Biodegradability measurements of the studied nanocomposites against the different foulant are presented in Fig. 17.10A and a comparison of the prepared nanocomposite smart coating with some commercial fouling release coatings is shown in Fig. 17.10B. Furthermore, antifouling coatings developed by incorporating butenolide derived from marine bacteria into biodegradable poly(ε-caprolactone)-based polyurethane were investigated by Ma et al. Mass loss measurements revealed that the polymer degraded in seawater and the degradation rate increased in the presence of marine organisms or enzymes. Butenolide was released from the biodegradable polymer for at least 3 months, and the release rate dependent on both the concentration of butenolide and temperature. With the incorporation of a naturally occurring resin (rosin) into the biodegradable polymer, the self-renewal rate increased, and afterwards, the release rate of butenolide improved. A field test indicated that the system had excellent antifouling properties [65]. 17.2.5.2 Antibacterial Coatings Several factors dominate bacterial interactions and one such factor is the surface charge. The bacterial surface charge varies depending on the species, the containing medium, the age of the bacteria, and the rod or round surface structure [66]. Bacteria survive by attaching to solid substrates via biofilms, where they can persist for extended periods, acting as a reservoir of pathogens and multiplying their pathways of transmission. In the biofilms, they are drastically more resistant to antibiotics and external forces and can withstand host immune responses. In-dwelling devices and implants, as well as surfaces in the nearpatient environment, play a significant role in the spread of hospital-acquired (nosocomial) infections. These nosocomial infections can be attributed to Gram-negative bacterial pathogens, for which there is an inadequate supply of antibiotics. The need for antibacterial coatings is strongly stimulated by the increasing urgency of identifying alternatives to the traditional administration of antibiotics. Hence, the development of controlled release strategies is vital to optimize therapeutic effects [67]. A study by Su et al. revealed that a shaking condition applied to the self-polymerization of dopamine in alkaline solution could facilitate the formation of roughened polydopamine (rPDA) coatings which remarkably enhanced antibacterial activities against Gram-positive Staphylococcus aureus, and

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FIGURE 17.10 (A) Represents biodegradability measurements of the virgin polydimethylsiloxane (PDMS) and filled ZnO SiO2/PDMS nanocomposites against different foulant. (B) A comparison of the prepared PDMS/ ZnO SiO2 nanocomposites with some commercial fouling release coatings. Source: Reproduced with permission M.S. Selim, M.A. Shenashen, A. Elmarakbi, N.A. Fatthallah, S. Hasegawa, S.A. El- Safty, Synthesis of ultrahydrophobic and thermally stable inorganic organic nanocomposites for self-cleaning foul release coatings, Chem. Eng. J. 320 (2017) 653 666. Copyright 2017, Elsevier.

Gram-negative Escherichia coli and Pseudomonas aeruginosa. The cell membranes of the bacteria incubated with the rPDA coatings were damaged by the contact-kill mechanism [68]. The gradual release of antibacterial agents, contact-killing, and antiadhesion/bacteriarepelling approaches are the three primary strategies for designing antibacterial coatings [67,69], as demonstrated in Fig. 17.11A C. As can be used with a low dose, recently nanoparticles have been reported as an alternative to common antibiotics. They could affect bacteria simultaneously through various processes, such as: (i) production of reactive oxygen species (ROS); (ii) electrostatic interaction with the cell membrane; (iii) ion release; (iv) internalization [70]. Many studies have

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FIGURE 17.11 Schematic of bacterial action in contact with (A) non-antibacterial coating; (B) antibacterial coating featuring contact-killing mechanism; (C) antibacterial coating featuring antiadhesive mechanism.

focused on the antibacterial coatings, which incorporated these antibiotic nanoparticles. In the case of noble metal nanoparticles, Kuma et al. reported that silver-nanoparticle (AgNPs) paints exhibited the high antimicrobial activity against both Gram-positive human pathogens (S. aureus) and Gram-negative bacteria (E. coli) [71]. Similarly, Ploux et al. reported the antibacterial action against E. coli of AgNPs-loaded plasma polymer coatings [72]. Regarding the metal oxide nanoparticles, nano-TiO2 and nano-ZnO particles are mostly used in the antibacterial polymer nanocomposite coatings [73,74]. To enhance the antibacterial activity of AgNPs, the hybridization of AgNPs and other oxide nanoparticles is a promising approach. Chudasama et al. reported that Fe3O4 Ag core shell nanoparticles have a better antibacterial activity against the Gram-negative bacteria (including E. coli) than that of AgNPs [75]. In the case of silver ferrite nanocomposites, Kondala et al. indicated that their antibacterial activity was higher than that of AgNPs and other antibacterial drugs [76]. Ngo et al. also reported that Fe3O4 Ag dumbbell-like hybrid nanoparticles had a higher antibacterial action against E. coli than the lone AgNPs [77]. On the other hand, for nano-TiO2 and nano-ZnO particles, their production of ROS could be enhanced by their hybridization with noble metal nanoparticles. In the case of Au TiO2 nanohybrids, Zhang et al. reported that the antibacterial action against E. coli was more than five times higher than that of pure TiO2. Chen et al. also indicated that AuNPs (5 nm) decorated nano-TiO2 particles had higher antibacterial activity against E. coli, than that of pure nano-TiO2 particles. Regarding the Au/ZnO nanohybrids, He et al. observed that the deposition of AuNPs onto nano-ZnO particles significantly increased the light-induced generation of hydroxyl radical, superoxide and singlet oxygen, holes, and electrons [78 80].

17.2.6 Fire-Retardants Coatings 17.2.6.1 Intumescent Coatings Intumescent coatings are used as a protective material for fire resistance by decreasing heat transfer to the substrate structure. An intumescent coating swells with heat exposure

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leading to increasing volume and decrease in density. They can be applied to structural members as a fireproofing paint. Intumescent paint contains many ingredients such as acid source, a charring agent, blowing agent, binder, etc. The binder is a crucial part of an intumescent coating. Its primary task is to bind all compounds, but it also acts as a carbon source and influences the foaming process. The role of different binders and their chemical reactivity have been investigated. It was found that the thermal insulation much improved when using a mixture of a linear copolymer presenting a good reactivity with the acid source and a cross-linked copolymer as the binder in the intumescent paint [81]. A series of intumescent coatings with different binders of vinyl ester/ethylene, vinyl acetate/ethylene, vinyl acetate/acryl ester, and vinyl acetate/vinyl ester of versatic acid from two different manufacturers were investigated regarding insulation, foaming, mechanical impact resistance, and residue morphology. The insulation performance had moderate influence with different binders, whereas the foaming dynamics, thickness achieved, and the inner structure of the residues showed a rich variety [82]. The selection of suitable flameretardant fillers influences the physical and chemical properties of the coatings. Intumescent coatings made of an acrylic binder and flame-retardant ingredients on steel substrates showed better fire protection as well as mechanical properties. It was found that the combination of aluminum hydroxide (Al(OH)3) and titanium dioxide (TiO2) had significantly improved the fire protection, thermal stability, and water resistance [84]. Intumescent coatings made of epoxy resin as a binder with variable concentrations of coconut fiber (CCN), wood waste (MDP), and peach stones (PEA) biomasses were studied. The optimum dry mass percentage of CCN and MDP was 9%, while 6% was ideal for PEA. The total expansion of the coatings after fire resistance testing was more than 600% for CCN9, 1500% for blank, 1300% for MDP9, and 1600% for PEA6. The biomass-based intumescent coatings showed better thermal insulation with the maximum temperatures on the back of substrate at 120 C, whereas it was at 474 C for uncoated steel [85]. The effect of fly ash cenospheres on the heat shielding performance was studied. Less feasibility of the heat transfer to the substrate was observed due to the large quantity of residual char (39.30 wt.%) containing titanium phosphate and boron phosphate [86]. Influence of kaolin clay on the expansion of coating and heat shielding was investigated. The coating with 5 wt.% of kaolin clay was enhanced up to 49% residual weight than that of the kaolin-free counterpart [87]. Intumescent flame-retardant coatings that incorporate chicken eggshell (CES) waste as the filler along with three flame-retardant additives, namely, ammonium polyphosphate phase II, pentaerythritol, and melamine in the acrylic binder has proven to be efficient in the protection of plywood against fire [83]. Epoxy resin coatings prepared with ginger powder, coffee husk, and other vegetable compounds as the carbon source showed better flame resistance [88]. 17.2.6.2 Nonintumescent Coatings Active nonintumescent coatings release fire-retarding species including gas phase radicals from the formulations of water-soluble salts containing phosphate, nitrate, halides sulfate, sulfamate, boric acid, and borax [89]. Cold plasma polymerized fluorinated acrylate on polyamide-6 was made by Errifai et al. for fire resistance. The deposit reduced both peak heat release rate (HRR) and time to ignition (tig) by 50%. This reduction was due to the dilution of combustible gases caused by the reaction of CFx radicals with the

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degraded polymer fragments [90]. Methacrylated phenolic melamine (MAPM) incorporated epoxy acrylate gave better flame retardancy. It was found that decomposition of melamine released nonflammable nitrogen volatiles like ammonia with no increase in char formation [91]. C ¸ akmakc¸i et al. prepared flame-retardant epoxy acrylate coatings with the incorporation of phosphate containing allyldiphenylphosphine oxide (ADPPO) [92]. Phosphoruscontaining UV-cured hyperbranched polyurethane acrylate showed a limiting oxygen index of 27.0. Synergistic effect of phosphorus and nitrogen at the phosphorus content around 0.7 wt.% gave the best value. The breaking of P O C bonds formed the P O P bonds. The presence of phosphorus in coating supported the char formation, which reduced the flame attack of underlying polymeric materials [93].

17.2.7 Other Smart Coatings 17.2.7.1 Antifingerprint Coatings These smart coatings provide a fingerprint-hiding effect. Antifingerprint coatings are widely used for touch screens in consumer electronic devices. With the release of surfactants or enzymes in response to surface contact, the deposited fingerprints become faded [5]. Furthermore, antifingerprint surfaces should have low energy, for the formation of an oleophobic weak boundary layer. Low energy surfaces are known to reduce attractive intermolecular forces [94]. The fingerprint problem on a touch screen surface is a pressing issue requiring antifingerprint coatings. The development of protective coating materials with amphiphobic (water and oil repellent) properties can resolve this with amphiphobic (water and oil repellent) properties. The amphiphobic property can be achieved by the construction of the morphological structure with re-entrant curvature in combination with the chemical composition and roughness on surfaces. Derived from a nano-scaled concave structure consisting of cavities, in which the capillary force produced at the liquid air interface inside the re-entrant can repel liquid (water or oil) from entering the void. In a study by Siriviriyanun and Imae, an attempt to make antifingerprint properties from nonfluorinated organosiloxane self-assembled monolayer-coated glass surfaces was achieved. The substrate with nonfluorinated (methyl terminated) organosiloxane (TMS glass) provided both amphiphobic and antifingerprint features on the surfaces. Also, the hybrid with both hydrophobic and nonfluorinated moieties (octadecyltrimethoxysilane, ODS, and trimethoxymethylsilane, TMS) imparted the same properties evaluated by the contact angle of oleic acid. It was concluded that antifingerprint is related to amphiphobicity [95]. Furthermore, the use of fluoride films on glass substrates of electronic panel devices providing antifingerprint effects experiences adhesion problems. A thin SiO2 layer should be inserted between the fluoride film and the glass to overcome the poor adhesion. An attempt to realize an enhanced mechanical durability of an antifingerprint coating by further incorporation of silver nanoparticles has been reported. For strong adhesion, the SiO2 layer was inserted between the silver nanoparticles and the glass substrate and the fluoride/SiO2 films deposited on that composition. An efficient antifingerprint property based on a wetting angle of about 116 was achieved [96]. Recently, both antifingerprinting and antibacterial effects from 30 nm thick ZnO thin films without an additional protective layer

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for smartphone panel application have been achieved. As a result of a simple annealing treatment of the film, high transmittance (B91.3%) comparable to that of a glass substrate at a wavelength of 550 nm was realized [97]. 17.2.7.2 Antireflective Coatings The idea of antireflective coatings in physical sciences dates back to Lord Rayleigh (John Strutt) in the 19th century when he observed the tarnishing on a glass increasing its transmittance rather than reducing it. However, Fraunhofer in 1817 produced antireflective coatings when he noticed that reflection was decreased due to etching a surface in an atmosphere of sulfur and nitric acid vapors. The quest for ways to maximize light collection efficiency has encouraged countless investigations for antireflective coatings to cater for the growing demand of optical and optoelectronic equipment in diverse areas of application [98]. With antireflection coatings, there is an increased performance of optical components fabricated from glass-based optical materials and a reduction in reflective losses at interfaces. Thus, antireflective coatings on flat-panel displays in electronics eliminate the effects of spurious images or veiled glares originating from stray and multiple reflections from optical surfaces. High-efficiency antireflection coatings fabricated from phaseseparated polyelectrolyte multilayer films that undergo a reversible pH-induced swelling transition have been realized [99]. Similarly, a double-layer coating based on λ/4 λ/4 index-gradient design wavelength system has been reported. Prepared with dense and porous silica films as high and low index layers separately. When coated on a glass substrate, a satisfactory broadband antireflective performance was realized for the demand of the amplifier blast-shields used in high power laser systems. With a subsequent NH3-heat treatment and trimethylchlorosilane (TMCS) post-treatment, weak mechanical property and optical instability of the coating were overcome [100]. Using modified nanosilica particles, Manca et al. fabricated the antireflective surfaces by a double-layer coating. This antireflection effect was attributed to the nanotextured topology producing by the nanoparticles-based top layer [49]. 17.2.7.3 Anti-icing Coatings In the modern society, icing is a source of a variety of problems. The icing on the wings and surfaces of aircraft could cause crash accidents. During flights, aircraft intercept supercooled water droplets through clouds or encounter freezing rain. The impacting water freezes rapidly to form deposits of ice. The ice accretion results in drag increase and sometimes may lead to dangerous loss of lift force, which may cause tragic aircraft crash accidents. Similarly, frost and ice accumulation in heat exchangers and refrigerators often results in a decrease of heat transfer efficiency up to 50% 75% due to the frost formation [101]. Icing can result in major electrical outages, interrupt offshore oil and gas production, and decrease the efficiency of wind power generation [102]. The need for anti-icing strategies and coatings cannot be overemphasized. Anti-icing coatings are coatings with properties that can prevent or delay freezing of the impacting and condensed water as well as decrease the ice adhesion on its surface. On such surfaces, condensed water droplets can spontaneously jump away before freezing due to its superhydrophobicity. In another scenario, the coating is impregnated with antifreeze lubricants [101,102]. Anti-icing properties of coatings may depend on the following (i) the state of the solid surface if colder than the

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air/vapor; (ii) how large the temperature gradient is; and (iii) if a thin film of water tends to form on the solid surface due to capillary effects, disjoining pressure, etc. [103]. An ideal superhydrophobic surface does not allow frost formation due to the coalescence induced self-removal of condensed water. But in reality, most of the superhydrophobic surfaces have been susceptible to the frosting and the initiation of any defect on the coating surface further encourages frost formation [101,102]. To improve the anti-icing performance of aircraft, the superhydrophobic anti-icing coatings should be effectively deposited on the aluminum and copper substrates. In a contribution to overcoming the shortcomings of antiicing coatings, projects such as ICECOAT project have developed new types of coating matrices, new nanoparticles for nanocomposites, and a new method for coating surface modification to produce anti-icing coating with superior hardness and erosion resistance [103]. Similarly, Zheng et al. have fabricated anti-icing coatings with low ice adhesion strength and higher contact angles for aluminum substrates in contribution to overcoming ice accumulation on aluminum surfaces at low temperatures [46]. Furthermore, focus on bio-inspired strategies for anti-icing are ongoing. A hybrid method of icing-prevention by antifreeze dispensing and by repelling impinging drops has been developed. The fabricated bio-inspired anti-icing coating was aimed at reducing the need for antifreeze liquid by dispensing antifreeze only when required and directed towards having significant economic and environmental benefits. The semipassive coating consisted of a porous superhydrophobic epidermis and wick-like underlying dermis that was infused with antifreeze liquid. The outer layer served as a barrier between the antifreeze and environment. The coating was architectured to respond remarkably when eventually iced over by releasing stored antifreeze liquid amongst other factors [102]. To reduce ice deposition, Li et al. have fabricated the hydrophobic polydimethylsiloxane/nano-silica hybrid coating. The authors indicated that their hybrid coating exhibited super hydrophobicity, both in terms of multiscale and low surface energy [104]. 17.2.7.4 Antifogging Coatings Fogging involves the formation of water droplets on transparent solid surfaces which scatters light and reduces optical transmission. Fogging is a challenge for the everyday use of eyeglasses, goggles, and windshields. Fogging also reduces the efficiency of solar energy panels, medical/analytical instruments, as well as other industrial equipment. In antifogging research, superhydrophilic surfaces with very low θS (,5 ) are well known to prevent water droplets by promoting the formation of a continuous thin film of water condensed from the air, and this has gained significant attention. However, these rely on external stimulation by UV-light. Highly transparent superhydrophilic antifogging coatings were prepared and coated on various substrates, such as glass slides, silicon, copper and poly(methyl methacrylate) (PMMA). The antifogging coatings fabricated with polyvinylpyrrolidone (PVP) and aminopropyl-functionalized, nanoscale clay platelets exhibited 90% transmission of visible light. The chemical/physical properties of the coated surface remained almost unchanged after several antifogging tests and exposure to humid air for 30 days [105]. Similarly, the improvement of water repellency by designing a micro- or nanostructure onto a low surface-energy material surface, with a water contact angle (CA) greater than 150 and a low sliding-off angle of less than 10 , enhances antifogging abilities. Antifogging coatings are therefore relevant for surfaces in cold and humid

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environments. Antifogging coatings with icing-delay properties have been investigated using poly(vinylidenedifluoride) (PVDF) polymer together with zinc oxide (ZnO) which produced ZnO-on-PVDF micro/nanostructure (ZP-MN) composite. On the ZP-MN surface, condensed water droplets were easily shed at 5 C for B1600 s via a slight wind or tilting. Interestingly, the droplets did not completely freeze on the ZP-MN surface at 10 C until B7360 s [106].

17.3 APPLICATIONS AND COMMERCIAL VIABILITY OF SMART COATINGS End-user applications of smart coatings are in the following areas: medical and healthcare, military, aerospace, automotive, marine, energy, consumer electronics, construction, oil and gas, packaging, textiles and apparel industries, etc. Smart coatings can be customized to meet the specific consumers’ requirements [107,3]. Self-cleaning thin nanofilms are applicable on household surfaces. They are safe for skin contact and protect the surfaces from dust or grease accumulation, scale deposition, and corrosion attack. From computer equipment to kitchen surfaces to shower screens these selfcleaning surfaces will minimize the use of detergents and time spent on cleaning [108]. Antifingerprint coatings have found application in optics, kitchens, cafes, restaurants, electronic display screens, textiles, floor sealants, automotive and HVAC (heating, ventilation, and air conditioning) systems, etc. [8]. Self-healing coatings for the aviation industry have been proposed by a team of researchers from the Center for Research in Ceramics and Composite Materials (CICECO), University of Aveiro. When applied to the fuselage of the aircraft, it repairs, as a skin, small discontinuities resulting from mechanical and environmental impacts suffered by appliances during flight by releasing its embedded healing molecules when required [109]. Likewise, an innovative nanopaint technology called Ultra-Ever Dry, a contribution to development in the world’s first selfcleaning car, has been tested by Nissan. It repels mud, rain, oil, sleet, dirt, and everyday road spray minimizing the car wash trips. Although there are no current plans to apply the self-cleaning paint to Nissan’s vehicle lineup as a standard feature, the technology is considered as a potential aftermarket option [110]. A few commercialized products are presented in Table 17.1.

17.4 CONCLUSION The purpose of this chapter is not to provide a complete summary of the literature on the present topic but rather to highlight some of the functions exhibited by these coatings with conceivable industrial relevance. As presented above, many of the smart coatings contain metal and metal oxide nanoparticles, which provide various functional properties and enhanced performance. Their hybrid nanoparticles are expected to produce the new multifunctional coatings. The area of smart or intelligent coatings is, in reality, a hot topic with a wide area of application due to the achievable multifunctionality. The list of coatings exhibiting

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TABLE 17.1 Selected Commercially Available Smart Coatings Products

Characteristics

Applications

Producers

Deletum 5000 & 3000

Antigraffiti paints

Coatings repellant to water and oil.

Victor Castan˜o

2C Marine Sealant PRO

Antifouling coating

Intended for gel coat marine surfaces with no or only minimal sign of wear.

Nanosafeguard

Burgundy Cukote

Antifouling coating

Cuprous oxide self-polishing coating

Sea Hawk

Smart solution

Copper free antifouling

Eco-friendly metal free bottom paint

Sea Hawk

Intercept 8500 LPP

Antifouling coating

Biocidal antifouling for marine vessels

AkzoNobel

ECONTROL

Intelligent solar control glass

Provides shading in the glass

EControl-Glas GmbH & Co. KG

NanoChar

Fire protection coating, VOC-free epoxy intumescent coatings based on nanotechnology

Protects steel amongst other nonmetallic substrates

Intumescent Associates Group

SPD-SmartGlass

Smart window coating

Changes the tint of any window, sunroof, or skylight

Research Frontiers

NANOMYTE

Waterborne, solventborne, superhydrophobic, self-healing, anticorrosion, acid resistant, etc.

Pretreatments, primers, and topcoats

NEI Corporation

Ultra-Ever Dry

Self-cleaning, antiicing, antiwetting, anticorrosion, etc.

Superhydrophobic and oleophobic coating that will repel most water-based and some oil-based liquids. For industrial use only

UltraTech International, Inc.

Victor Castano: http://www.nanotechproject.org; Nanosafeguard: http://www.nanotechproject.org; Intumescents Associates Group: http://www.advancedepoxycoatings.com; NEI Corporation: http://www.neicorporation.com/products/coatings/ anticorrosion-paints-coatings/; Sea Hawk: www.seahawkpaints.com; UltraTech International, Inc.: http://www.spillcontainment. com/; AkzoNobel: https://www.akzonobel.com; Research Frontiers: http://www.smartglass.com; EControl-Glas GmbH & Co. KG: www.econtrol-glas.de/en/company.

intelligent characteristics is gradually becoming inexhaustive, especially when dual to multiple functionalities can now be achieved in most categories due to coating formulation maximizing its potential. With recent developments focused mainly on smart coatings for anticorrosive applications, one of the essential approaches for self-healing coatings is apparently the use of nanocontainers capable of being loaded with active agents and having shells possessing controlled permeability specific to several triggers. Sustainability/durability is related to a continuous healing action. Likewise, leading approaches for self-cleaning coatings involves high superhydrophobicity with extreme repellency to varying corrosive droplets which are maintained irrespective of the metal substrate and also UV stability. Essential strategies for the active corrosion-sensing coatings involve the inclusion of fluorescent or color indicators responsive to varying environmental triggers. These could be any of the

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following: pH, ion, or redox indicators. Approaches for enhancing antifouling coatings are concerned with self-cleaning, water-resistant and superhydrophobic surfaces with lowsurface free energy, low microroughness, and ultrasmooth topology to prevent fouling settlements when biocides are not utilized. Otherwise eco-friendly biocides are encouraged. Besides the self-cleaning and water-resistant properties, superhydrophobic surfaces support antifouling characteristics alongside anti-icing, antiscratching, and antibacterial properties which contribute immensely to anticorrosion [111]. Smart materials and coatings can also be categorized into high-performance materials, property-changing materials, and energy-exchanging materials. Processed as a combination of several constituents to exploit the best properties of the individual constituents in the passive and active modes [112]. Smart coatings containing nanoparticles are versatile in formulation and functionalities. Nanocontainers of various types of materials coupled or loaded to obtain different active centers gain more attention for the innovations and application in various smart systems from drug delivery through bioactive surfaces to corrosion protection and beyond. Presently, efforts are devoted to the up-scaling of nanocontainer production and evaluation [113]. The challenge of translating smart coatings concepts in fundamental academic research into practical coating systems with commercially viable industrial applications is still of great concern. This obviously requires a lot more dialog and collaboration between academics and industrialists to bridge the gap between the technological progress and the market demand. Within the industry, there is continued product development, and the focus is on higher performance, not only to fulfill customer needs but also to comply with regulatory requirements. Whereas, the results from the academia aided by computer simulations may contribute to providing some leverage to addressing certain technical difficulties in the coatings industries. Furthermore, durability is a key challenge to advancing current smart coating technologies for external and internal applications in the different sectors where it is being utilized. Similarly, adding functionality to coatings often increases cost which is another challenge. But, effective multifunctional smart coatings in real-time service will boost market demands far beyond predictions [5,114].

17.5 SOURCES OF FURTHER INFORMATION The following articles and patents provide additional insights and advances in antibacterial coatings [115,116], antifouling coatings [117,118], antigraffiti coatings [119], antireflective coatings [120,121], anti-icing coatings [122,123], and antifogging coatings [121,124]. Also, some website sources include [125] (www.coatingsworld.com), [126] (www.european-coatings.com), and [127] (www.paint.org).

Acknowledgments Sarah B. Ulaeto acknowledges Council of Scientific and Industrial Research (CSIR) and The World Academy of Science for Developing Countries (TWAS) for the award of CSIR TWAS Postgraduate Fellowship. The authors appreciate Knowledge Resource Center of CSIR NIIST for access to some valuable articles utilized in the preparation of this manuscript.

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