[
    {
        "id": 1,
        "chunk": "# A review on protective polymeric coatings for marine applications  \n\nShatakshi Verma $\\textcircled{1}$ , Smita Mohanty, S. K. Nayak $\\circleddash$ American Coatings Association 2019  \n\nAbstract The main objective of this review is to discuss the recent research on polymer-based surface coatings contributing to the protection against marine biofouling based on the knowledge available in literature, supplemented by means of figures, mechanism illustrations, mathematical models, and equations. A need for studies on the mathematical behavior of such coatings is emphasized, composed of quantitative evaluation of foul-release performance of coatings using mathematical equations of the concerned parameters. Apart from the synthesis of protective polymeric coatings, understanding the relationship between characteristics of coating materials and accompanying foulrelease and antifouling mechanism is important. In this regard, efforts have been made to equally evaluate, simulate, and measure the appropriate performance of the coatings. By examining the physicochemical and mechanical properties of the polymers, adhesion behavior has been found to be one of the prerequisites for the success of polymeric coatings for marine applications. The potential development of a broad spectrum of methods used to evaluate the foul-release performance of polymeric coatings depending on adhesion behavior of fouling organisms with the coatings has been discussed. From the analysis of the factors affecting degradation of coatings, environmental interference is declared a key factor for complete degradation of polymeric coatings. This review opens up new research directions to improve the adhesion performance of polymeric coatings for ship hulls designed with tunable viscoelasticity by the incorporation of elastomeric polymers (like polydimethylsiloxane) into stiff polymers (like epoxy resins) with and without the utilization of additives, modifiers, and nano-fillers. In addition, it provides methods to improve the foul-release performance of such coatings that work on adhesion behavior of biofouling species attached to the coating surface.",
        "category": " Abstract"
    },
    {
        "id": 2,
        "chunk": "# Graphical abstract  \n\n![](images/7494b8dd659e0c7c8fff507d02cb4278732ec2691f79b76121fe834f13ba325a.jpg)  \n\nKeywords Polymeric coatings, Foul-release, Adhesion strength, Epoxy–PDMS (polydimethylsiloxane) hybrids",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# Introduction  \n\nMarine biofouling is the undesirable adhesion and settlement of various marine species (flora and fauna) classified in two broad categories: microorganisms (bacteria, algae, diatoms, sponges) and macroorganisms (mussels, balanus, barnacles, hydroids, etc.).1–3 The adverse effects of marine biofouling on the performance of marine vessels include deterioration of the vessel’s surface, increase in weight of the vessel, reduction in speed, increased fuel consumption, increase in the emission of $\\mathrm{SO}_{x}$ , $\\mathrm{NO}_{x}$ , $\\mathrm{CO}_{2}$ ), causing a threat to the marine environment, leading to the worldwide problem related to loss of billions of dollars per year on fuel and maintenance.2,4 Polymeric materials play a vital role in manufacturing of surface coatings, which endeavor to protect and preserve various substrates from corrosion, wear, or any sort of chemical and biological invasion.5  \n\nPolymers entered the field of advanced application, during the last 15 years, noticing an abrupt increase in their demand in the field of research and development contributing in several ways to the betterment of mankind. The results of a survey on scientific production from the years 2000 to 2016 revealed that half of the global scientific production deals with polymers and polymer modifications.6 Most of the protective coatings contain several types of long-chain polymers. Altering the nature of polymers, under the influence of inorganic and/or organic precursors, they are often processed from multicomponent solutions, in which macromolecular and solvent species interact in complicated ways to influence material structures and functions to bring about new technologies for the coating industry.7,8 Surface coating is a simple and efficient method for the protection of metals by outsourcing polymeric materials, normally to serve the following purpose—to protect the surface, or to control friction and wear, or to alter physical properties, by modifying the reactivity of polymers by adding one or more functionalities to them.9 Coatings are indispensable to the industrial sector of every nation’s economy. The amelioration in numerous advanced coating technologies can be done by the combination of advantageous bulk properties of polymeric materials with surface-selective chemical conversions or surface modifications.5  \n\nThe prerequisite for designing multifunctional polymeric coatings is to manufacture a material with conflicting properties like flexibility, hardness, ease of handling, thermal stability, low surface energy, corrosion, abrasion, chemical, and foul resistance along with good adhesion to the substrate.10 The primary concern for the success of polymeric coatings is its appropriate and sufficient curing behavior resulting in threedimensional crosslinked networks. In this regard, Pathania et al.11 studied the polymer coatings with respect to their crosslinking phenomenon, along with the various parameters that control crosslinking in polymeric coatings, like diffusion studies. Polymer coatings can be synthesized by various methods and the solvent plays a crucial role, due to the presence of variety of polymers available to mankind from rubbery to glassy polymers.11  \n\nThe synergistic effect of polymeric coatings opens a new arena of research in various directions, likewise, the combination of superhydrophobic and biomimetic polymeric materials results in a large number of potential applications. Polymer coatings incorporated with foreign materials like superhydrophobic particles or nanofillers serve as a boon for the coating industry.1 Guangzhao et al.13 studied the self-renewing ability of certain polymeric resins like rosin, acrylic resin, and chlorinated rubber, giving rise to the selfsmoothening surface of the protective coating. The polymer coatings developed were able to form a selfpolishing surface, possessing antifouling capability in both static as well as dynamic conditions.13 Wooley et al.14 studied another excellent class of polymers for the formulation of robust, sustainable polymer coatings with long-term retention in properties. To enhance the general coating properties like flexibility, hardness, and adhesion, liquid crystalline polymers were employed for their excellent environmental, chemical, and weather resistance, wide thermal stability range, and fracture toughness. Block copolymers have attracted many researchers since the development of the first living polymer which was synthesized 50 years ago.14 The surface tension properties of polymer surfaces were greatly exploited for the design of hydrophobic segments by the past researchers as stated by Wynne et al.15 The increasing capability of polymers is being continuously tailored to fulfill the specific needs of the society, by chemically modifying their surfaces and/or bulk properties via wet chemistry or by following the route of physical or biological modifications as described in the literature.6 In a similar context, Wei Huang et al.16 described a means of modifying solid polymers at their surface for partial fulfillment of potential applications that was proposed in their work, which intends to incorporate a few surface-active additives at the time of polymer fabrication.16 A variety of polymers like surface-active, zwitterionic, polysaccharides, and hydrophilic polymers (such as polyethylene glycol), have been used to synthesize functional polymer coatings and brushes, polycationic, and fouling-release coatings for marine applications.1 Garcia et al.18 extensively reviewed research contributing to the utilization of polysiloxanes, acrylic and methacrylic polymers, hyperbranched and dendritic polymers, as potential and safe precursors for the synthesis of environmentally benign marine coatings. Du et al.19 in their study on extensively used polymeric materials to design and formulate antifouling surfaces and coatings against protein and bacterial adsorption reported about hydrophilic, hydrophobic, zwitterionic, polysaccharides and other potential polymers with respective mechanism of antifouling action. 19–21 The synergistic effect of amphiphilic polymer coatings employing liquid crystalline polymers gives rise to superior coating technologies, stable in environmentally challenging conditions, resulting in dynamic and robust crosslinked polymer coatings.22 Ferrari and Benedetti23 thoroughly studied the role of self-healing polymers for the development of surface-finishing superhydrophobic surfaces. Thus, polymeric coatings serve as a type of green technology and thereby find potential applications in the fields of fouling and corrosion prevention.23  \n\nAccording to the American Coatings Association (ACA), out of several coatings (Fig. 1),24 marine coatings have been explored extensively as highperformance coatings, employing state-of-the-art technologies. These coatings have been based on advanced polymer technologies, manufacturing effective products brought to the market on a regular basis by leading coating industries.25 The major class of foulingrelease coatings is a group of synthetic polymers generally called low surface energy polymers (like siloxane and fluoropolymers).26 Several excellent reviews $^{1-4,27-33}$ have covered existing trends on marine protective coatings based on polymeric materials being the potential monomers of coatings either employed in a matrix or reinforcement form. In both forms, polymers fill the implementation gap of the conventional TBT or metal-based coatings, thereby being nonhazardous for marine environments.  \n\nMost studies dealing with the development of antifouling and foul-release coatings lack studies on innovation in performance evaluation of such systems through the development of self-sufficient laboratory setups to measure the antifouling and foul-release performance of the formulated paints in terms of toxicity levels, adhesion strength, drag-reduction efficiency, bacterial adhesion, etc. Therefore, in this regard to bridge the gap between the prevailing systems and the future developments in the field of marine coatings, this review lays emphasis on the performance evaluation of such coatings. More emphasis on the determination of variables like attachment strength, rate of adhesion, hydrolytic degradation of polymer, velocity gradient determination, antidrag performance, growth of algal bloom, removal force, and swelling ratio has been given. The methodology for evaluation of mechanical strength using shear lag model, measurement of vertical force from the buoyancy force, dispersion studies, and encapsulation time of fillers using Taylor’s model in the case of filled polymer systems, role of binders and antioxidants in the design of antifouling and foul-release polymeric coatings have been discussed. This article reviews the recent progress on protective coatings for marine applications and suggests the quantitative approach to judge the antifouling as well as foul-release efficacy of the designed surface coatings.",
        "category": " Introduction"
    },
    {
        "id": 4,
        "chunk": "# Polymeric coatings for marine applications  \n\nPolymeric surface coatings can bestow a wide range of functionalities, and depending upon the functionality, they also make a way for suitable applications. Suitable here denotes the ability of the polymeric coatings to tune them according to the desired profile of application. Apart from this, the coating must fulfill several requirements like adhesion to the substrate, desired mechanical and functional properties, viz. scratch/wear resistance, hydrophobicity/hydrophilicity, antibacterial, antifouling, antistatic, chemical resistance, etc.6 Another important condition for suitability is temperature of coating application, without damaging the substrate onto which coating is applied. Pertaining to their extended usages and increased applications in marine paint industry, these have been grouped into three broad categories: superhydrophobic, antifouling, and foul-release coatings, which have been used for centuries and engineered recently for specialized protective applications.34  \n\n![](images/641ffcc2bb279dffbd33b3506b74ee335ba10857ea04f15990077fbc56167cb2.jpg)  \nFig. 1: The types of coatings as per ACA and US Census Bureau Current Industrial Report MA 325F",
        "category": " Introduction"
    },
    {
        "id": 5,
        "chunk": "# Superhydrophobic coatings  \n\nA drop of water placed on a surface of the polymer is good enough to determine wettability of the surface and can be distinguished as hydrophilic on spreading, whereas hydrophobic on beading or rolling off (Fig. 2). Moreover, the receding or advancing contact angle values can be obtained by simply tilting the polymer surface on which the drop rests, not more than 10\u0003.15,35 Superhydrophobicity in nature defined in the terms of Fihri et al.36 is a physicochemical phenomenon inherited by low surface energy, rendering the surface extremely difficult to wet. The theory of superhydrophobicity is well expressed in terms of the most traditional and essential equation in the science of wetting. The Young’s equation (1) explains the inverse of water repellency, i.e., wettability in terms of contact angle (h)36,37  \n\n$$\n\\cos\\theta=\\frac{\\gamma_{\\mathrm{sv}}-\\gamma_{\\mathrm{sl}}}{\\gamma_{\\mathrm{lv}}}\n$$  \n\nwhere $\\gamma_{\\mathrm{sv}},~\\gamma_{\\mathrm{sl}}$ , and $\\gamma_{\\mathrm{lv}}$ refer to the interfacial surface tension values of solid, liquid and gas, respectively. The Young’s equation is suitable for predicting the contact angle values of flat, perfectly smooth surfaces with the homogeneous interface, defined by three-phase contact line at water, air, and polymer surface,15,36,37 whereas for the wetting of heterogeneous interface, more complicated route needs to be followed as developed by Wenzel and Cassie–Baxter.36 For rough and chemically heterogeneous surface36 the contact angle can be determined by the Wenzel model which assumed that the water droplets penetrate inside the roughness troughs and tend to completely wet the surface35 with negligible effect of gravity. The contact angle is given by the Wenzel’s equation37  \n\n$$\n\\cos\\theta=R_{\\mathrm{f}}\\cos\\theta_{\\mathrm{L}}\n$$  \n\nwhere $R_{\\mathrm{f}}$ denotes the ratio of the actual surface area to its flat projected area, and $\\theta_{\\mathrm{{L}}}$ denotes the contact angle of the liquid on the surface.  \n\nFor rough and heterogeneous surfaces composed of two fractions or more, the value of contact angle is given by the Cassie–Baxter model35 assuming the droplet sitting on top of asperities creating air pockets trapped among themselves generating a composite solid–air–liquid interface as depicted in Fig. 3. The wetting behavior of this regime is well suited for liquidrepellent surfaces, and the value of contact angle is determined by the following equation37  \n\n$$\n\\mathbf{cos}\\theta=f_{1}\\cos\\theta_{1}+f_{2}\\cos\\theta_{2}\n$$  \n\nwhere $f_{1}$ and $f_{2}$ are the fractional area with respective contact angle $\\theta_{1}$ and $\\theta_{2}$ . Similarly, for composite interface the contact angle can be calculated using Cassie–Baxter equation comprised of solid–liquid fraction and liquid–air fraction where $f_{1}$ is replaced by $f_{\\mathrm{SL}}$ and $\\theta$ by $\\theta_{\\mathrm{{L}}}$ for solid–liquid fraction and $f_{2}$ by ${f_{\\mathrm{LA}}}^{37}$  \n\n![](images/ff66910c0787dad24943ca1360964fbe0ec480abe546016437543f7882b3d3b5.jpg)  \nFig. 2: Schematic representation of the various types of wetting regimes between substrate and water droplet  \n\n![](images/c90ebfe48d06389edd4851c327615bc4cd8763b5901c28b183e685d4e7148f23.jpg)  \nFig. 3: Progress of wetting behavior for different wetting regimes of a droplet on a flat surface (a) Young’s model, (b) Wenzel model, (c) Cassie–Baxter model with different surface roughness.36 Copyright 2017, reproduced with kind permission from Elsevier  \n\n$$\n\\cos\\theta=R_{\\mathrm{f}}\\cos\\theta_{\\mathrm{L}}-f_{\\mathrm{LA}}(R_{\\mathrm{f}}\\cos\\theta_{\\mathrm{L}}+1).\n$$  \n\nStatic contact angle measurement is not sufficient for the determination of unexpected wetting behavior of the polymeric coatings. In this regard, Wynne et al.15 came forward with an approach on Wilhelmy balance method for dynamic contact angle (DCA) evaluation, which helps to eliminate the ‘‘contamination gap’’ for marine waters by examining the ‘‘leaching’’ of contaminants in water. The leachates tend to alter the contact angle values by reducing the surface tension of water. This method reports the change in the values of contact angle according to the depth and zones of immersion and emersion of hydrophobic (Fig. 4a) and hydrophilic coatings (Fig. 4b).15  \n\n![](images/347763b64914e08caa90eee331b2beafd12dd22fdcfa7c8df38e6375f26e43f8.jpg)  \nFig. 4: Pictorial representation of sequence of immersion and emersion phenomenon observed in (a) hydrophobic and (b) hydrophilic coatings. Where $I_{1},I_{2},I_{3}$ denote the progressive stages of immersion in water contact and $\\pmb{\\cal E}$ denotes the emersion. $)_{A1},\\theta_{A2},\\theta_{A3}$ denotes the advancing contact angles at different stages of immersion, whereas $\\theta_{\\mathsf{R}}$ denotes the receding contact angle  \n\nFurther, for recording the dynamics of contact angle measurement, efforts were made by Nair et al.15 to calculate the dynamic contact angle $\\mathbf{\\dot{\\rho}}(\\theta)$ , employing an experimental setup with a glass surface coverslip coated with the polymeric coating; hung from a sensitive electrobalance with vertical force, $F_{\\ast}$ given by the following equation  \n\n$$\nF=P\\gamma L\\cos{\\theta}-F_{\\mathrm{b}}\n$$  \n\nwhere $P$ denotes the perimeter of coated slides and $F_{\\mathrm{b}}$ denotes the buoyancy force proportional to the depth of immersion.15  \n\nIn context of fabrication of superhydrophobic coatings, a tremendous increase in superhydrophobicity was achieved in a study by Huang et al.,38 reporting WCA greater than $170^{\\circ}$ by dip coating nano- $\\mathrm{TiO}_{2}$ (P25) on hastealloy substrate, the process shown in (Fig. 5). Mostly, the film was suitable for marine applications since it showed superhydrophobicity even after being corroded with strong acids.38  \n\nMartin and Bhushan35 described a means of modifying the surface of polydimethylsiloxane from hydrophobic to superhydrophobic by coating it with $\\mathrm{SiO}_{2}$ nanoparticles incorporated in methylphenyl silicone resin binder. The results found that the novel formulation possesses self-cleaning, antifouling, lowdrag, and antismudge properties apart from superhydrophobicity.35 The induction of superhydrophobicity in any polymeric surface coating is followed by added advantages of anticorrosiveness, abrasion resistance, cohesion strength, and many more. In this regard, improvement of other coating properties brought at the advent of superhydrophobicity has been discussed in the findings of Fihri et al.36 on polymer-based superhydrophobic coatings on steel substrates, listed in Table 1.  \n\n![](images/a3e59a9df5752fe32676854dbfd57055969291d60cf65c5befe4d57acdbdd808.jpg)  \nFig. 5: $\\pmb{\\Tilde{\\Pi}}\\pmb{0_{2}}$ suspension containing $\\pmb{\\Tilde{\\mathbf{liO}}_{2}}$ nanoparticles as precursor, leading to a hierarchical structure with high water contact angle of $173.7^{\\circ}$ .38 Copyright 2012, reproduced with kind permission from Elsevier  \n\nTable 1: Improvement in the hydrophobicity of polymer coatings on value addition of nanoparticles36   \n\n\n<html><body><table><tr><td rowspan=\"2\">S. no.</td><td rowspan=\"2\"> Polymer matrix</td><td rowspan=\"2\"> Nanoparticles embedded</td><td rowspan=\"2\">%Wt loading/ratio</td><td colspan=\"2\">Contact angle (°)</td><td rowspan=\"2\"> Properties improved</td><td rowspan=\"2\">Referen nos.</td></tr><tr><td>Pristine</td><td>Blend</td></tr><tr><td>1.</td><td>Polyurethane</td><td>Molybdenum disulfide</td><td>20-55.6</td><td>87</td><td>157</td><td>Surface roughness, abrasion resistance, superhydrophobicity</td><td>39</td></tr><tr><td>2.</td><td>Fluorinated polysiloxane</td><td>Steric acid-modified ZnO</td><td>13:7</td><td>一</td><td>166</td><td>Excellent durability, corrosion resistance</td><td>40</td></tr><tr><td>3.</td><td>Teflon tailings</td><td>Tetrafluoroethylene and hexafluoropropylene</td><td></td><td>一</td><td>> 165</td><td>Uniformity,superhydrophobicity</td><td>41</td></tr><tr><td>4.</td><td>Fluoride latex</td><td>Phosphating material</td><td></td><td>155</td><td>168</td><td>Corrosion resistance, good stability in salt spray environment</td><td>42</td></tr><tr><td>5.</td><td>PTFE</td><td>Polyphenylene sulfide</td><td>40 vol%</td><td>一</td><td>165</td><td>Good cohesion strength, high and low temp. resistance</td><td>43</td></tr><tr><td>6.</td><td>Epoxy resin</td><td>Fatty acids and epoxidized oleic acid</td><td>5</td><td></td><td>160.5</td><td>Corrosion barrier properties</td><td>44</td></tr></table></body></html>  \n\nWang et al.45 asserted an enormous increase in superhydrophobicity of the coatings by employing very rarely used nanofiller, electrochemically exfoliated graphite (EEG) incorporated into polydimethylsiloxane matrix. The prepared surfaces obeyed the Cassie– Baxter model with WCA value of $160^{\\circ}$ indicating water droplets in a suspended state instead of penetrating the surface and exhibited excellent self-cleaning properties, able to withstand water and sand-impact tests. The contact angle measurements captured spherical droplets, whereas the SEM images revealed a nanoflower structure on the surface of Al alloy and steel substrates, as demonstrated in Fig. 6.45  \n\nWang et al.46 discussed the synthesis of PDMS–ZnO nanocomposite coating having surface microstructures able to restore themselves on account of strong mechanical stability along with robust abrasion resistance toward any surface degradation. The coating qualified as a potential candidate to be used in protective applications such as anti-icing, superhydrophobic, as well as nonfouling (Fig. 7).46  \n\nIn continuing theme of polymeric superhydrophobic marine coating research, Mo et al.47 reported a facile approach for fabricating protective coating from methylhydrosilicone oil and stearic acid-modified $\\mathrm{TiO}_{2}$ nanoparticles with the highest water contact value of $1\\dot{5}1.5^{\\circ}$ .47 Cong et al.48 formulated PDMSbased superhydrophobic coatings and successfully compared the effect of concentration of $\\mathrm{TiO}_{2}$ and $\\mathrm{SiO}_{2}^{-}$ nanoparticle incorporated into PDMS matrix on the repellent properties. The contact angle values were reported as follows—(a) $\\mathrm{PDMS}/\\mathrm{TiO}_{2}$ -NPs (30 $w t\\%$ )— $154.69^{\\circ}$ (b) $\\mathrm{PDMS}/\\mathrm{SiO}_{2^{-}}$ NPs (40 $\\mathrm{wt\\%}$ )— $152.46^{\\circ}$ . The TEM results supported the microroughness of fractured surfaces revealing that the $\\mathrm{TiO}_{2}$ nanoparticles showed more dispersion. Moreover, superhydrophobic property was imparted to the coatings by low surface energy of PDMS moieties and microscale roughness of $\\mathrm{TiO}_{2}$ nanoparticle clusters along with the photocatalytic property stable up to 6 months.48 Similar findings were reported by Bokobza and Diop49 by generating titania nanoparticles synthetically in post-crosslinked networks by using a novel protocol. The TEM images revealed two-phase structure, and the fillers were dispersed homogeneously in the PDMS matrix. There exists a strong interface between the polymer and the filler leading to excellent mechanical properties.49  \n\nSurface tension plays a crucial role, in altering the hydrophobic nature of the surface coatings when any liquid phase comes in contact with the solid-coated substrate. In this regard, the Owens–Wendt–Kaeble approach was employed by Martinelli et al.50 for surface tension calculation of the coatings, with the help of predetermined contact angle (h) values. The geometric mean of the interfacial surface tension c) is given by the following equations50  \n\n![](images/1f6642da97bc95dc469b02c4534c5286830b5126f670b3a1a0eaf10e54f68125.jpg)  \nFig. 6: (a) Appearance of water droplets on superhydrophobic surface (b) contact angle measurement reporting $160^{\\circ}$ (c) Contact angle measurement at tilt angle of ${\\mathfrak{s o}}$ (d), (e) SEM images of superhydrophobic composite coating representing nanoflower structures (f) Colorful magnification.45 Copyright 2016, reproduced with kind permission from Elsevier  \n\n![](images/9db5dcd9d2f3bc1d554b4ffa0aadacb2af50ac8c510323e4b447c4f521c08077.jpg)  \nFig. 7: The water contact angle values recorded for (a) uncoated substrate (b) coated substrate (left) and the surface regeneration mechanism of PDMS-ZnO superhydrophobic coating (right).46 Copyright 2015, reproduced with kind permission from Springer  \n\n$$\n\\begin{array}{l}{\\gamma=\\gamma^{\\mathrm{d}}+\\gamma^{\\mathrm{p}}}\\\\ {\\gamma_{12}=\\gamma_{1}+\\gamma_{2}-2\\big(\\gamma_{1}^{\\mathrm{d}}\\gamma_{2}^{\\mathrm{d}}\\big)^{0.5}}\\end{array}\n$$  \n\nCombining equation (7) with Young’s equation (1), equation (7) can be rewritten as follows:  \n\n$$\n\\gamma_{\\mathrm{L}}(1+\\cos\\theta)=2\\Big[\\big(\\gamma_{\\mathrm{S}}^{\\mathrm{d}}\\gamma_{\\mathrm{L}}^{\\mathrm{d}}\\big)^{0.5}+\\big(\\gamma_{\\mathrm{S}}^{\\mathrm{p}}\\gamma_{\\mathrm{L}}^{\\mathrm{p}}\\big)^{0.5}\\Big]\n$$  \n\nwhere $\\gamma^{\\mathrm{d}}$ denotes the nonpolar component of interfacial surface tension, i.e., dispersive and hydrogen bonding component, and $\\gamma^{\\mathrm{p}}$ denotes the polar component representing dipole–dipole interactions. Equation (8) consists of two unknowns, to be solved via simultaneous equations, provided that the value of contact angle (h) has been determined by sessile drop technique.50–52  \n\n![](images/703c5790dd49223166bb185e06c548d63c720b9429f78dd26609f103ed77796f.jpg)  \nFig. 8: Schematic illustration of Marine biofouling—its reasons and consequences. (a) Image of deteriorated coating system (b) Coating delamination because of Biocorrosion-Exposure to atmosphere (c) Deposited hard fouling over ship hull-Exposure to seawater  \n\nThe roll-off angle is defined as the tilting angle at which the droplet of any solvent (water, methylene iodide, etc.) used in sessile drop technique begins to roll-off on a surface. The expression for determining the roll-off or sliding angle was given by Brockway et al.53 as follows:  \n\n$$\nF_{\\mathrm{ad}}=2R\\gamma(1+\\cos\\theta)\\sqrt{\\varnothing_{\\mathrm{T}}}=\\rho V g\\sin\\alpha\n$$  \n\nwhere $F_{\\mathrm{ad}}$ is the lateral adhesive force between the droplet and the solid surface, $R$ is the mean radius of the contact line, $\\gamma$ is the surface tension of water, $\\sqrt{\\varnothing_{\\mathrm{T}}}$ is the wetting solid fraction, $\\rho$ is the density of water, $V$ is the volume of the droplet, $g$ is the acceleration due to gravity, and a is the roll-off angle.53,54  \n\nA simpler approach for the measurement of surface energy was reported by Zhou et al.55 by utilizing the equation of state method, as follows55  \n\n$$\n\\sigma_{\\mathrm{s}}=\\gamma_{\\mathrm{sl}}-\\sigma_{\\mathrm{l}}\\cdot\\cos\\theta\n$$  \n\n$$\n\\gamma_{\\mathrm{sl}}=\\sigma_{\\mathrm{l}}+\\sigma_{\\mathrm{s}}-2\\sqrt{\\sigma_{\\mathrm{l}}\\cdot\\sigma_{\\mathrm{s}}}\\cdot\\mathrm{e}^{-\\beta\\left(\\sigma_{\\mathrm{l}}-\\sigma_{\\mathrm{s}}\\right)^{2}}\n$$  \n\n$$\n\\cos\\theta=-1+2\\sqrt{\\frac{\\sigma_{\\mathrm{s}}}{\\sigma_{\\mathrm{l}}}}\\cdot\\mathrm{e}^{-\\beta(\\sigma_{\\mathrm{l}}-\\sigma_{\\mathrm{s}})^{2}}\n$$  \n\nwhere $\\sigma_{\\mathrm{{s}}}$ denotes the surface energy of the solid, $\\sigma_{\\mathrm{l}}$ denotes the surface energy of the liquid, $\\gamma_{\\mathrm{sl}}$ gives the interfacial tension of the solid/liquid phase and $\\theta$ is the contact angle.",
        "category": " Results and discussion"
    },
    {
        "id": 6,
        "chunk": "# Antifouling coatings  \n\nThe process of bacterial colonization is an irreversible adhesion consisting of many stages of recruitment of microorganisms on the surface of marine structures, whether protected or unprotected, making it impossible to reduce biofouling once it has been formed over the solid surface (Fig. 8). Antifouling (AF) coatings are essential for preventing the growth of fouling on immersed structures due to the environmental and economical benefits. Thus, there is an urgent need of the hour to develop effective coatings to protect the surface of underwater objects from adhesion of various microorganisms.19,56 As per the norms imposed by IMO, new antifouling systems qualifying the implementation gap for AF polymer coating have been designed as viable alternatives to banned TBT paints and other tin-free biocide-based replacements. The green strategies for AF coatings were discussed as a part of two approaches (1) detachment of biofoulants and (2) preventing biofoulants attachment in an extensive review using triangular approach on amphiphilicity, superhydrophobicity and topographic nature of marine coatings by Ayda et al.33  \n\n![](images/92471e3ffaa3530fe1fb3efe344a7f073ff0a3c13cd4444e3e12ad0543d37700.jpg)  \nFig. 9: Various approaches to design antifouling surfaces.30 Reproduced with kind permission from JOHN WILEY AND SONS LICENSE, 2010  \n\nThe various approaches used to classify antifouling coatings are represented in Fig. 9. These approaches are based on the type of biofouling species attached and the concurrent type of polymer employed for the synthesis of such coatings to impart resistance against the attachment of a particular biofoulant.30  \n\nAF coatings are the paints applied on the solid hull surface to prevent it from unwanted attachment of microorganism through biocide release either from porous films or from ablative paints that continuously ablate biocides in water with time by dissolving themselves into marine water.1 According to Bressy et al.,4 AF coatings can be categorized into two types: chemically active coatings (generally toxic) and nontoxic coatings. The chemically active coatings are further divided into biocide-based and enzyme-based coatings, both functioning on dissolution mechanism. This explains the intensive search for new effective biocide-based polymer composites that can be as effective as those containing tin and copper.4 A concise review on the fundamental structure–property relationship and mechanism of antifouling polymers with their capability of being a fascinating class is presented with a particular focus on recent developments by Zhang and Chiao.31",
        "category": " Results and discussion"
    },
    {
        "id": 7,
        "chunk": "# Progress in antifouling technology  \n\nThe progress made in the interdisciplinary study on fabrication of robust AF surfaces may bring new ideas to the current research on multidefense AF coatings for marine applications. The AF performance of the fabricated coating surfaces depends upon the following three factors33  \n\n1. The length scale of coating roughness, ascribed to the topography,   \n2. The percentage of air incursions entrapped at the interface as per the Cassie–Baxter model,   \n3. The capability of the coating to hold and to stick to the interface.  \n\nHydrophilic polymers like glycocalyx-mimetic peptoids have been used as efficient antifouling materials due to the formation of a strong hydration layer at polymer–water interface providing higher interfacial strength. Subsequently, this prevents bacterial adhesion, recruitment, and colonization along with protein adsorption by inhibiting the exchange of enzymes and lipids among the microorganisms.19,21 A layer of hydration develops due to the formation of hydrogen bonds between the hydrophilic polymers and water interface, whereas ionic solvation takes place for zwitterionic polymers.19,20,56 T he hydrophobic polymers exhibit different mechanisms for fouling prevention, similar to that of lotus-leaf with microfibrils on its surface representing hierarchical nanostructures preventing the coated surface from fouling.19,57 Du et al.19 described a valuable means of synthesizing antifouling paints by grafting silicificated polyaniline nanofiber arrays (SPNAs) over the solid base of HPAPD film. The resultant formulation showed excellent adhesion to steel substrate, corrosion resistance to ASW (artificial seawater) and possessed remarkable antifouling properties, repelling marine bacteria like E. coli, and deterring the adhesion of proteins.19  \n\n![](images/cc2c81f0c07102091193c66f15dca276a525dee5fc921b938f19ec28f089f6f1.jpg)  \nFig. 10: General framework for the characterization of antifouling performance of the copolymers. Reprinted with permission from reference (58). Copyright 2017, American Chemical Society  \n\nThe pioneers well-known for the development of antifouling coating technologies, Duong et al.58 reported a generalized framework for the synthesis and characterization of polysiloxane-based copolymers using RAFT agents59 emphasizing that the antifouling performance of the diblock copolymers were better than that of the triblock copolymers, (Fig. 10).58  \n\nThe predetermined time-based study to carry out the hydrolytic degradation test of polymer films was conducted by incubating the samples in ASW. The mass loss (in $\\mathrm{wt\\%}$ ) was determined using the following equation  \n\n$$\n\\mathrm{loss}(\\%)=\\frac{w_{o}-w_{t}}{w_{o}-w_{\\mathrm{pvc}}}\\times100\\\n$$  \n\nwhere $w_{o}$ and $w_{t}$ are the initial and final (at time $=t$ ) weights of the PVC substrate coated with synthesized copolymer compositions and $w_{\\mathrm{pvc}}$ represents the weight of the substrate foil. After the bacterial attachment studies were conducted, the crystal violet (CV) solution was extracted from the inoculated wells and the relative rate of adhesion was measured by the following equation58  \n\n$$\n\\mathrm{\\adhesion=\\frac{\\left(OD_{with\\bacteria}-O D_{b l a n k}\\right)_{c o a t i n g}}{\\left(O D_{w i t h\\ b a c t e r i a}-O D_{b l a n k}\\right)_{P S c o n t r o l}}\\times100}\n$$  \n\nwhere ODwith bacteria and $\\mathrm{\\Gamma_{OD_{blank}}}$ represent the optical density of the CV solution with bacteria and without bacteria.58 Antifouling field studies as per French standard (NF T 34-552) led to the determination of an efficacy parameter $N$ , evaluated from a distance of $1\\mathrm{cm}$ from the edges of the panel. The parameter, $N$ , was defined by the following equation  \n\n![](images/db1be9c040c66d7c6ba30b9ee26ca3ae69021d2c5abe23982a28a7e0e181c72f.jpg)  \nFig. 11: Antifouling field studies performed at Toulon Bay, France. D and T stand for diblock and triblock, $5k$ and $10k$ represents the molecular weight of PDMS. Ref. stands for the reference standard. Reprinted with permission from reference (58). Copyright 2017, American Chemical Society  \n\n$$\n{\\cal N}=\\Sigma(\\mathrm{IF}\\times\\mathrm{SF})\n$$  \n\nwhere SF is defined as the severity parameter, responsible for the frictional drag penalty of ship hulls generated on account of increased surface roughness due to foulers, whereas IF is the intensity factor which gives an estimated percentage of the surface covered by variety of macrofoulers, directly proportional to the molecular weight of the polymer employed for coating fabrication as shown in Fig. 11.  \n\nIn the succeeding work, they reported about the thermal characteristics of such well-defined diblock copolymers60 in which the thermal behavior and stability of PDMS-based diblock copolymers was extensively studied. After obtaining the DSC thermograms, it was found that these diblock copolymers have two distinct $T_{g}$ values. Therefore, the value of $T_{\\mathrm{g}2}$ (second Tg) was obtained from Fox–Flory equation61,62 given as follows:  \n\n$$\nT_{g}=T_{g\\infty}-{\\frac{K}{M_{n}}}\n$$  \n\n$$\n\\frac{1}{T_{g}}=\\frac{W_{1}}{T_{g1}}+\\frac{W_{2}}{T_{g2}}\n$$  \n\nwhere $W_{1}$ and $W_{2}$ are the weight fractions of each component.  \n\nAnother work by Bressy and team63 was comprised of the synthesis of environmentally benign hybrid antifouling coatings known as FRC/SPC antifouling coatings. The field studies conducted post-synthesis showed excellent antifouling properties after 7 months of immersion and, also good foul-release properties were obtained.63  \n\nPretti et al.64 studied the effect of bismuth catalyzed coatings on leaching rates and AF performance of marine coatings under toxicity analysis and its effect on microorganisms was studied subjected to ecotoxicological assessment. The blended three layer AF coatings revealed lipophobic character, attributed to the low surface energy of fluorinated polymers. It was concluded from the toxicity assessment that the tin-based ions exhibit acute toxicity of lower degree to different species of marine lives, fisheries, invertebrates, and juvenile, to name a few.64–67 When compared to Cubased leachates/catalyst, bismuth is found to be an ecosustainable alternative to the traditionally used catalysts for the preparation of surface-active polymeric coatings.64 Song et al.68 reported a case study on marine pollution caused by leachates at South Yellow Sea of China. They fabricated an antifouling coating by incorporating a mixture of sodium benzoate (NaB) and sodium pyrithione (NaPt) as potential biocides into commercial silicone matrix (Sylgard 184). The nylon and bamboo coated substrates were prevented against macroalgae ( $U.$ Prolifera), by inhibiting the entire growth of the propagules. The rate of adhesion $(r)$ was calculated from the density of settlement of microorganisms using the following equation68  \n\n$$\nr={\\frac{\\left(C_{\\mathrm{b}}-C_{i}\\right)}{C_{\\mathrm{b}}}}\n$$  \n\nwhere $C_{\\boldsymbol{\\mathrm{b}}}$ and $C_{\\mathrm{i}}$ denote the density of micro-propagules in blank and experimental containers. The plotted results obtained from the Tukey test suggested that NaB formulated soluble matrix antifouling paint was optimized for $1\\ \\mathrm{wt\\%}$ of NaB incorporation as durable antifouling paint without rapid leaching of its constituents into the marine environment.68  \n\nThe coating science on microtopography and nanotopography surface design goes with the evolution of coatings on biomimetic structures inspired from sharks, whales, shells, oysters, etc. by the incorporation of natural products, to yield long-lasting antifouling systems. In this regard, Chambers et al.69 introduced a new methodology to incorporate natural products into the coating system and later analyze their performance layerwise. The present approach comprised an antifouling additive, a crude Chondruscrispus extract incorporated into the resin system as a booster biocide. The results of the rigorous field immersion test carried out for 42 and 105 days of immersion were reported in Fig. 12.69  \n\nA similar research was carried out by Svenson and team70 by exploiting the antibacterial property of polygodial (drimane sesquiterpene) as the natural product transformed into alkenes which undergo epoxidation to provide a series of 11 drimane compounds. The macrofouling activity of the 11 synthesized materials was inspected under the influence of 12 different types of biological species. The successful biofouling inhibition was witnessed in artificial and in natural seawater environments inhibiting the growth of micro- and macrofoulers at the surface.70 Another recent work on incorporating natural products for antifouling coating fabrication was reported by Ding et al.71 using nonleaking capsaicin (active component of chili peppers) as the natural product blended with PDMS-block copolymer. The capsaicin particles were bonded chemically with $\\mathrm{CoFe}_{2}\\mathrm{O}_{4}^{-}/$ gelatin nanospheres, on account of which the synthesized coatings possessed nonleaking environment-friendly AF approach. The optical study showed the lowest cell settlement for Navicula subminuscula due to an active layer of capsaicin inhibiting the settlement of microorganism without leaching out of the coating surface.71  \n\nChitosan-based nanocomposite antifouling coatings were recently fabricated by blending chitosan nanoleaves with biocides (basically, transition metal/metal oxide nanoparticles) like zinc oxide72 and copper oxide.73 Abiraman and team73 synthesized chitosan coatings by incorporating copper oxide into chitosan matrix under different reaction conditions and their feasibility with various parameters as a function of antifouling performance was studied deeply. The synthesized nanoleaves were blended into the polymers with accurate mechanical properties (like polyurethane) to enhance the durability of the system, coated on three different substrates: wood, mild steel and cement. The SEM results revealed the formation of micro-size domains of copper oxide nanoparticles. The HRTEM results revealed that the copper oxide embedded chitosan nanoleaves were $14~\\mathrm{nm}$ wide and $98~\\mathrm{nm}$ to $171\\ \\mathrm{nm}$ long. Outstanding biofouling efficiency was witnessed against a set of three algae, namely Arthrospira, Chlorella, and Amphora. The growth of algal bloom was determined by quantifying the amount of chlorophyll derived from a scratched portion of algae from the fouled coating surface, utilizing a method reported by Lichtenthaler.74 The algae cell count can be measured using Neubauer hemocytometer. The pigment concentration was calculated from the following equation, assuming that the factors responsible for algal growth temperature, pressure and surface roughness remains unperturbed:  \n\n$$\n\\mathrm{Chl}-a\\Big(\\frac{\\mathrm{mg}}{\\mathrm{L}}\\Big)=11.24\\times A_{661.6}-2.404\\times A_{644.8}\n$$  \n\nwhere the pigment concentrations were obtained by inserting the measured absorbance values denoted by $\\cdot_{a}\\cdot{}$ in micrograms per milliliter of plant extract solution. The above equation is based on the redetermined specific absorption coefficients, denoted by $\\boldsymbol{A}_{661.6}$ and $A_{644.8}$ , listed in reference (74). The results proved the efficiency of chitosan nanoleaves coating as environmentally benign coatings by prohibiting leaching out into the marine atmosphere.73 Similar findings were reported by Al-Naamani et al.,72 when they studied the incorporation of zinc oxide $(Z\\mathrm{nO})$ biocide into chitosan polymer matrix for the optimization of antifouling efficiency of the fabricated coatings. The swelling and solubility studies reported a steep fall in the swelling ratio attributed to more compact, tightly bound and fully crosslinked three-dimensional structures formed due to nano- $.Z_{\\mathrm{{nO}}}$ particles. A controlled release of biocidal $\\mathrm{znO}$ nanoparticles was ensured by ICP analysis, rendering it safe to blend with chitosan polymer. The immediate effect of photocatalysis under the influence of white light for contact killing of microorganisms lasted for a shorter span, thus proving it to be less harmful to the marine environment.72  \n\n![](images/e5d9c34487ec0dd4d66183c51e28f3f9a9c5d8b4bc255c30a4d34da5059fd7d4.jpg)  \nFig. 12: Field-immersion test images of the coated specimens.69 Copyright 2014, reproduced with kind permission from Elsevier  \n\nBinders are known to be the film-forming component of paints, mostly referred to as resins, which combine with solvents to deliver a successful paint technology with desired dry film thickness (DFT). This result-oriented approach of the binders serves as a boon for the coating industry, especially for antifouling paints. Generally, these resins are polymeric compounds divided into two categories—convertible and nonconvertible, which undergo polymerization reaction after being applied onto the substrate, whereas the latter already exists in polymerized form, and only needs to be coated after blending with the solvent.75 The role of binders in successful functioning of AF paints was discussed in a patent by Proudlock and Dennington76 who claimed to possess an invention on synthesis of a binder system with optimum rate of hydrolysis to produce two methacrylate polymers with pigment volume concentration (PVC) between 30 and $50~\\mathrm{wt\\%}$ , using zinc oxide pigment to make the system antifouling active.76  \n\n![](images/6ead712561204bbe0b6dda5ee086a9dd7b1474dc0e7a4e3ef11fdc1d5199c2bb.jpg)  \nFig. 13: Laboratory setup to promote the growth of mussels, plaque, and shellfish in a $\\sim12,000\\mathrm{-}\\mathsf{L}$ aquarium, where siphon was used to produce surge and other parts to produce adhesion. Reprinted with permission from reference (77). Copyright 2016, American Chemical Society  \n\nThe role of antioxidants to deter marine biofouling was studied by Wilker et al.77 by taking a step toward environmental sustainability. The health hazards imposed by AF coatings can be assessed by a novel technique, reported as follows—a total of ten marine biofouling species were maintained in an aquarium setup (Fig. 13), exposed to the immediate vicinity of the coated specimen observing their survival for 3 days. After undergoing a rigorous procedure, the weight of the extracted samples of species was rounded up to three decimal place and the condition index $(C I)$ was calculated using the following equation77,78  \n\n$$\n\\mathrm{CI}={\\frac{\\mathrm{dry}\\ m e a t\\ w e i g h t}{\\mathrm{dry}\\ \\mathrm{shell}\\ w e i g h t}}\\times100\n$$  \n\nUnder the category of filler-based antifouling coatings, the following research contributions are noteworthy. The fabrication of PDMS–FTDS (perfluorodecyltrichlorosilane) antifouling coatings was carried out by Li et al.79 in one-step procedure by incorporating zinc oxide as nanofiller for imparting antifouling efficacy to the nanocomposite coatings coated onto steel (Q-235) substrates. The SEM analysis confirmed the presence of FTDS on the surface of the coating specimen by forming a layer over the protrusions caused by $\\dot{Z}\\mathrm{nO}$ nanoparticles, filling the air pockets. The wettability measurements revealed that the critical surface tension values obtained from Zismann plot were very low on the order of $19\\mathrm{\\mN/\\Omega}$ m and in case of complete wetting also, the coatings were able to resist fouling.79 Oldani et al.80 reported a facile route for impregnating the perfluoropolyether (PFE) film with ceramic oxides, where $\\mathrm{TiO}_{2}$ and $\\mathrm{ZrO}_{2}$ particles were used to formulate multilayer AF coating. The particulate fouling test was conducted to evaluate the resistance of coatings toward $\\mathrm{CaSO_{4}}$ particles, where the solution was heated in a tank and pumped through a tube sample, internally lined by sample coatings (Fig. 14). The foulants deposited were quantified by the weight differences and then normalized in function in terms of surface area and time of exposure. After $^{72\\mathrm{~h~}}$ , a fouling of only $2\\%$ was observed on $\\mathbf{Z}\\mathbf{r}\\mathbf{O}_{2}$ -based coating, whereas it was $10\\%$ for $\\mathrm{TiO}_{2}$ -based multilayer coating.80  \n\n![](images/1cc97c1966651164d2fb5a4c3efee77995503ada7920dc912c461a97c32d90b0.jpg)  \nFig. 14: Schematic of the test rig used for fouling mitigation assessment. R-heating element, TCthermocouple, $\\pmb{P}$ -pump, FM-float flowmeter.80 Copyright 2015, reproduced with kind permission from Elsevier  \n\n![](images/4fcd3e0658df131e73d2e2b6f720031ce47cc582cc31faff39c9926ee1a89c62.jpg)  \nFig. 15: The structure of novel hybrid biocide-incorporated PDMS coating-preparation and characterization.81 Copyright 2017, reproduced with kind permission from Springer  \n\nThe novel dual-nature multifunctional antifouling coatings based on biocide incorporation using allyltrimethoxysilane as precursor of the reaction, coated onto mild steel (S-36) substrates were synthesized by Suleiman et al.81 (Fig. 15). The five different types of biocides namely Irgarol, 1,1-dimethylbiguanide hydrochloride, silver nanoparticle dispersion, titanium nanosize powder, 1-hydroxycyclohexyl phenyl ketone, followed by MOLY-white 101 used as the corrosion inhibitor, were incorporated into the PDMS polymer matrix. The SEM analysis revealed dense, homogeneous and microcrack-free surface. The biocide-embedded PDMS hybrid coatings were declared antifouling and anticorrosion driven; they were ideal and suitable for the overall protection of marine structures.81",
        "category": " Results and discussion"
    },
    {
        "id": 8,
        "chunk": "# Foul-release coatings  \n\nMany foul-release (FR) systems are available commercially; the development of an efficient product entirely based on the FR properties is still a far cry. A rational study needs to be done to completely understand the actual polymeric materials, post-determining from a massive screening of available polymers to locate the materials having actual role in releasing the growth of various microorganisms. Still, the broadspectrum activity of foul-release polymers is questioned by the huge diversity of polymers; their structure–property relationship and attachment mechanisms has not been found. Therefore, in this regard, literature revealed that no systems would ever prevent fouling of a surface totally; yet potential attempts can be made to reduce it to a large extent. The hurdles in the path of development of foul-release coatings can be slower testing time, cost of fabrication, time-consuming environmental assessments, government regulations and the dependency of market on antifouling coating technology. On the other hand, polysiloxane-based FR coatings already yield good results on fast-moving vehicles. Further studies on the influence of mechanical and surface properties on adhesion phenomenon will orientate the research on polysiloxane-based foul-release coatings for navigation of vessels at a minimum or slower speed. The most significant advances in the field of foul-release coatings have been recently collected in an excellent review by Selim et al.3  \n\nThe usage of foul-release coatings prevents the accumulation of heavy metals like Cu, Zn, and their oxide nanoparticles from settling into the deep-sea beds in the form of sediments, different from antifouling systems. Mikael et al.82 consolidated the valuable feedback on the ban of copper-based biocide leachate coatings through a survey targeting a particular questionnaire answered by a team of experts comprising of academician, researcher, marine legislation, marine product specialist and shipping industry management. All favorable responses from different professional roles were collected upon cost analysis and efficiency revealing the future prospects of nonstick coatings. This comprehensive information provides a valuable insight into a fruitful research on foul-release coatings and suggests the development of upcoming marine antibiofouling green technologies.82 The understanding of the mechanism of foul-release coatings is essential for the development of such systems and to differentiate them from the biocide-based antifouling coating systems, as depicted symbolically in Fig. 16.",
        "category": " Introduction"
    },
    {
        "id": 9,
        "chunk": "# Silicone elastomer-based foul-release polymer coatings  \n\nSilicone-based elastomers possess the properties required for good fouling release, like low surface energy on the order of less than $22~\\mathrm{mN/m}$ and low modulus within a range of $1.4{-}3\\ \\mathrm{MPa}$ .50 The foulants adhere weakly to the surface of these elastomers, facilitating easy release of marine fouling with the aid of hydrodynamic shear of moving water. In the past decades, multifarious properties of specialty polymers like siloxane and fluoropolymers have been exploited to develop marine foul-release coatings (Fig. 17)3,14,83,84 leading to the formation of not only block but also linear, graft and hyperbranched structures.85 Fluoropolymers are the second most widely used polymers after siloxane class moieties for foul-release applications due to their high electronegativity and low polarizability, which in turn leads to weak London dispersion forces, cohesive and adhesive forces, which contribute to low surface energy and higher water contact angle values.35  \n\n![](images/13296ee077f6491e3fa64c19f306f4146284b7dcc369f253463d14c834140084.jpg)  \nFig. 16: Schematic illustration of mechanism of (a) foul-release and (b) antifouling coating systems  \n\nPolydimethylsiloxane along with its different functionalities has been widely used nowadays as the basis of commercially available ‘‘foul-release coatings,’’ enabling the researchers to develop more out of it and implement such systems by extending their usage for industrial coating applications. The release rate efficiency of functionalized and nonfunctionalized PDMS-based coatings (blends, composites, block copolymers) was estimated to be five times higher than that of other hydrophilic surfaces, establishing low interactions with proteins and other enzymes.83,86 In the context of proving polydimethylsiloxane as the potential candidate for foul-release marine coating application, Stevens and team87 took efforts to prove PDMS as harmless for the marine environment. Although PDMS fluids ultimately sink as sediments in deep-sea beds, there were no adverse effects found on a wide range of marine species. After an in-depth study of the sediments generated by the sludge disposal from various sources, it was found that the dry weight concentration of PDMS was between 0.03 and $2.3~\\mathrm{{mg/}}$ kg only, details of the findings are mentioned in Table 2.87  \n\n![](images/41a05d1bbd8ddf3f5e2dede146b57c8e9b1976717e9b56f8317a6f350779bc77.jpg)  \nFig. 17: The schematic representation of multitudinous properties of foul-release polymers  \n\nAll the data cited above, obtained from the analysis, reveal that PDMS is the optimum and the safest polymer to be employed for environment-friendly marine coating research. Since PDMS with high sheer and physical size of the molecule is used for the synthesis of marine polymeric coatings, it restricts its absorption into other organisms.  \n\nThe hydrophobic nature of PDMS, depicted in Fig. 18 is attributed to the presence of methyl groups at the surface. When PDMS comes in contact with three-phase contact line of water, a partial/incomplete wetting is observed on hydrophobic PDMS surface. The low surface energy of PDMS and the low surface tension, on the order of less than $19\\ \\mathrm{mN/m}$ , makes it viable for foul-release applications by reducing the interaction between marine organisms and the coating surface. The microorganisms that are weakly adhered onto the coatings surface via van der Waals forces are easy to release, since they are loosely bound to the coating surface.  \n\n![](images/e1eca6f7828e3ca683efc87b1e98f40f29e09c39c32034ac8ce811fe971d2d0c.jpg)  \nFig. 18: Water-repellent nature of PDMS due to the presence of hydrophobic $(-C H_{3})$ segments which leads to phase separation between the two distinct phases  \n\nTable 2: Concentration of PDMS in marine surface sediments from areas used for disposal of sewage sludge87 Copyright 2001, reproduced with kind permission from Elsevier   \n\n\n<html><body><table><tr><td>S. no.</td><td>Sample location</td><td colspan=\"2\">Concentration (mg/kg dry wt)</td><td>References</td></tr><tr><td></td><td></td><td>Mean</td><td>Max.value</td><td></td></tr><tr><td>1.</td><td>Uncontaminated area</td><td>< 0.03</td><td>0.04</td><td>CEFAS88</td></tr><tr><td>2.</td><td>Liverpool Bay</td><td>0.33</td><td>2.3</td><td>CEFAS88</td></tr><tr><td>3.</td><td>Boston Harbour, USA</td><td>16.6</td><td>34.2</td><td>Powell et al.89</td></tr></table></body></html>  \n\nCallow et al.90 described a means of depositing nanocomposite siloxane films using PACVD (plasmaassisted CVD) technique over glass slide to determine the biofouling resistance potential of foul-release coatings. The XPS analysis revealed that upon immersion, the carbon content kept on decreasing layer by layer from the coating surface. Adhesion strength was measured with the help of biological assays, growth of sporelings of macroalgae (Ulva linza) and marine bacteria (Pseudomonas fluorescens) was counterchecked by measuring the wall shear stress of the coatings, when subjected to water jet. Focusing on the deposition parameters, authors suggested that extended cleaning time and temperature of deposition affected the degree of crosslinking of the polymer film resulting in production of more robust polymeric coatings suitable for foul-release applications.90 Navabpour et al.91 emphasized the mechanical properties of the siloxane-based marine coatings deposited on glass and steel substrates by using a hybrid of PACVD–PVD deposition techniques. The results from scratch test indicate a high level of mechanical robustness by resisting scratch tracks up to $35\\mathrm{~N~}$ , except for coating (B) which started to wear beyond the load of $35\\mathrm{~N~}$ under higher rates of shearing (Fig. 19).91  \n\nAntioxidants stop the formation of glue by terminating the crosslinking reactions at the coating–water interface which hinders formation of linking pathways between the extracellular polymeric substance (EPS) secreted by microorganisms and the coating surface.77,92 A potential attempt was made by Wilker and team77 to curb marine pollution by reinventing the surface chemistry through surface modification by replacing harmful biocides with harmless antioxidants, equally potential antifoulants as biocides. In the present work, three different types of antioxidants (1) anisole, (2) 2, 6-di-tert-butyl-4-methylphenol (BHT), and (3) 2, 6-di-tert-butylphenol (DTBP) were blended with a solvent and coated on aluminum panels with the help of epoxy primer. The foul-release performance of the designed coatings was accessed by making a comparison between the reference control compound, 3, 5-di-tert-butyltoluene (DBT), and the novel designed system. The results revealed that such a system was based on the notion that a variation in the wt% loading of the antioxidant will alter foul-release performance of the system.77  \n\n![](images/d0c56ef9a6a2f2837939ec35809b9f50e53bf1df1d12c58ad73f47c5693b1e22.jpg)  \nFig. 19: Scratch tracks for various coatings deposited on M42 steel, obtained using a $200\\mathrm{-}\\upmu\\mathrm{m}$ -diameter diamond tip.91 Copyright 2010, reproduced with kind permission from Elsevier",
        "category": " Results and discussion"
    },
    {
        "id": 10,
        "chunk": "# Amphiphilic block copolymer-based foul-release coatings  \n\nThe limitations of foul-release coatings being not suitable for stagnant environment and eventually failing at sailing speeds below 20–25 knots for most vessels. This fact makes way for the instigation of amphiphilic coatings as the prospective research in foul-release coating field having the properties of both hydrophobic as well as hydrophilic surfaces. These coatings provide a topological and chemically compositional platform and complex surface to the fouling organisms (like larvae), reducing their adhesion by obstructing both hydrophilic as well as hydrophobic interactions between the organism’s secretions and the coating surface.50 An extensive review by Yilgor93 emphasized the silicone containing diblock and triblock copolymers synthesized using the following polymerization techniques: (1) anionic polymerization, (2) ring-opening polymerization, (3) atom-transfer radical polymerization, (4) step-growth polymerization and (5) chemical combination of preformed blocks, focusing on the functionality and the reactivity of the silicone polymers.93  \n\nBlock copolymerizaton is a breakthrough technology used to develop desired properties with fusion of siloxane or/and fluorinated polymers, well-known to possess the following characteristics: low elastic modulus, low surface energy,94 higher surface enrichment and microphase separation, high purity,95 and chemical and weather resistance. It is a controlled polymerization technique,96 with benefits of complete consistent wetting, thermotropic mesophase formation, wellordered liquid crystalline surface domains, oleophobicity, thermodynamic and surface stability.97 Nevertheless, the surface chemistry of the twin polymers remains misapprehended, and as a result, it is unable to reach the desired level of effective surface control.98 Therefore, in order to understand the surface chemistry depicting in detail the surface aggregation, accumulation and segregation of two or more polymer matrices, the recent developments by various researchers are amalgamated and reported as follows.  \n\nDiblock copolymers of PDMS and poly(perfluorooctylethyl acrylate) were synthesized by Martinelli et al.85 leading to the formation of smectic structures. A probe length of $4\\mathrm{nm}$ of GISAXS equipment confirmed the presence of fluoroalkyl helices as the main constituent at the surface, and at the same time the presence of PDMS below $4\\mathrm{nm}$ level can be speculated. DCA value of $110^{\\circ}$ was reported owing to the hydrophobic nature of PDMS. The obtained coatings were highly oriented with approx. $1\\mathrm{nm}$ thickness of fluoropolymer aggregation on the surface, giving excellent release properties suitable for marine coating application.85 Albert et al.99 in a conference proceeding reported the significance of amphiphilic block copolymers that are well-known to impart hydrophilicity by altering the surface properties of PDMS prepolymer by the embodiment of zwitterionic polymers for enhancement in foul-release performance of the coatings.99,100 Similar findings on amphiphilic networks were reported by DeSimone et al.101 They synthesized copolymer networks from polyethylene glycol (PEG) and perfluoropolyether (PPFE) and monitored the effect of humidity on curing behavior resulting in different surface properties. The highest dynamic contact angle value reported was $110.8^{\\circ}$ for high humid condition cured coatings. Such coatings cured under ambient humid ( $24\\%$ -low and $57\\%$ -high) conditions were reported as unique foul-release coatings since the fouling generated can be removed easily from the coating substrate by the application of water jet.101 Bodkhe and team102 reported the fabrication of amphiphilic PDMS-polyurethane coatings capable of fighting the microorganisms (N. incerta, ${\\bar{H}}.$ pacifica, C. lytica) and the macroorganisms (adult barnacles and macroalga) when exposed to artificial seawater (ASW) environment. The surface stratification and surface reconstruction properties led to the generation of more hydrophilic structures successful in deterring a wide variety of marine fouling organisms delivering a smooth foul-release performance.102  \n\nEkin and Webster103 synthesized triblock copolymers of PDMS and polycaprolactone crosslinked with polyurethane revealing good underwater performance with low surface energy and desired toughness.103 Tingfa et al.104 intermixed polyurethane and acrylic latex to yield poly(siloxane-ether-urethane)-acrylic hybrids. The hybrid coatings consisted of properties of waterborne emulsions as well as adhesives. The surface hydrophobicity was reported to increase from $72.5^{\\circ}$ to $101^{\\circ}$ , attributed to phase separation, which gives rise to self-stratification and enhances foulrelease properties. Because of excellent elongation, the water resistance and surface roughness properties also improved.104  \n\nThe problem of surface accumulation of polymers over one another was common and has been explored by many researchers in the past. To overcome the prevailing issue Liu et al.13 formulated PDMS and polyurea copolymeric coatings with good mechanical properties. The images of the coated panels, after immersion at South China Sea for a duration of 60 days, are represented in Fig. 20. The results are valid up to a speed of 18 knots for $200\\mathrm{~h~}$ under navigation and also in suspended conditions since the coatings adhered strictly onto the substrate.13  \n\nSimilar findings on siloxane-polyurethane amphiphilic block polymer coatings were reported by Sommer et al.105 using ground titanium dioxide as the potential pigment copolymerized along with the monomers. The results of pseudobarnacle adhesion and leachate toxicity analysis revealed that the percentage removal of marine bacteria remained unchanged but the gloss performance increased dramatically.105 In continuing theme of titanium dioxide embodiment into polymeric materials, a study on the effect of photocatalysis on curing characteristics of the synthesized copolymeric coatings was reported by Safty et al.106 (Fig. 21). The polarized light microscopy results revealed surface homogeneity since the bactericidal titanium dioxide nanoparticles were dispersed homogeneously inside the polymer matrix. The laboratory biological assays and field immersion tests demonstrated that the results were on par with the desired foul-release performance of the commercial coatings.106,107  \n\n![](images/aeb1d66f648d4f92cd7dd8eecea4fe9f1763ffd1eec62d9a4d0e7c7faadb405a.jpg)  \nFig. 20: Photographs of coated panels after static immersion in South China Sea for 60 days. Reprinted with permission from.13 Copyright 2016, American Chemical Society  \n\n![](images/f86082f637a4f3f953c04f5276acba5acd59fef410708e7bed739ccb404403c0.jpg)  \nFig. 21: Photocatalysis of PDMS/TiO2 nanocomposite coating under the influence of UV light.106 Copyright 2016, reproduced with kind permission from Elsevier  \n\nIn extensive research based on high-throughput (HT) combinatorial approach,108 a total of 75 FR coatings were synthesized by Majumdar et al.109 by incorporating quaternary ammonium salts (QAS) into siloxane matrix reporting a tremendous improvement in hydrophobicity and foul-release performance against marine bacteria, C. lytica. Minimum leachate toxicity was reported for 69 coatings out of the 72 investigated coatings. In addition, the combinatorial approach was successful in determining the optimum and broad-spectrum antimicrobial activity of trimethoxysilane functional QAS (QAS-TMS) incorporated PDMS coatings out of 60 unique coating compositions. The strong antimicrobial performance exhibited by synthesized coatings was attributed to the segregation of QAS rich domains over the coating–air interface.110 In the continuing theme, an advanced combinatorial high-throughput (C/HT) FR laboratory assay was utilized in consecutive work by Majumdar et al.111 to synthesize QAS-functional alkoxysilane incorporated PDMS AF/FR hybrid coatings, cured using methyltriacetoxysilane. A total of 24 different formulations were exposed to high-throughput bacterial and diatom assays reporting strong antimicrobial performance manifested by ethoxy silane groups of QAS molecules.111 In another work, copolymer of methylhydrosiloxane–dimethylsiloxane (PMHS– PDMS) was amalgamated with tethered QAS, later moisture cured and cast over bare and primed aluminum disks for the evaluation of AF performance against ASW bioassays. The effect of hydride equivalent weight of copolymer on AF/FR hybrid performance of moisture-cured coatings was evaluated and $29\\ \\mathrm{wt\\%}$ QAS concentration was reported as optimized composition with maximum biocidal activity.112 The combination of different biocides can bring about a significant amelioration in protein adhesion and biofilm retraction performance of the siloxane tethered AF coatings. These findings were corroborated in consecutive research by Ye et al.113 through the incorporation of triclosan and $\\mathbf{C{-}14\\mathrm{~OAS}}$ into PDMS matrix tested against fibrinogen as model globular protein. The research outcome confirmed restructured surfaces by biocide incorporation into PDMS matrix which resulted in biocidal retraction, increased AF performance and reduced biofouling growth.113 In addition, the sum frequency generation (SFG) spectroscopy was utilized to study polymer-structured surfaces of silanol-terminated PDMS-QAS tethered hybrid coatings. Also, a track of coating surface interaction with air, water, solvent, and nutrient– growth medium (NGM) interface was maintained using SFG. The antimicrobial performance of the biocide-tethered coating toward E. coli, S. aureus, and C. albicans immersion assays was found to be dependent on alkyl chain length of the QAS molecule.114 The previously discussed research contributions serve as a breakthrough in siloxane-urethane coating chemistry for protecting marine structures from biofouling.",
        "category": " Results and discussion"
    },
    {
        "id": 11,
        "chunk": "# Nonfouling technology based on (epoxy resins-PDMS) hybrids  \n\nEpoxy resins have been used in coating industries since inception, for protecting vessels with highquality paint technology applied onto ship hulls as well as deck and superstructures subjected to salt spray, weather conditions, and cargo spillage. Although the painted deck surface is continuously abraded by foot traffic and cargoes, it is able to withstand hard wear and repeated cleaning.115,116 In the past era, before the discovery of purely foulrelease polymer (PDMS) for potential foul-release applications, epoxy coatings were recoated by chlorinated rubber forming an impervious coat to water for preventing fouling.116 The unique properties like thermal stability, multiphased chemistry, biocompatibility, environment friendliness, and low toxicity, enables PDMS to be utilized as an effective material to be blended with epoxy resins.117 PDMS has been used as the potential precursor for the synthesis of protective polymer coatings because of reduction in its frictional drag resistance, elevation in corrosion and marine biofouling resistance and hydrophobicity of the designed system. The downside of polysiloxane-based coatings has been accompanied by poor adhesion and mechanical properties resulting in easy damage during navigation, ultimately reducing the performance and lifetime of the coatings. In this regard, one of the effective synthesis pathway to improve the mechanical properties of the protective coatings is either the introduction of epoxy resin segments and/or the incorporation of additional fillers and modifiers into the base polymer matrix.33 The possibility of combining the advantages of polysiloxane and epoxy resins has attracted extensive attention of many researchers in the past decade. This combination would endow fabricated polymeric coatings better thermal, oxidative stability, and lower temperature flexibility than neat epoxy resin and better mechanical and abrasion properties than neat polydimethylsiloxane.  \n\n![](images/195008768068683349a21a853d0ef836b56dab8fd0f46c16102debeb8202ee61.jpg)  \nFig. 22: Combined catalogue of the properties of epoxy– PDMS-filled and unfilled systems",
        "category": " Introduction"
    },
    {
        "id": 12,
        "chunk": "# Nonfouling (epoxy resin-PDMS)-unfilled coating systems  \n\nThe blending avenue of epoxy resins with siloxane polymers opened multidirectional research areas for many researchers in the past decade reporting numerous reasons for blending the two polymers. A possibility to combine the advantages of both the polymers would bestow enhancement in the following properties as represented in Fig. 22.16,55,118–122 A paradigm shift in the attractive silicone polymer chemistry was studied by Yilgor et al.93 revealing numerous advantages of silicone polymers, entitling them to be blended with thermoset polymers like epoxy resins to foster unique copolymers suitable for eclectic applications.93  \n\nIn the nineteenth century Riffle et al.123 gave noteworthy contributions in the field of fabricating elastomeric epoxy networks by utilizing polysiloxane modifiers via equilibrium polymerization technique. The types of siloxane modifiers depending on the endterminated functionality were discussed along with an in-depth reaction kinetics of the two polymers. The results from conversion vs time plots showed that the siloxane polymers had higher reactivity when compared to amines, used as curing agents. The resultant films were turbid, tough and durable, suitable for hardcore applications for safeguarding marine structures.123  \n\nCopolymers of epoxy resins and polydimethylsiloxane with different functionalities like hydroxyl, aminopropyl were synthesized, deploying an assorted range of PDMS with molecular weights (between 650 and  \n\n![](images/e40cc5b004ddc98496930870b75145663acf981c6da2fdcc79e85323808a1e46.jpg)  \nFig. 23: The synthesis route of epoxy–PDMS core–shell microspheres.124 Copyright 2014, reproduced with kind permission from Springer  \n\n$24{,}000\\ \\mathrm{g/mol})$ . The intercrosslinking network (ICN) mechanism was employed for the formulation of foulresistant coatings using epoxy resins and silicone phosphorous polymers, by Kumar et al.118 The immunity against fouling was accessed by determining the growth size (in mm) of the barnacle species (Balanus variegatus) and the volume $\\%$ of growth on the mild steel coated substrates recorded against time frame of 30–200 days. It was reported that the minimum and maximum size of the barnacles grown over the substrate surface was $4~\\mathrm{mm}$ and $8~\\mathrm{mm}$ , with $10\\%$ of barnacles found dead. The use of different curing agents for epoxy resins and the incorporation of silicone in epoxy resin matrix led to an improvement in the antifouling performance of the coatings.118  \n\nThe reactive compatibility of the epoxy–PDMS system for various applications was investigated by Baselga et al.120 to develop a tougher thermoset material system exhibiting gradient structure initially phase separated, later diminished on the phase boundaries along with the curing profile. As evident from SEM and $\\mathbf{X}$ -ray microanalysis results, the microhardness and the domain size of gradient were homogenized over the course of time portraying miscible and fully compatible microstructures.120 The dispersion of polysiloxanes into epoxy resin matrix was improved in a research study by Rajagopal et al.124 by employing core–shell microsphere analogy using suspension polymerization technique (Fig. 23). The siloxane core and epoxy shell led to the formation of elastomeric microspheres.124  \n\nThe presence of spherical cavities was confirmed by studying the fractured surfaces obtained from mechanical tests. The SEM images along with the elemental composition of a single microsphere revealed that the maximum amount of silicon was present in the core as compared with the composition of shell (Fig. 24).  \n\nThe Halpin–Tsai model124,125 and the Lewis–Neilson model124,126 were used for prediction of modulus of epoxy–composites under the influence of adhesion between the polymer (epoxy resin) and the modifier/toughening agent (PDMS). The results of the study were in close agreement with rubber cavitation mechanism resulting in development of toughened epoxy systems attributed to incorporation of siloxane class.124 Similar findings on IPN coating networks of epoxy and siloxane were reported by Jia et al.127 revealing no clear interface between the two phases ranging in nanoscale, marking the successful polymerization of IPNs suitable for marine coating applications.  \n\nThe surface modification of such epoxy–PDMS blends was carried out by Jannesari et al.121 where they transformed the partial compatibilization of epoxy–PDMS blends to full compatibilization by segregating the dual PDMS-rich and epoxy-rich phases into an individual phase. Later, the irregular dispersion in the blended specimen was cleared after overcoming the following factors responsible for immiscibility of the blends as reported by ${\\mathrm{W}}{\\mathrm{u}}^{128}$ difference in (1) matrix viscosity, (2) mixing shear stress, (3) interfacial tension of two polymers, and (4) viscosity ratio of dispersed phase to continuous phase. The Taylor’s dispersion model was employed for dispersion studies, to find a relation between dispersed phase particle size and the viscosity ratio given by the following equation  \n\n$$\nr={\\left({2\\Gamma p^{\\mp0.84}}\\right)}/{\\eta_{m}\\gamma}\n$$  \n\n![](images/3935eb142beee20b490682fd9e3a5cbe2a17e49836923a0c204092e750fec903.jpg)  \nFig. 24: (a) Cross-sectional SEM image of a PDMS-epoxy core–shell elastomeric microsphere, (b) EDX analysis of the shell, (c) EDX analysis of the core.124 Copyright 2017, reproduced with kind permission from Springer  \n\nwhere $r$ is the number-average particle size, $\\gamma$ is the shear rate, $\\Gamma$ is the interfacial tension between the two components resulting in dispersion, $p$ is the viscosity ratio of the dispersed and matrix phase viscosities. The case study depending upon the value of $p$ was discussed as follows128  \n\nCase-I: For all $p>1$ ; the exponent is positive. Case-II: For all $p<1$ ; the exponent is negative. Case-III: For $p=1$ ; blend of equiviscous components, acquires the finest morphology.  \n\nFurthermore, surface studies revealed that the hydrophilic surface of the neat epoxy coating specimen transforms to a hydrophobic surface upon the incorporation of hydroxy-terminated PDMS fulfilling a prerequisite of foul-release coatings which can be attributed to the water-repellent nature of PDMS.121 Similar findings were reported by Romo-Uribe et al.129 describing the dispersion of PDMS as rubber phase in droplet form, dispersed into epoxy-rich matrix. The droplet’s size summed up from histograms of droplet diameters fostered from AFM micrographs lies between 0.6 and $0.8~{\\upmu\\mathrm{m}}$ , initially phase separated leading to the interpenetration of siloxane moieties into epoxy phase. The increase in toughness was attributed to the energy absorption capacity of functional rubber-rich phase, imparting the blend’s desirable mechanical and thermal stability, making it suitable for structural coating applications.129  \n\nThe advantages of modification of epoxy resins with PDMS of amine functionality have been reported by Huang et al.16 leading to oleophobic upgradation of the surfaces.16,130 The present approach provided a way to synthesize triblock copolymers of polyether and PDMS by modifying epoxy resins. The ESCA investigation estimated the degree of PDMS accumulation over the substrate trio (PTFE, steel and silicone rubber). The results revealed that the formulated blends cast over stainless steel possess stick-flip phenomena along with increased hydrophobicity and lower value of coefficient of static friction $(\\mu_{\\mathrm{s}})$ .16 Zhou et al.55 synthesized epoxy–siloxane hybrid coating systems via silicone intermediate synthesis with appropriate methoxy content and without the incorporation of any internal reinforcement. It possessed strength equivalent to that of the filled systems coated on aluminum substrates. The epoxy–siloxane-grafted polymer coatings were synthesized with better mechanical properties, heat resistance and amphipathic nature contributing toward foul-release applications.55  \n\nApart from superior mechanical properties, adhesion to the substrate is one of the prerequisites for maintaining the stability and lifespan of protective coatings, independent of the substrate surface and its make. The poor adhesion of coating to the hull substrate leads to coating delamination, which gives rise to several prospects of ship hull coating disintegration. The most likely phenomenon to take place is corrosion, under the influence of highly humid atmospheric conditions with high moisture content in and around the sea. This is depicted in Fig. 25, which shows ship hull degradation in possible ways, subjected to static and dynamic exposure of consecutive cycles of high and low tides conditions prevailing for prolonged periods, captured under the influence of Bay of Bengal, India.  \n\nThe adhesion strength is mostly dependent on the physical and chemical interactions between the substrate and the polymeric coating. The adhesion strength and pull of behavior of the epoxy–silicone dual-layered antifouling coatings were discussed by  \n\n![](images/3fb093830b7ac5adf32d049d58605d1d883495b80afa774dffec5006923f4fdd.jpg)  \nFig. 25: (a) Degradation of ship hull due to the breakdown of protective coating layer leading to biocorrosion, (b) the visible underwater growth suspended onto the ship hull surface, (c) growth of freshwater algae and seaweeds attached to the hull, (d) the on-shore growth of adult barnacles and mussels adhered strongly to the concrete surface, (e) the continuous and steady recruitment of biofouling progressively led to the permanent settlement of macroorganisms  \n\nKohl et al.131 to emphasize thickness of coating which is directly proportional to the coating toughness, attributable to the generation of more stiffer bonds between the copolymers. The structure of the duplex coating is illustrated in Fig. 26.131  \n\nThe extension of the Kendall model (1971)132 was carried out to determine pull-off force of a metal cylinder attached to the dual-layered epoxy–PDMS antifouling coating. The adhesion strength of elastomers depends upon the elastic properties, surface energy, bulk modulus, and contact radius. The following equation was developed by Kendall to quantify the critical pull-off force $(\\bar{P}_{\\mathrm{c}})$ required to detach a rigid cylinder attached to thin elastomeric glue film on a metal substrate:  \n\n$$\nP_{\\mathrm{c}}=\\pi a^{2}\\bigg(\\frac{2w_{\\mathrm{a}}K}{t}\\bigg)^{1/2}\n$$  \n\nwhere $t$ denotes the thickness of the coating, $w_{\\mathbf{a}}$ denotes the Dupre’s work of adhesion between the cylinder and the elastomer, $K$ denotes the bulk modulus, and a is the contact radius.132",
        "category": " Results and discussion"
    },
    {
        "id": 13,
        "chunk": "# Nonfouling (epoxy resin–PDMS)-filled coating systems  \n\nTo impart additional benefits of improved hydrophobicity, foul-resistance and durability to the existing epoxy–PDMS coating system, it needs to be blended with various additives. These additives can be added in different concentrations in terms of $w t\\%$ or $1\\%$ playing the role of either a potential filler or a modifier focusing to instigate the desired property into the polymer coating system. The most frequently used inorganic fillers to modify the properties of prevailing coating systems are fumed silica, calcium carbonate, titanium dioxide, iron oxide, carbon black, natural sepiolite, MWCNT, graphene and fluorographene.133,134 In the past few studies, these fillers were used to improve the tensile modulus and hydrophobicity of the surface coatings.93,121,135 Beyond mutual copolymerization, the basic polymers (epoxy resin and PDMS) were also blended with engineering plastics like polyamide-6 to impart permanent surface modification to existing polymeric structures, attributed to the bond chemistry and the linkages among them.93,136 The reported methods in literature extend significantly to the available toolbox used for the incorporation of additives (both polymer and metal) into the epoxy–PDMS coating system.  \n\n![](images/9ceb3b88d8bba2cbe8293661315bb2c10ab1a4812690f7297b5503568563c621.jpg)  \nFig. 26: Epoxy–silicone (PDMS elastomer) duplex coating on steel substrate.131 Copyright 1999, reproduced with kind permission from Springer  \n\n![](images/8f5e83fbdbd787d56fdd44e6818a0949d4ef17ddfb06db1f6ceef3f53ab89b3b.jpg)  \nFig. 27: Dispersion of silica nanoparticles in PDMS-Epoxy nanocomposite coatings.138 Copyright 2017, reproduced with kind permission from Elsevier  \n\nSaravanan et al.137 synthesized epoxy–PDMS-filled nanohybrid systems using titanium dioxide as the nanofiller coated on mild steel substrates. The morphological studies revealed the uniform distribution of $\\mathrm{TiO}_{2}$ nanoparticles, and the salt spray test confirmed the fruitful addition of $\\mathrm{TiO}_{2}$ by inhibiting corrosion. High crosslinking density and consequent hydrophobicity of the coatings repelled and resisted fouling products. The disinfectant property of titanium dioxide nanoparticles adds up to the antibacterial performance of coatings entitling them for effective nonfouling applications.137 Ammar et al.138 fabricated epoxy– PDMS hybrid coatings and utilized finely powdered silicon dioxide as a nanofiller, incorporated via solution intercalation method in different weight ratios. As evident from SEM micrographs (Fig. 27), uniform dispersion of silica nanoparticles throughout the host polymeric matrix with rough surface was obtained. The contact angle results revealed remarkable enhancement in hydrophobicity of the filled hybrid coatings with a contact angle value of $132^{\\circ}$ and superior electrochemical properties leading to better anticorrosive performance.138  \n\nXu et al.139 formulated epoxy–PDMS-filled systems consisting of silicon dioxide as potential filler along with the aid of two compatibilizers (GPTMS and ATS) added in predetermined proportions. The research emphasized microcrack prevention by modulating the sequence of addition of monomers and coupling agents, and later transparent (gel-like) cured hybrid systems were obtained. The increased hydrophobic character of the formulations was attributed to methyl groups, which penetrated well into the epoxy–filler matrix. The solvent-free $\\mathrm{SiO}_{2}$ -modified epoxy–siloxane hybrids led to the development of nonporous hydrophobic segments suitable for marine structural protective applications.139  \n\nDuraibabu et al.140 synthesized tetrafunctional epoxy resin, later copolymerized with synthesized nano-zinc oxide ( $1\\ \\mathrm{nm}$ to $50\\ \\mathrm{nm}$ ) coated on mild steel substrates. The TEM results revealed spherical morphology and monodispersity of the zinc oxide nanoparticles. Antimicrobial tests were conducted employing the bacterial culture (E. coli) (code-ATCC 8739), utilizing inhibition zone method. The antibacterial performance was ascribed to the rough surface texture and the electrostatic interaction of $\\scriptstyle z_{\\mathrm{nO}}$ nanoparticles with the cell surfaces of microorganisms. Similar findings were reported by Ramesh et al.141 on the synthesis of epoxy–PDMS blend coatings along with the incorporation of zinc oxide nanoparticles in varied proportions via solution intercalation method to yield nanocomposite coatings with improved hydrophobicity (max. CA value $=128^{\\circ}$ ) and anticorrosive performance.141  \n\nThe adhesion strength of the epoxy–PDMS-filled systems was evaluated by Esfandeh et al.142 by studying the different ways to coat aluminum substrates by using top coat and tie coat layers, under the influence of various adhesion promoters. The SEM image revealed good compatibility and adhesion between the two polymers, as depicted in Fig. 28. From the field immersion studies conducted for a duration of 7 months and at a depth of $11\\mathrm{~m~}$ beneath the sea, it was found that the intermediate tie coat of silicone– epoxy with $1\\ \\mathrm{wt\\%}$ silane top coat showed no delamination and maintained better fouling resistance.142  \n\n![](images/6d219aa37a8634cb174a18ecba633024a1aedb6f824393a77e2ee42b03f50b91.jpg)  \nFig. 28: SEM image at the interface of epoxy base coat and intermediate layer (silicone/epoxy, 1 $w t\\%$ silane).142 Copyright 2010, reproduced with kind permission from Elsevier",
        "category": " Results and discussion"
    },
    {
        "id": 14,
        "chunk": "# Role of fillers and additives in PDMS-based coatings  \n\nThe role of fillers and additives added in the form of nanoparticles to alter wettability of the base matrix yield either self-cleaning or superhydrophobic surfaces as is well discussed in the literature. The consequence of addition of fillers and additives on the properties of the polymer coating system is discussed as follows.  \n\nFor PDMS polymer the swelling ratio analysis was done by Bokobza and Diop49 reporting swelling ratio inversely proportional to the degree of interaction between the rubber and the filler. Swelling is undesirable and swelling ratio gives the measure of resistance offered by vulcanizates to swell within the solvents. Theory of swelling is generally applied to evaluate the total network chain density, which is given by the following equation49  \n\n$$\n\\nu=\\nu_{\\mathrm{r}}+\\nu_{\\mathrm{f}}\n$$  \n\nwhere $\\nu_{\\mathrm{r}}$ represents the number of effective network chains in the unfilled rubber and $\\nu_{\\mathrm{f}}$ represents the number of additional chains produced by bonding to the filler. The significance of swelling measurement over stress–strain measurements is that the amount of additional crosslinking can also be determined, affected by the presence of filler which is free from hydrodynamic reinforcement.49 In order to calculate the equilibrium swelling ratio $(Q_{\\mathrm{r}})$ of the rubber phase, it is assumed that the filler particles do not swell in the solvent phase, given by the following equation  \n\n$$\nQ_{\\mathrm{r}}={\\frac{Q-\\phi}{1-\\phi}}\n$$  \n\nwhere $\\phi$ is the volume fraction of the filler and $\\boldsymbol{Q}$ is the equilibrium ratio of the composite, expressed as follows:  \n\n$$\nQ={\\frac{V}{V_{\\mathrm{d}}}}\n$$  \n\nwhere $V$ denotes the volume of the sample and the solvent, and $V_{\\mathrm{d}}$ denotes the volume of the dry sample. It was observed that the increase in the filler content resulted in consequent decrease in the swelling ratio on account of restricted alignments due to the incorporation of filler at higher loadings.49  \n\nThe mechanical strength of PDMS was increased by incorporation of ‘‘closite $20\\mathrm{A}^{,,}$ nanoclay into HTPDMS (hydroxy-terminated polydimethylsiloxane), the analysis was done via shear lag model of indentation and FEA (finite element analysis). The objective was to derive the interfacial shear stress function of composite films and to measure the shear strength of nanocomposites reinforced by organically modified nanoclay platelets. As the indenter tip penetrates the elastic surface of the film, it deforms at the interface, and the interfacial shear stress can be quantified by the following equation  \n\n$$\n\\tau=\\tau_{\\mathrm{{max}}}\\sin\\left(\\frac{\\pi r}{\\frac{\\lambda_{o}}{2}}\\right),\\quad0\\leq r\\leq\\frac{\\lambda_{o}}{2}\n$$  \n\nwhere $\\tau_{\\mathrm{max}}$ denotes the maximum stress at the interface before yield. It was found that the interfacial shear strength is inversely proportional to the percentage weight of the nanoclay content, related to the shear thinning behavior of nanoclay particles. Moreover, the authors also reported optimum filler loading percentage as $5\\ \\mathrm{wt\\%}$ of nanoclay, since the maximum elastic modulus value was obtained for the same.143  \n\nWouters et al.144 fabricated a nanocomposite coating by incorporating inorganic, anisotropic, functionalized nanoparticles into the PDMS matrix. Two pathways for functionalizing sepiolite nanoparticles were used, namely ion-exchange and covalent functionalization. The coatings were subjected to biofilm formation, and later the obtained biofilms were exposed to the shear forces under turbulent flow and the quantification was done using the following equation144  \n\nThe surface topology results were in accordance with the Cassie–Baxter model, stating that interfacial tension played a significant role in altering the surface chemistry of the coatings. The interaction between two particles and between particle and coating determines the morphology and topology to account for aggregation, agglomeration and uniform dispersion of the nanoparticles throughout the polymer matrix.144 Zhang and team145 reported a mathematical approach for the determination of time of interaction of hydrophilic silica microparticles with an oligomer. The filler particles were rapidly encapsulated in PDMS matrix under the effect of capillary drag along with the gradient of surface energy as the driving force, $\\Delta\\gamma$ . By dimensional analysis, the time needed for encapsulation is represented by the following equation  \n\n$$\n\\tau\\sim\\frac{\\eta L}{\\Delta\\gamma}\n$$  \n\nwhere $\\eta$ represents the viscosity of the elastomer (PDMS) which retards the encapsulation process, and $\\mathrm{~L~}$ denotes the diameter of silica microparticle.145",
        "category": " Results and discussion"
    },
    {
        "id": 15,
        "chunk": "# Mathematical evaluation of foul-release performance  \n\nThe macrofoulers such as invertebrates, adult barnacles and their spores tend to adhere to certain selective surfaces according to the topography suitable to furnish sufficient adhesion with the coating surface. The interfacial properties responsible for the initial settlement of marine fouling and its subsequent growth are topography, wettability, and chemical heterogeneity; also considered as factors determining the foul-release performance.50 The joint-use of short-time rotary experiments, polymer reaction engineering and kinetic data studies can be brought together to provide empirical inputs for the development of mathematical models. They can be utilized to simulate the lifetime performance of the synthesized coatings in a study of just a couple of minutes.2 The flexible development of mathematical equations employed for the evaluation of foul-release performance of the coatings is a need of the hour.  \n\nJiang et al.83 referred the coating–water interface as a synthetic surface where foul-release activity takes place and described its durability mechanism with respect to modes of adhesion failure. The release ability of foulants is said to be directly proportional to the surface free energy of the substrate and to the modulus (gE)1/2,83,146 whereas in terms of fracture mechanics, the stress required to separate a foulant from the surface of the coating is given by the following equation  \n\n$$\n\\mathrm{Stress}={W E_{\\mathrm{c}}}{}_{/}^{1/2}\n$$  \n\nwhere $W$ is the work of adhesion, $E_{\\mathrm{c}}$ is the composite modulus of the adhesive matrix and the coating, and $a$ is the radius of contact.50 The minuscule contact angle of water droplets serves as a sharp knife, which scrapes off fouling from the substrate surface, releasing the microorganisms that were stabilized via electrostatic and weak van der Waals forces, resulting in the simple removal of micro- and macrofoulers.37 It is worthwhile to notice that degree of fouling assessment is dependent on the following parameters and can be determined by using any one of the ways37,147  \n\n1. Determination of critical velocity.   \n2. Measurement of adhesion strength.   \n3. Modeling the flow induced forces.   \n4. Measurement of wear resistance of surface.   \n5. Measurement of elastic stress.  \n\nLarsson et al.147 described a means of evaluating the mechanical adhesion strength of barnacle species (Balanus improvisus) by developing a hydrodynamic model based on assessment of local flow velocities close to the vessel’s hull, confirmed by the foulingrelease measurements for three different levels of barnacles. High-velocity sweeps were preferred over mean velocities for the organism detachment prediction, which were later expressed in terms of the boundary layer and fracture properties.147 The velocity gradient is described in the model of hydrodynamic forces acting on settled barnacles over a relatively smooth surface and is given by the following equation  \n\n$$\nu(z)=\\frac{u_{*}}{K}\\textcircled{1}\\frac{z}{z_{o}}\n$$  \n\nwhere $u(z)$ is the mean velocity parallel to the substrate at a distance $z$ above the surface, $u_{*}$ represents the friction velocity, K is von Karman’s constant and $z_{o}$ denotes the roughness parameter.147 Wouters et al.144 provided a simple tool for fouling-release performance evaluation of the benchmark coatings on the basis of percentage of biofilm formation; stating that the biofilm formation does not solely depend on surface wettability, as parameters like swelling characteristics need to be determined as well. The bar graph represents the performance of various formulated coatings in comparison to the reference siloxane coatings and the epoxy standard, which showed $100\\%$ performance from biofilm formation (Fig. 29).144  \n\n![](images/dbb03f4f6e490316238f56f00fad003a06dd991b85c536d5692a2002c4ed2ac9.jpg)  \nFig. 29: Performance of the coatings in relation to the respective benchmark coatings. Patterned bar $\\mathbf{\\sigma}=\\mathbf{\\sigma}$ formation of biofilm; unpatterned bar $\\mathbf{\\sigma}=\\mathbf{\\sigma}$ biofilm release.144 Copyright 2010, reproduced with kind permission from Elsevier  \n\n![](images/e97615161c29b5c08f65385f6fc6a139b79fe32c71181eeecf575f516ea32118.jpg)  \nFig. 30: A schematic of the rheometer apparatus.148 Copyright 2016, reproduced with kind permission from Elsevier  \n\nFeng Zhou et al.148 proposed an excellent way of evaluating the foul-release characteristics of the designed coatings by using rheometer apparatus (Fig. 30). This work supplies valuable evidence for the evaluation of drag-reduction efficiency for several potential applications such as foul-release, antifouling, superhydrophobicity and antidrag characteristics.148  \n\nThe antidrag performance on account of apparent boundary slippage can be evaluated by detecting a lower shear stress value, exerted over the bottom surface of the coating recorded by a process controller or an installed computer. Therefore, the drag-reduction efficiency can be estimated by the following equation  \n\n![](images/dde18c9badcaee39ab99224ac47fca548200abc8c147c82cf64e9644c8ac4d25.jpg)  \nFig. 31: Representation of the adhesion test carried out by clamping thread of the plaque, pulled apart from the substrate surface. Reprinted with kind permission from reference (77). Copyright 2016, American Chemical Society  \n\n$$\nD_{\\mathrm{E}}=\\frac{\\tau_{\\mathrm{non-slip}}-\\tau_{\\mathrm{slip}}}{\\tau_{\\mathrm{non-slip}}}\\times100\n$$  \n\nwhere $\\tau_{\\mathrm{non-slip}}$ and $\\tau_{\\mathrm{slip}}$ denote the shear stress exerted at the coating wall at no-slip and slip boundary conditions, respectively.149–152  \n\nWilker et al.77 reported a method for adhesion study which significantly extends the available toolbox used for foul-release assessments. The test was carried out to check the pull-off adhesion force of the mussels adhered to the surface of the coated panels, by clamping the threads uniformly in between the platens of the Instron 5544 materials testing machine, as depicted in Fig. 31.  \n\nThe maximum stress value obtained at the pulling rate of $10\\ \\mathrm{mm/min}$ was considered as the magnitude of removal force. Next, the adhesion of individual plaques was calculated by dividing the removal force by the area occupied by the plaque, mathematically expressed as follows:  \n\nZhang et al.94 performed an in-depth comparative analysis of five different types of commercial foulrelease coatings for their performance evaluation against two microrganisms, namely diatom and Ulva spore. Spearman’s rank correlation test was used to establish correlation and consistency between laboratory and field immersion tests. Evaluation of static and dynamic foul-release performances was done as per ASTM D 3623 and ASTM D 4939, respectively. The results of the study revealed that the value of Spearman’s coefficient $(r_{\\mathrm{{s}}})$ was between 0.975 and 0.949 for both (diatom and Ulva spore) which did not show much difference in the values, hence proving the validity of correlation.94 Kohl et al.131 enlisted the mechanical factors contributing toward the release behavior of the epoxy–silicone antifouling coatings where the release behavior of the microorganisms over the coating surface was accessed using a standard pulloff test. The significance of such tests lies in the determination of mechanical factors (like fracture energy) and thickness of the coating layer establishing an efficient mode of release of marine biofouling.122  \n\n![](images/6b33c8652d490ec41896da5b117d376113bbd2b918025d32649fb81117a10c43.jpg)  \nFig. 32: Antifouling micromixer surfaces thwarting biofouling  \n\nIn a recent research by Balazs et al.153 the surface topography of the coating when maintained like sawtooth structure, can be a breakthrough in inhibiting marine fouling without harming the environment. They assumed the coating surface as sawtooth structure and the microorganisms adhering to the surface as mobile microcapsules (Fig. 32). A constant shear force (replicated as the hydrodynamic drag of water) was applied to rupture bonds between two microcapsules, and this force also transported the separated capsules away from the water layer. This computation modeling was based on ‘‘herringbone chaotic mixers’’ to optimize the micromixer coating surfaces, utilizing the two models: the lattice Boltzmann method (LBM) for fluid dynamics and the lattice spring method (LSM) for the micromechanics of capsules. The calculated force acting between the inter-capsule bonds was termed as the Hookean spring force, and the formation or rupture of bonds was modeled through ‘‘Bell model.’’153",
        "category": " Results and discussion"
    },
    {
        "id": 16,
        "chunk": "# Conclusions  \n\nScience has given the world an increasingly diverse range of polymeric coatings that improve the performance and durability of metals and other industrial building materials. With an increased focus on sustainability, there has been a growing demand for highquality industrial coatings that can be applied to prefabricated and custom-built structures. The technology of long-chain molecular coatings has improved drastically since the 1960s. In the current era, marine industries use a variety of surface coatings to shield the interior and exterior of important structures. Polymeric coatings protect the treated substrate from weather, UV rays, corrosion and most importantly from marine biofouling.34 Polymeric materials play a vital role in the fabrication of protective coatings for various applications. In the field of marine application, the major contribution is made by the siloxane and fluoropolymers directing the research in safeguarding marine structures to the core. The diversified types of metals mainly aluminum, mild steel and their alloys are protected against the threats of corrosion and fouling by the implementation of surface coatings. Although much has been accomplished, urgent demands and great challenges remain to create more robust antifouling/foul-release coatings for shielding marine vessels from the adverse effects of biofouling.  \n\nPolydimethylsiloxane (PDMS)-based coatings are used to protect the base of ship hull against marine biofouling exposed for long time. The elastomeric potential application of pristine PDMS polymer is limited by the risk of adhesion failure due to its low surface energy and extremely low chemical reactivity. Thereby, with the objective of imparting desired mechanical stability by increasing the mechanical strength of PDMS, it is blended with polymers like epoxy resins. It prevents the blend coating from microcracking and establishes cohesion at the coating–metal interface. The converse of blending PDMS with epoxy resins also holds true, since the brittle character of epoxy thermosets needs to be overcome by imparting toughness through the embodiment of elastic siloxane backbone.  \n\nAmphiphilic polymers are very promising as current generation antifouling materials due to their outstanding dual-nature properties. The metallic inorganic nanoparticles and their oxides have been incorporated into the blend of polymeric systems on account of interactive filler/polymer interface. By imparting functionalization to the pristine polymers, it leads to the successful fabrication of cost-effective and reliable nonstick marine nanocomposite coatings for ship hulls.  \n\nThe role of such potential fillers has been discussed and quantified using mathematical and analytical tools. Such models help in determining the controlled biocide release mechanisms and their quantification, which are later distinguished as toxic and nontoxic systems. Also, their development would help to cut down long time duration taken by traditional empirical methods to formulate new foul-release systems. Thereby, the mathematical studies dealing with the influence of surface properties and process parameters on adhesion phenomena, responsible for easy release of fouling organisms are oriented and discussed in the last section of the review. To avoid the serious problem of ice formation on ship hulls, systems such as temperature independent foul-release coatings should be designed to remain stable and functional under deep-sea freezing temperatures. Another desirable system would include the combination of oil fouling and biofouling resistant coatings since at the stage of oil fouling, the polymeric coating properties like superhydrophobicity, mechanical stability, and others degrade and eventually fail.46 Therefore, the underlying principle of foulrelease performance evaluation, adhesion, and mechanical strength measurement under complex marine conditions should be further studied to sustain the development of robust nonfouling polymeric surface coatings for protecting marine structures.",
        "category": " Conclusions"
    },
    {
        "id": 17,
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        "category": " References"
    }
]