applied sciences

Article Preparation and Characterization of Electrosprayed Nanocapsules Containing Coconut-Oil-Based Alkyd for the Fabrication of Self-Healing Coatings

Roya Malekkhouyan 1, Saied Nouri Khorasani 1,*, Rasoul Esmaeely Neisiany 2 , Reza Torkaman 3, Mohammad Sadegh Koochaki 1 and Oisik Das 4,*

1 Department of Chemical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran; [email protected] (R.M.); [email protected] (M.S.K.) 2 Department of Materials and Engineering, Faculty of Engineering, Hakim Sabzevari University, Sabzevar 9617976487, Iran; [email protected] 3 Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran; [email protected] 4 Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Luleå, Sweden * Correspondence: [email protected] (S.N.K); [email protected] (O.D.)

 Received: 18 March 2020; Accepted: 29 April 2020; Published: 1 May 2020 

Abstract: In the present study, the preparation of nanocapsules using the coaxial electrospraying method was investigated. Poly(styrene-co-acrylonitrile) (SAN) was used as a shell material and coconut-oil-based alkyd resin (CAR) as a core. Chemical structure, thermal stability, and morphology of nanocapsules were characterized by Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), and field emission scanning electron microscopy (FE-SEM), respectively. In addition, the formation of the core–shell structure was approved by transmission electron microscopy (TEM) and FE-SEM micrographs of the fractured nanocapsules. Furthermore, differential scanning calorimetry tests (DSC) were carried out to investigate the reactivity of released healing agents from the nanocapsules. The prepared nanocapsules were then incorporated into the epoxy and applied on the surfaces of the steel panels. The effect of capsule incorporation on the properties of the coating was evaluated. The self-healing performance of the coatings in the salty and acidic media was also assessed. The FTIR results revealed the presence of both shell and core in the prepared nanocapsules and proved that no reaction occurred between them. The morphological studies confirmed that the electrosprayed nanocapsules’ mean diameter was 708 252 nm with an ± average shell thickness of 82 nm. The TGA test demonstrated the thermal stability of nanocapsules to be up to 270 ◦C while the DSC results reveal a successful reaction between CAR and epoxy resin, especially in the acidic media. The electrochemical impedance spectroscopy (EIS) test results demonstrate that the best self-healing performance was achieved for the 2 and 1 wt.% nanocapsules incorporation in the NaCl, and HCl solution, respectively.

Keywords: self-healing coatings; coaxial electrospraying; encapsulation; corrosion resistance; coconut-oil-based alkyd resin; epoxy resin

1. Introduction Metals are extensively utilized in numerous industries due to their superior physical and mechanical properties [1]. However, metallic structures are usually susceptible to corrosion, wear, and erosion causes financial damages. According to the World Corrosion Organization, the global annual cost of

Appl. Sci. 2020, 10, 3171; doi:10.3390/app10093171 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 17 Appl. Sci. 2020, 10, 3171 2 of 17 annual cost of corrosion is approximately 1.3 trillion Euros, or 3.1% to 3.5% of a country’s Gross National Product [2]. Therefore, the corrosion process of metallic structures is considered as a global economiccorrosion isissue. approximately Polymeric coatings 1.3 trillion are Euros, considered or 3.1% as to one 3.5% of ofthe a most country’s facile Gross andeconomical National Product strategies [2]. toTherefore, protect themetals corrosion from processcorrosion. of metallic The polymeric structures co isatings considered provide as a barrier global economic action as issue. well Polymericas active corrosioncoatings are inhibition considered due as to one their of the low most permeability facile andeconomical to corrosive strategies chemicals. to protect Polymeric metals coatings from corrosion. act as anThe obstacle polymeric between coatings the providemetallic barriersubstrates action and as the well corrosive as active environment corrosion inhibition around them due toand their extend low thepermeability lifetime toof corrosivethe metallic chemicals. structures Polymeric [3,4]. coatingsHowever, act asthe an polymeric obstacle between coatings the are metallic susceptible substrates to damages,and the corrosive once they environment encounter temperature, around them UV and ra extendys, and the external lifetime mechanical of the metallic shocks. structures The created [3,4]. damagesHowever, consequently the polymeric deteriorate coatings are the susceptible barrier toperfor damages,mance once of the they coatings. encounter Therefore, temperature, self-healing UV rays, coatings,and external as smart mechanical coatings, shocks. have The been created developed damages to consequentlyenhance the deterioratelifespan of thethe barrier polymeric performance coating andof the consequently coatings. Therefore, the metallic self-healing structures coatings, underneath. as smart coatings, have been developed to enhance the lifespanIn recent of the years, polymeric several coating strategies and consequently for the development the metallic of self-healing structures underneath. have been employed, such as theIn release recent of years, healing several agents, strategies reversible for the bonds development and also ofnanoparticles self-healing with have the been ability employed, to migrate such [5,6].as the It release has been of healing proved agents, that the reversible release of bonds the healing and also agent nanoparticles is the most with well-studied the ability method to migrate among [5,6]. theIt has others been [7,8]. proved When that a the crack release initiates, of the healingthe release agent of isthe the healing most well-studied agent protects method the crack among from the propagationothers [7,8]. Whenand decreases a crack initiates, the penetration the release of of theoxyg healingen, water, agent and protects ions theinto crack the fromsubstrate. propagation Three strategiesand decreases for thestoring penetration the healing of oxygen, agent water, are and as ions follows: into the the substrate. usage Threeof micro/nanocapsules, strategies for storing micro/nanofibersthe healing agent are[9], as and follows: micro/na the usageno vascular of micro [10]/nanocapsules, based systems. micro In/ nanofibersrecent years, [9], many and micro attempts/nano havevascular been [10 made] based to systems.produce Inan recent optimized years, self-hea many attemptsling coating have using been madethe capsules-based to produce an optimizedsystem. It wasself-healing reported coating that the using nanocapsules the capsules-based showed system.better self-healing It was reported performance that the nanocapsules in comparison showed with microcapsulesbetter self-healing [11,12]. performance in comparison with microcapsules [11,12]. Among the several employed healing agents, the al alkydkyd resins resins have have recently recently gained gained considerable considerable attention. Alkyd Alkyd resins resins are are special special types types of of polyeste polyestersrs with with distinctive distinctive properties properties that that are are made made by by thethe polycondensation polycondensation of of three three kinds kinds of of monomers, monomers, including including , polyols, polybasic polybasic acids, acids, or or anhydrides, anhydrides, fattyfatty acidsacids obtainedobtained from from triglyceride oils oils (or (or vegetable vegetable oils). oils). [13 ,14[13,14].]. There There are twoare typestwo types of fatty of acids:fatty acids:drying drying alkyds alkyds with enough with enough unsaturated unsaturated fatty acids fatty that acids cure withthat oxygen,cure with and oxygen, non-drying and non-drying alkyds with alkydsa lower with amount a lower of unsaturated amount of un fattysaturated acids that fatty cannot acids be that cured cannot with be oxygen. cured with Non-drying oxygen. alkydNon-drying can be alkydused as can a healing be used agent as a due healing to its crosslinkingagent due to sites its (thecrosslinking presence ofsites carboxyl (the presence and hydroxyl of carboxyl groups) and [15], hydroxylwhile they groups) are stable [15], against while oxidation they reactionsare stable due against to the lackoxidation of unsaturation reactions bonds due [ 16to]. Consideringthe lack of unsaturationsome properties, bonds including [16]. Considering low cost, safe some to work properties, with, a including renewable low source, cost, and safe eco-friendly, to work with, these a renewabletypes of oils source, can be extensivelyand eco-friendly, used in these anticorrosive types of coatings oils can for be di ffextensivelyerent industries. used Figurein anticorrosive1 shows a coatingsplausible for structure different of industries. the alkydresin. Figure According 1 shows a to plausible Figure1, structure the hydroxyl of the and alkyd carboxyl resin. groups According can topossibly Figure react1, the with hydroxyl other and functional carboxyl groups, groups i.e., can residual possibly epoxy react groups,with other or oxygenfunctional to formgroups, a solid i.e., residualbinder,making epoxy groups, it an eco-friendly or oxygen to healing form a agent solid forbinder, the epoxy making coatings it an eco-friendly [17]. In addition, healing in agent the realfor thecondition, epoxy coatings the reaction [17]. isIn catalyzed addition, byin the real protons cond andition, possibly the reaction chloride is catalyzed ions, which by the are protons available and in possiblythe environment chloride [ions,18]. which are available in the environment [18].

FigureFigure 1. 1. ProbableProbable structure structure of of alkyd alkyd resin resin ( in triglyceride oil is shown by R).

Since the introduction of self-healing coatings, several methods, i.e., in-situ in-situ and interfacial polymerization, multi-stagemulti-stage emulsion emulsion polymerization, polymerization, and solventand solvent evaporation, evaporation, have been have developed been developedto encapsulate to encapsulate healing agents healing within agents micro /withinnanocapsules micro/nanocapsules [19,20]. These methods[19,20]. These are expensive, methodstime are expensive,consuming, time and consuming, need purification. and need To purification. address the aforementionedTo address the aforementioned shortcomings, the shortcomings, electrospraying the Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 17 Appl. Sci. 2020, 10, 3171 3 of 17 electrospraying method has been used for the encapsulation of materials, while most researches were carried out for other applications, such as drug delivery. Electrospray, known as method has been used for the encapsulation of materials, while most researches were carried out for electrohydrodynamic atomization (EHDA), has attracted attention recently due to its high other applications, such as drug delivery. Electrospray, known as electrohydrodynamic atomization encapsulation yield and loading efficiency [21]. This method consists of liquid acceleration and cone– (EHDA), has attracted attention recently due to its high encapsulation yield and loading efficiency [21]. jet formation, which is the consequence of force balances [22]. The parameters that affect This method consists of liquid acceleration and cone–jet formation, which is the consequence of force electrospraying are high voltage, flow rate and distance of particle collector in addition to the balances [22]. The parameters that affect electrospraying are high voltage, flow rate and distance of particle characteristics of polymer solution, namely viscosity and concentration [23]. collector in addition to the characteristics of polymer solution, namely viscosity and concentration [23]. In the present study, CAR was encapsulated in SAN nanocapsules. SAN (as the shell of capsules) In the present study, CAR was encapsulated in SAN nanocapsules. SAN (as the shell of capsules) is used due to its superior encapsulation yield, which is an important factor for shell materials [24]. is used due to its superior encapsulation yield, which is an important factor for shell materials [24]. The coaxial electrospraying method was employed for the preparation of the capsules. The prepared The coaxial electrospraying method was employed for the preparation of the capsules. The prepared nanocapsules were then incorporated into epoxy resins and applied on the steel panels to develop a nanocapsules were then incorporated into epoxy resins and applied on the steel panels to develop self-healing coating. The effect of nanocapsule incorporation on the properties of the coating was a self-healing coating. The effect of nanocapsule incorporation on the properties of the coating was evaluated and the self-healing performance of the coating was investigated using EIS tests in the 3.5 evaluated and the self-healing performance of the coating was investigated using EIS tests in the wt.% NaCl and 0.5 M HCl media. 3.5 wt.% NaCl and 0.5 M HCl media.

2.2. Materials and Methods

2.1.2.1. Materials CARCAR withwith thethe viscosityviscosity of of 5.6 5.6 stocks stocks (5.6 (5.6 ×10 10−44 m22/s),/s), acid valuevalue ofof 1010 mgmg KOHKOHg g−11 and oil length × − − ofof 62%62% waswas purchasedpurchased fromfrom AriaAria ResinResin Co.,Co., Tehran,Tehran, Iran.Iran. SANSAN (Mw(Mw == 185 kDa, acrylonitrile 30 wt.%) andand NN,,NN-dimethylformamide-dimethylformamide (DMF,(DMF, 99.8%)99.8%) werewere obtainedobtained fromfrom Sigma-AldrichSigma-Aldrich (St.(St. Louis, MO, USA). TheThe dichloromethanedichloromethane (DCM)(DCM) waswas providedprovided byby MerckMerck (Darmstadt,(Darmstadt, Germany).Germany). AllAll thethe materialsmaterials andand chemicalschemicals werewere usedused asas receivedreceived withoutwithout anyany furtherfurther purification.purification. For the preparation of the coatings, EPONEPON 828,828, from from Hexion Hexion (Columbus, (Columbus, OH, OH, USA) USA) with with the the viscosity viscosity of 11–15 of 11–15 Pa.s Pa.s was usedwas used as epoxy as epoxy resin. Diglycidylresin. Diglycidyl etherof ether 1,6-hexanediol of 1,6-hexanediol (ED 180) (ED with 180) the with viscosity the viscosity of 0.015–0.025 of 0.015–0.025 Pa.s was Pa.s purchased was purchased from Inchemfrom Inchem Ltd. (Budapest, Ltd. (Budapest, Hungary) Hungary) and used asand the us reactiveed as diluent.the reactive Polyaminoamide diluent. Polyaminoamide (Merginamide A280)(Merginamide with amine A280) value with of amine 0.25–0.29 value g of KOH 0.25–0.29/g and g viscosity KOH/g and of 1–2 viscosity Pa.s was of 1–2 provided Pa.s was by provided Hobum Oleochemicalsby Hobum Oleochemicals (Hamburg, Germany)(Hamburg, and Germany) utilized and as the utilized curing as agent the curing of epoxy agent resin. of epoxy resin.

2.2.2.2. Encapsulation Process FigureFigure2 2 schematically schematically represents represents the the employed employed coaxial coaxial electrospray electrospray setup setup for for the the encapsulation encapsulation process.process. TheThe sprayingspraying home-made home-made nozzle nozzle consists consists of twoof two concentric concentric stainless stainless steel steel needles: needles: a core–shell a core– needleshell needle with gaugewith gauge 16 and 16 anand inner an inner needle needle with with gauge gauge 24. The24. The needle needle was was charged charged to theto the cathode cathode of highof high voltage voltage supply supply and and the the anode anode was was connected connected to the to platethe plate covered covered with with aluminum aluminum foil (actingfoil (acting as a collectoras a collector of the of nanocapsules). the nanocapsules).

Figure 2. Schematic illustration of the employed coaxial electrospraying setup. Figure 2. Schematic illustration of the employed coaxial electrospraying setup. Appl. Sci. 2020, 10, 3171 4 of 17

Coaxial electrospraying was carried out using SAN solution (5% w/v) at the outer needle (as a shell), while the CAR was filled in the inner needle. The blend of DCM:DMF with a ratio of 50:50 was used for the preparation of the SAN solution. The flow rate of the shell and the core solutions were changed from 0.5 to 0.7 mL/h and 0.04 to 0.06 mL/h, respectively. The applied high voltage was varied between 18 and 26 kV, and the distance between the tip of the needle and the collector was set at 15 cm.

2.3. Preparation of Self-Healing Coatings for Corrosion and Mechanical Tests Epoxy resin was mixed with reactive diluent with a 3:1 weight ratio to reduce its viscosity. The produced nanocapsules were dispersed into diluted epoxy resin using mechanical stirring with a speed of 200 rpm for 5 min. Then, the curing agent was added with a proportion of 100:50 (mass ratio of epoxy to polyaminoamide). The nanocapsules’ content inside the prepared mixture was set at 1, 2, and 4 wt.%. Before applying the coating on the steel panels, their surface was polished with sandpaper and washed by acetone to eliminate any contaminants on the surfaces. The panels were coated by epoxy mixtures using a film applicator (Zehtner Zau 2000.80, Zurich, Switzerland) and the thickness of the coatings was controlled to 100 µm. The coatings were allowed to cure for seven days at room temperature. For the self-healing evaluations, the cured coatings were scratched using a scalpel blade. The scratched coatings were then kept at room temperature for seven days to enable the healing process of the coating. The control panels were prepared by the same procedure but without embedding any nanocapsules into the resin.

2.4. Chemical Structure Evaluation The chemical structure of SAN, CAR, and nanocapsules were investigated separately using a 1 1 FTIR spectrometer (WQF-510A, China), with 32 scans from 4000 to 500 cm− at the resolution of 4 cm− . The samples of empty nanocapsules (only shell material) and those containing CAR were prepared by grinding with KBr, while, for the CAR sample, a thin layer of the CAR was spread onto the KBr pellet.

2.5. Evaluation of Encapsulation Yield and Core Content To investigate the amount of core content in the prepared nanocapsules, the extraction of CAR from nanocapsules was performed according to previous research [15]. Therefore, a known weight of the nanocapsules was crushed with mortar and pestle. As a result of crushing, the shells of nanocapsules were broken and the core was subsequently washed by ethanol. After dissolving the core in the solvent, the shell was filtered, washed with ethanol several times and dried at 40 ◦C for 24 h in an oven. The practical percentage of the core-content (Wpractical) was calculated according to Equation 1).

  Wca Wsh %Core content Wpractical = − 100 (1) Wca × where Wca refers to the weight of nanocapsules and Wsh refers to the weight of the shell [15]. The theoretical core content (Wtheoretical) can be also assessed by using the flow rates and densities of electrosprayed solutions according to [25]. Consequently, the yield of encapsulation can be obtained through Equation 2). Wpractical %Encapsulation yield = 100 (2) Wtheoretical ×

2.6. Morphological Studies The surface morphology, shape, and size of the prepared nanocapsules were investigated by a FE-SEM (QUANTA FEG 450, Graz, Austria). To confirm the successful encapsulation of the core by the shell, the thermodynamic behavior of polymer solution, shell, and CAR (core) should be assessed. In general, the tendency of a polymer phase to spread on a liquid or solid substrate for the formation of a shell layer can be explained by the spreading coefficient λij (Harkin’s equation [26]): Appl. Sci. 2020, 10, 3171 5 of 17

λ = γ γ γ (3) ij j − i − ij where γi, γj, and γij are the surface tensions of polymer phase and CAR, and the interfacial surface tension between the two phases, respectively. The Harkin’s equation is for describing the propagation of a liquid on a solid, and then the equation was extended for two immiscible solutions in a third immiscible phase [27]. Based on Harkin’s equation, the spreading of phase i on phase j will occur when the spreading coefficient is positive and when negative, the spreading is reversed [28]. To perform the calculation of the spreading coefficient in Equation 3), the interfacial surface tension can be approximated by employing the surface tension components directly (Equation 4)) [29]:  p 2 γ = √γ γ (4) ij i − j These evaluations are an approximation in equilibrium condition, which is not easily approachable in electrospraying, so there may be differences between experimental and theoretical results [30]. The surface tensions are measured using tensiometer (DCAT 11-Dataphysics, Filderstadt, Germany). The transmission electron microscopy (DS-960A DSS, Zeiss, Germany), operating at 120 kV was employed to practically confirm the core–shell structure of the prepared nanocapsules. Therefore, the nanocapsules were electrosprayed onto a Lacey Formvar/carbon-coated copper grids. Furthermore, the core–shell structure of capsules was studied through the investigation of the fractured capsules in the composite using FE-SEM. Therefore, the capsules were electrosprayed onto the epoxy resin and the composite was prepared by the addition of epoxy resin curing agent. The mixture was poured into a mold and cured at room temperature for three days. After three days, the composite was fractured in liquid nitrogen and was investigated by FE-SEM.

2.7. Thermal Stability Evaluations The thermal stability of the prepared nanocapsules, CAR, and neat SAN, were analyzed using a thermogravimetric analyzer (Perkin Elmer STA 6000 TGA system, Waltham, MA, USA) under an argon atmosphere. For the evaluation of the thermal behavior of the prepared nanocapsules, they were firstly washed with methanol three times and then dried in a vacuum oven for 24 h at 40 ◦C. For all 1 TGA tests, the heating rate was adjusted at 10 ◦C min− in the temperature range of 25 to 700 ◦C.

2.8. Evaluation of Healing Reaction Heat In order to investigate the reactivity of the embedded core material (the healing agent) inside the nanocapsules with epoxy resin during any damages, DSC tests were evaluated using a BÄHR-DSC302 (Hüllhorst, Germany) from room temperature to 300 ◦C. The DSC tests were carried out under nitrogen atmosphere.

2.9. Evaluation of the Coating Properties For investigating the effect of embedding nanocapsules on the adhesion strength of the coatings, pull-off adhesion tests were carried out according to ASTM D 4541 using a PosiTest AT-M from DeFelsko (Ogdensburg, NY, USA). Each sample was tested three times and the average of measurements was calculated and reported. In addition, the elongation at break of the coatings containing nanocapsules was evaluated through the bending tests according to ASTM D 522 test method A (conical mandrel bending tester SHEEN SH801-UK).

2.10. Evaluation of the Self-Healing Performance of the Coatings To assess the anticorrosive and self-healing ability of the coatings, salt spray testes were accomplished on the scratched control coating and coatings containing nanocapsules according to ASTM B117. The total exposure time of the coatings was about 72 h. It should be noted that the scratched coatings were kept at room temperature for seven days before testing to provide enough Appl. Sci. 2020, 10, 3171 6 of 17 time for the release of the healing agent and healing reaction. In addition, the self-healing ability of the coatings containing nanocapsules was evaluated in the NaCl and HCl solutions through the electrochemical measurements. The corrosion resistance of the coated substrates was determined by EIS tests using an Ivium–potentiostat (Ivium Technologies, Eindhoven, Netherland). A conventional three-electrode arrangement was utilized with a KCl saturated calomel electrode (SCE) as a reference electrode, a platinum sheet as a counter electrode and scratched coatings as the working electrode. Before electrochemical evaluation, the scratched coatings were kept in 3.5%wt. NaCl solution for 8 days separately to be stabilized at open circuit potential (OCP). The EIS measurements were carried 1 out in a frequency range between 100 and 10 mHz with an amplitude of 10 mV rms− . The results of the corrosion tests were estimated by extrapolating the polarization curve based on ASTM-G102-89. The obtained EIS data were analyzed in terms of electrical equivalents circuits utilizing the Z-View software. In addition, the EIS tests were also conducted on the scratched coatings immersed in a 0.5 M HCl solution, to evaluate the self-healing performance of the coatings in the acidic media.

3. Results and Discussion

3.1. The Electrospraying Process Observation To achieve a stable cone-jet, the effective parameters in electrospinning or electrospraying process, namely, the concentration of the shell solution, the solvent characteristics, applied voltage, flow rates, and the distance between the needle tip and collector, should be optimized [31,32]. The concentration of the shell solution is important not only to control the viscosity, electrical conductivity, and interfacial tension of the solution but it also affects the stability of the tailor cone. It is difficult to obtain an intact shell with a low concentration of polymer and is challenging to achieve a stable cone-jet mode with high polymer concentration. It may also cause the formation of fibers instead of capsules, as shown in Figure S1 in the supplementary file. Therefore, the shell concentrations of 4, 5, and 6 wt.% were examined. The applied voltage also plays a critical role in the formation of core–shell capsules. The stable cone–jet can be obtained by changing the applied voltage. The dripping mode occurs when low voltage is applied, and, by applying high voltage, multi-jet occurs in the electrospray process [31]. Therefore, the applied voltages of 18 to 24 kV were examined to obtain the optimum applied voltage. Figure S2a, of the supplementary file, shows the FE-SEM image of the collector in dripping mode (low voltage), and Figure S2b shows the FE-SEM images of the prepared capsules in the multi-jet mode (high voltage). In addition, the inner and the outer flow rates in coaxial electrospray are the other effective factors for cone–jet stability and droplet size. A stable core–shell structure can be formed for a small range of inner/outer liquid flow rates [31]. Therefore, in this research, several feed 1 1 rates were investigated, 0.5 to 1 mL.h− as a shell feed rate and 0.01 to 0.06 mL.h− as a core feed rate. Some of the unsuccessful encapsulations are shown through the FE-SEM images of Figure S3. Another important process parameter is the distance between the tip and the collector. At a constant voltage, a shorter distance is advantageous for generating a higher electrical field strength and consequently smaller particles are prepared. However, very short distance results in low solvent evaporation and, consequently, the coalescence and aggregation of wet particles at the collector. On the other hand, the long collector to tip distance would need higher applied voltage to compensate for the reduced electrical field strength. In general, one could expect to obtain larger particles compared to particles produced at a shorter collector to tip distance setup [33]. In this research, 13, 15, and 17 cm were examined as the distance of the needle to the collector. Figure S4 shows the FE-SEM images of the prepared capsules in two needle-to-collector distances. Using DMF as the solvent of shell material keeps the collector wet due to its slow evaporation rate, while DCM solvent leads to the formation of bigger particles (micro-scale) that is not appropriate for coating applications. Therefore, using a mixture of these solvents with equal amounts addressed the issue by optimizing solvent evaporation rate and electrical conductivity of the solution, which is reported in previous studies [34,35]. Figure S5 depicts the FE-SEM images of the prepared capsules when the neat DCM and DMF were employed as shell solvent. Appl. Sci. 2020, 10, 3171 7 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 17

OtherOther effective effective parameters parameters were were optimized optimized by by changing changing a a parameter parameter and and keeping keeping the the other other constant.constant. The The parameters parameters were were chos chosenen as as optimum optimum at at those those values values where where the the electrospraying electrospraying process process isis stable andand ledled to to the the preparation preparation of theof the nanocapsules nanocapsules in a sphericalin a spherical shape shape with the with minimum the minimum average averageof the capsule’s of the capsule’s diameter. diameter. The optimized The optimized parameters parameters obtained obtained were 22 were kV, 0.622 kV, and 0.6 0.05 and mL 0.05/h, mL/h, 15 cm, 15and cm, 5 w and/v % 5 for w/v applied % for voltage,applied flowvoltage, rates flow of the rates shell of andthe coreshell solutions,and core thesolutions, distance the of distance needle and of needlecollector, and and collector, the concentration and the concentration of the shell solution,of the shell respectively. solution, respectively.

3.2.3.2. The The Chemical Chemical Structure Structure of of the the Prepared Prepared Capsules Capsules FigureFigure 33 shows shows the the FTIR FTIR spectra spectra of the of neat the shell neat material, shell material, the neat CAR, the andneat crushed CAR, nanocapsules.and crushed 1 −1 nanocapsules.The FTIR spectrum The FTIR of neat spectrum SAN shows of neat the characteristicSAN shows the peaks characteristic at 2237 and peaks 1600 cmat −2237correspondingand 1600 cm to correspondingC N and C=C into styreneC≡N and ring C=C bending, in styrene respectively ring bending, [24,36]. respectively All the absorption [24,36]. of All SAN the can absorption be observed of ≡ 1 1 −1 −1 1 SANin spectrum, can be observed such as 700, in spectr 2900,um, 3025 such cm− asfor 700, CH, 2900, 2852 3025 cm− cmfor CHfor 2CH,and 2852 540, cm 760, 1490for CH cm2− andfor 540, CH 760,aromatic 1490 cm ring−1 [for37]. CH aromatic ring [37]. −11 CharacteristicCharacteristic bands bands of CAR are observed at 1740 cm− forfor ester ester groups groups and and small small twin twin peaks peaks at at −11 1604,1604, and 1584 cm − correspondingcorresponding to to C=C C=C stretching stretching vibratio vibrationn of of the the aromatic aromatic ring ring originated originated from from phthalatephthalate groups that formed thethe alkydalkyd resinresin (as(as shown shown in in the the plausible plausible structure structure of of the the alkyd alkyd resin resin in inFigure Figure2). In2). addition, In addition, the aromaticthe aromatic C-H bendingC-H bending arising arising from this from aromatic this aromatic functional functional group appears group 1 −1 1 −1 appearsat 720 cm at− 720as cm a sharp as a peak.sharp Thepeak. broad The broad stretching stretching bands bands at 3455 at cm3455− cmconfirm confirm the presencethe presence of free of 1 −1 freehydroxyl hydroxyl and carboxyland carboxyl groups, groups, while while the peaks the atpeaks 2850 at and 2850 2923 and cm 2923− are cm attributed are attributed to C-H aliphatic to C-H 1 −1 aliphaticstretching. stretching. Peaks are Peaks also observedare also observed at 1008–1240 at 1008–1240 cm− for thecm C-O-C for the stretching C-O-C stretching of ester that of ester supports that supportsthe structure the structure of CAR [ 15of]. CAR [15].

Figure 3. FTIR spectra of (a) neat SAN nanocapsules; (b) neat CAR; (c) and crushed nanocapsules Figure 3. FTIR spectra of (a) neat SAN nanocapsules; (b) neat CAR; (c) and crushed nanocapsules obtained from coaxial electrospraying. obtained from coaxial electrospraying. The FTIR spectrum of crushed nanocapsules shows all the characteristic peaks of neat SAN The FTIR spectrum of crushed nanocapsules shows all the characteristic peaks of neat SAN and and CAR, while no more peak can be discerned. It can be concluded that both SAN and CAR are CAR, while no more peak can be discerned. It can be concluded that both SAN and CAR are available available in the prepared nanocapsules, considering no reaction occurred between them during the in the prepared nanocapsules, considering no reaction occurred between them during the encapsulation process. encapsulation process. The practical core content of the prepared nanocapsules was measured to be 32 wt.% using the The practical core content of the prepared nanocapsules was measured to be 32 wt.% using the extraction method, while the theoretical core content was calculated to be 46 wt.% according to the feed extraction method, while the theoretical core content was calculated to be 46 wt.% according to the rate of the shell and core solutions. Therefore, the encapsulation yield can be obtained from Equation 2) feed rate of the shell and core solutions. Therefore, the encapsulation yield can be obtained from at 69%, confirming the high efficiency of the electrospray method in comparison to other researches Equation (2) at 69%, confirming the high efficiency of the electrospray method in comparison to other with an encapsulation yield of 10–20% [38]. researches with an encapsulation yield of 10%–20% [38].

3.3. Morphological Studies Appl. Sci. 2020, 10, 3171 8 of 17

3.3. Morphological Studies Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 17 Figure4a,b represent FE-SEM micrographs of the nanocapsules at two di fferent magnifications. SphericalFigure nanocapsules 4a,b represent are clearly FE-SEM seen, micrographs which guarantee of the nanocapsules easy dispersion at two into different the resins magnifications. before applying coatingSpherical [15]. Innanocapsules addition, the are rough clearly outer seen, surface which ofguarantee the capsules easy dispersion provides enhanced into the resins adhesion before to the resinapplying and metallic coating substrate. [15]. In addition, The rough the surface rough ofou capsulester surface can of be the attributed capsules toprovides the phase enhanced change of CARadhesion to solid to during the resin the encapsulationand metallic substrate. process. The To investigaterough surface the of mean capsules diameter can be and attributed size distribution to the of synthesizedphase change nanocapsules, of CAR to solid ImageJ during software the encapsul was usedation and process. the average To investigate of 50 particles’ the mean diameter diameter was determined.and size distribution Nanocapsules of synthesized with a mean nanocapsules diameter, ofImageJ 708 software252 nm was were used found. and Figurethe average4c shows of 50 the ± sizeparticles’ distribution diameter of the was prepared determined. nanocapsules. Nanocapsules It can with be aseen mean that diameter the coaxial of 708 ± electrospraying 252 nm were found. method led toFigure the formation4c shows the of nanocapsulessize distribution with of the a wide prepar rangeed nanocapsules. of size distribution. It can be The seen capsules that the with coaxial a wide electrospraying method led to the formation of nanocapsules with a wide range of size distribution. range in the size distribution showed great potential in self-healing performance due to covering and The capsules with a wide range in the size distribution showed great potential in self-healing healing all sizes of cracks, from nano to microscale [39]. performance due to covering and healing all sizes of cracks, from nano to microscale [39].

Figure 4. FE-SEM micrographs of the prepared nanocapsules at: (a) 5000× and (b) 20,000× Figure 4. FE-SEM micrographs of the prepared nanocapsules at: (a) 5000 and (b) 20,000 magnification; magnification; (c) the size distribution of nanocapsules; (d) FE-SEM image× of the fractured× capsules (c) the size distribution of nanocapsules; (d) FE-SEM image of the fractured capsules in the epoxy in the epoxy composites; and (e,f) TEM images of the prepared nanocapsules. composites; and (e,f) TEM images of the prepared nanocapsules. Appl. Sci. 2020, 10, 3171 9 of 17

The fractured surface of the epoxy composite is shown in Figure4d. According to Figure4d, the core–sheath structure, along with a smooth inner layer of the nanocapsules, was formed, which confirms the FTIR results regarding the presence of CAR in nanocapsules without any reaction with SAN shell materials. Moreover, Figure4d reveals an increased bonding of nanocapsules and the epoxy matrix, which is attributed to the high roughness of nanocapsules and the chemical reaction of unencapsulated CAR on the surfaces of the nanocapsules and epoxy matrix. Figure4e shows the TEM images of nanocapsules, where the dark and bright fields are observed in these images. It proves the formation of the core–shell structure of nanocapsules while the single-core structure of nanocapsules can be observed. According to the TEM and FE-SEM images of the fractured capsules, the thickness of shells is in the range of 35–95 nanometers, and, consequently, a scratch longer than 95 nm has the ability to break the capsules’ shell [40]. From these measurements, the volume fraction of the core was calculated to be in the range of 27–33%. Knowing the density of CAR and SAN solution (1.027 g/mL and 1.08 g/mL, respectively) led to the weight percentage of the core content in the range of 27% to 32% [40]. These values are in good agreement with the measured core content from the extraction method (32 wt.%).

Thermodynamic of the Encapsulation Process In the coaxial electrospraying method, the surface tensions of the components are important factors for the successful encapsulation process [30]. The surface tensions of the core and shell materials were measured and listed in Table1. According to Table1 and Equations (3) and (4), the spreading coefficient was calculated to be 3.02 mN/m. The positive spreading coefficient proves the spreading of the shell solution on CAR and confirms the successful encapsulation of CAR within the SAN shell [30].

Table 1. Surface tension of core and shell materials.

Material Surface Tension (mN/m) 5 wt.% SAN solution in an equal ratio of DMF and DCM (shell) 30.8 CAR (core) 33.9 The interfacial tension between CAR and the 5% SAN solution 0.074 (calculated from Equation 4)) Spreading coefficient 3.02 > 0

3.4. Thermal Stability of the Nanocapsules TGA tests were carried out to investigate the thermal stability of the prepared capsules. Figure5 represents the TGA and derivative thermogravimetric analysis (DTGA) diagrams of capsules containing the healing agent, neat SAN capsules (without core), and neat CAR. The TGA curve of neat SAN nanocapsules shows that the first weight loss (around 100 ◦C) of the neat SAN nanocapsules can be attributed to the evaporation of adsorbed moisture and residual solvent on the surface of the nanocapsules. There was a significant thermal decomposition event that initiated at 303 ◦C and continued till 452 ◦C. The thermal decomposition at 303 ◦C is associated with nitrile oligomerization, leading to the production of volatile products, e.g., NH3, HCN, CH3CN, etc., which are the components of acrylonitrile sections of the copolymer [41]. The CAR thermal decomposition occurs between the temperature range of 230–500 ◦C. The thermal decomposition of the core–shell nanocapsules started at 270 ◦C, contributed to the decomposition of the shell and continued up to 500 ◦C due to the decomposition of encapsulated CAR. It can be seen that the thermal decomposition curve of core–shell nanocapsules is contiguous with the mass loss curve of the shell (SAN) in the beginning and is adjacent to the mass loss curve of the core (CAR) at the end of the analysis. This proves the presence of both the components in the prepared capsules (Figure5a). In addition, the temperature decomposition range reveals the high thermal stability of the produced nanocapsules. Appl. Sci. 2020, 10, 3171 10 of 17 Appl.Appl. Sci. Sci. 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 1010 of of 17 17

FigureFigure 5. 5. ( a((a)a ))TGA TGA TGA and and and ( b( (b) ))DTGA DTGA DTGA curves curves of of neat neat SAN, SAN, neat neat CAR, CAR, and and the the prepared prepared nanocapsules. nanocapsules.

3.5. Evaluation of the Reactivity of the Encapsulated Healing Agent 3.5.3.5. Evaluation Evaluation of of the the Reactivity Reactivity of of the the Encapsulated Encapsulated Healing Healing Agent Agent The heat flow and existence of an exothermic peak in the DSC diagram of epoxy and the healing TheThe heat heat flow flow and and existence existence of of an an exothermic exothermic peak peak in in the the DSC DSC diagram diagram of of epoxy epoxy and and the the healing healing agent was reported as evidence for the healing reaction [24,42]. Figure6a represents the DSC diagram agentagent was was reported reported as as evidence evidence for for the the healing healing reacti reactionon [24,43]. [24,43]. Figure Figure 6a 6a repr representsesents the the DSC DSC diagram diagram of of of epoxy and CAR mixture without any catalyst. The endothermic peak around 40 C is attributed to epoxyepoxy and and CAR CAR mixture mixture without without any any catalyst. catalyst. The The endothermic endothermic peak peak around around 40 40 °C °C is is◦ attributed attributed to to the the the melting of CAR (this peak is also observed in two other diagrams). Figure6b demonstrates that meltingmelting of of CAR CAR (this (this peak peak is is also also observed observed in in two two other other diagrams). diagrams). Figure Figure 6b 6b demonstrates demonstrates that that the the DSC DSC the DSC curve of epoxy and CAR resin, with the presence of HCl as a catalyst, shows a considerable curvecurve of of epoxy epoxy and and CAR CAR resin, resin, with with the the presence presence of of HCl HCl as as a a catalyst, catalyst, shows shows a a considerable considerable exothermic exothermic exothermic peak at 54 C, confirming the reaction between epoxy and CAR. In addition, Figure6c peakpeak at at 54 54 °C, °C, confirming confirming◦ the the reaction reaction between between epoxy epoxy and and CAR. CAR. In In addition, addition, Figure Figure 6c 6c represents represents the the DSC DSC represents the DSC diagram of the crushed epoxy composite containing 2 wt.% of nanocapsules in the diagramdiagram of of the the crushed crushed epoxy epoxy composite composite containing containing 2 2 wt.% wt.% of of nanocapsules nanocapsules in in the the presence presence of of HCl. HCl. It It can can presence of HCl. It can be discerned that this curve also shows an exothermic peak at 54 C, which bebe discerned discerned that that this this curve curve also also shows shows an an exothermic exothermic peak peak at at 54 54 °C, °C, which which proves proves the the healing healing◦ process process proves the healing process and the reactivity of the released healing agent from crushed nanocapsules. andand the the reactivity reactivity of of the the released released healing healing agent agent from from crushed crushed nanocapsules. nanocapsules.

FigureFigure 6. 6. DSC DSC diagrams diagramsdiagrams of of ( a(a) ) epoxyepoxy epoxy andand and CAR CAR CAR in in in the the the absence absence absence of of anyof any any catalyst; catalyst; catalyst; (b) ( ab(b mixture) )a a mixture mixture of epoxy of of epoxy epoxy resin and CAR in the presence of HCl; and (c) a crushed epoxy composite containing nanocapsules with HCl. resinresin and and CAR CAR in in the the presence presence of of HCl; HCl; and and ( c(c) )a a crushed crushed epoxy epoxy composite composite containing containing nanocapsules nanocapsules 3.6.with Evaluationwith HCl. HCl. of the Coating’s Properties

3.6.3.6. Evaluation EvaluationThe results of of the the of Coating’s pull-oCoating’sff adhesionProperties Properties strength and elongation at break of the control sample and self-healing coatings are summarized in Table2. Pull-o ff adhesion tests were carried out to evaluate The results of pull-off adhesion strength and elongation at break of the control sample and self- the adhesionThe results strength of pull-off of coatings. adhesion According strength to and the elongation results, increasing at break the of capsulethe control content sample in the and matrix self- healing coatings are summarized in Table 2. Pull-off adhesion tests were carried out to evaluate the ledhealing to a coatings decrease are in thesummarized adhesion strengthin Table 2. of Pull-off the coatings adhesion that tests can bewere attributed carried toout the to reductionevaluate the in adhesionadhesion strength strength of of coatings. coatings. Acco Accordingrding to to the the results, results, increasing increasing the the capsule capsule content content in in the the matrix matrix ledled to to a a decrease decrease in in the the adhesion adhesion strength strength of of the the coatings coatings that that can can be be attributed attributed to to the the reduction reduction in in Appl.Appl. Sci. Sci.2020 2020, ,10 10,, 3171 x FOR PEER REVIEW 1111 of of 17 17

contact areas between the coating and its substrate [11,44]. Furthermore, elongation at break was contactmeasured areas for between all of the the samples coating thorough and its a substrate conical mandrel [11,43]. bending Furthermore, test. The elongation results revealed at break that was measuredthe bending for elongation all of the samples at break thorough decreased a conicalby increa mandrelsing the bending nanocapsule test. The conten resultst. This revealed result thatcan be the bendingdue to the elongation negative ateffect break of decreasedadding nanocapsules by increasing on thethe nanocapsuleadhesion strength content. of the This coatings result canas well be due as totheir the negativeagglomeration effect ofat higher adding contents, nanocapsules which on act the as adhesiondefects instrength the coating of the matrix coatings [15]. as well as their agglomeration at higher contents, which act as defects in the coating matrix [15]. Table 2. Impact of nanocapsules content on the properties of the coatings. Table 2. Impact of nanocapsules content on the properties of the coatings. Sample Nanocapsules Content Adhesion Strength Total Bending SampleCode Code Nanocapsules(wt.%) Content (wt.%) Adhesion(MPa) Strength (MPa) TotalElongation Bending Elongation (%) (%) AA 0 2.59 2.59 24.8 24.8 BB 1 2.11 2.11 19.8 19.8 CC 2 2.10 2.10 19.6 19.6 D 4 1.43 18.8 D 4 1.43 18.8

3.7. Evaluation of the Self-Healing Ability of the Coatings 3.7. Evaluation of the Self-Healing Ability of the Coatings 3.7.1. Salt Spray Tests 3.7.1. Salt Spray Tests The results of the scratched coatings after 72 h of salt spray corrosion tests are presented in The results of the scratched coatings after 72 h of salt spray corrosion tests are presented in Figure7. The control panel and the panel without capsules showed severe corrosion after exposure to Figure 7. The control panel and the panel without capsules showed severe corrosion after exposure the salt solution (Figure7a). It can be seen in Figure7 that with increasing the capsule content in the to the salt solution (Figure 7a). It can be seen in Figure 7 that with increasing the capsule content in coatings, the corrosion in the scratch area was considerably decreased. This can be due to the healing the coatings, the corrosion in the scratch area was considerably decreased. This can be due to the reaction occurring when capsules rapture in the crack area. However, it can be seen that by increasing healing reaction occurring when capsules rapture in the crack area. However, it can be seen that by capsuleincreasing content capsule to 4 content wt.%, the to number4 wt.%,of the dark number spots representingof dark spots the representing porosity has the increased porosity in has the matrix,increased due in to the capsule matrix, agglomeration due to capsule [12]. agglomeration According to Figure[12]. According7, the coating to Figure containing 7, the 2 wt.%coating of capsulescontaining showed 2 wt.% the of most capsules efficient showed corrosion the protectionmost efficient of the corrosion coating afterprotection scratch of and the exposure coating inafter the corrosivescratch and environment exposure (5 in wt.% the NaClcorrosive solution). environmen The anticorrosivet (5 wt.% propertyNaCl solution). of this sample The anticorrosive is because of theproperty efficient of healingthis sample reaction is beca betweenuse of the epoxy efficient and releasedhealing reaction CAR from between capsules epoxy and and the released formation CAR of a newfrom film capsules on the and substrate the formation [44]. of a new film on the substrate [45].

FigureFigure 7. 7.Images Images ofof thethe scratchedscratched coatingscoatings afterafter saltsalt spray test: ( (aa)) Sample Sample A; A; ( (bb)) Sample Sample B; B; (c (c) )Sample Sample C;C; ( d(d)) Sample Sample D. D. Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 17

3.7.2. Evaluation of the Self-Healing Ability by EIS Tests in NaCl and HCl Solutions The EIS measurements were carried out to assess the self-healing performance of the coatings. Figure 8 shows the Nyquist and bode diagrams for the scratched samples, which were immersed in 3.5 wt.% NaCl solution for 8 days. The corresponding fitted curves according to the proposed electrical equivalent circuit are also presented. Similarly, the same results for the coatings that were immersed in 0.5 M HCl solution are presented in Figure 9. Comparing the results for the samples immersed in the NaCl solution reveals that Sample C, the coating containing 2 wt.% of the nanocapsules, has the best corrosion resistance, while acidic media sample B showed the best self- healing performance. According to the EIS results, adding the capsules increased the corrosion resistance of scratched coatings for both the immersion solutions. However, increasing the nanocapsule content more than 2 and 1 wt.%, reduced the Nyquist semi-circle radii for the coatings immersed in NaCl and HCl solutions, respectively, indicating the reduction in the corrosion resistance of the samples. Figure 10 shows the equivalent electrical circuit utilized to fit the EIS data. In this circuit, Rs, Rcoat and Rcorr are the solution resistance, the coating resistance, and charge transfer resistance, respectively. Table 3 represents the impedance data extracted from the equivalent electrical circuit of the samples immersed in the 3.5 wt.% NaCl solution. In addition, Table 4 summarizes those acquired for the samples immersed in the 0.5 M HCl solution. In Nyquist plots, the high-frequency time constant is attributed to the capacitive behavior of the epoxy coating containing nanocapsules. Whereas the low-frequency one can be related to the charge transfer at the interface of the metallic substrate and self-healing coating. According to Figure 8a, in the Nyquist data of sample C, there are Appl. Sci. 2020, 10, 3171 12 of 17 two capacitive loops that are attributed to the adequately released healing agent for the self-healing process. In fact, there is a significant difference between the semi-circle radii of sample C and the other3.7.2. samples, Evaluation indicating of the Self-Healing the effectiveness Ability of by the EIS corrosion Tests in NaCl protection and HCl of Solutions the self-healing coating. When nanocapsules in the epoxy coating increased from 2 to 4 wt.%, the corrosion resistance clearly The EIS measurements were carried out to assess the self-healing performance of the coatings. decreased due to smaller dimensions of the capacitive loop. This is due to the agglomeration of the Figure8 shows the Nyquist and bode diagrams for the scratched samples, which were immersed in nanocapsules in the epoxy coating leading to localized corrosion and penetration of the corrosive 3.5 wt.% NaCl solution for 8 days. The corresponding fitted curves according to the proposed electrical NaCl solution into the surface of the self-healing coated substrates. Generally, the agglomerated area equivalent circuit are also presented. Similarly, the same results for the coatings that were immersed provides a localized form of corrosion through some defects, such as holes, cavities, and cracks [46]. in 0.5 M HCl solution are presented in Figure9. Comparing the results for the samples immersed It can be seen that the radius of the capacitive loop increased when the self-healing nanocapsule was in the NaCl solution reveals that Sample C, the coating containing 2 wt.% of the nanocapsules, has incorporated in the epoxy coating, proving the self-healing agent’s role in the protection of the the best corrosion resistance, while acidic media sample B showed the best self-healing performance. metallic surface. Comparing the corrosion resistance, the coatings with various amounts of According to the EIS results, adding the capsules increased the corrosion resistance of scratched nanocapsules show that the corrosion resistance of the samples in the NaCl solution was significantly coatings for both the immersion solutions. However, increasing the nanocapsule content more than 2 higher than those acquired for the samples immersed in the HCl solution. For example, Rcorr for the and 1 wt.%, reduced the Nyquist semi-circle radii for the coatings immersed in NaCl and HCl solutions, sample C in NaCl solution was 697,000 Ω cm2, which reduced to 10,100 Ω cm2 when it was immersed respectively, indicating the reduction in the corrosion resistance of the samples. in the HCl solution.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 17

Figure 8. ((a) Nyquist Nyquist and ( b) bode diagrams of the scratched samples immersed in the NaCl solution for 8 days.

Figure 9. (a) Nyquist and (b) bode diagrams of the scratchedscratched samples immersed in the HCl solution for 8 days.

Figure 10 shows the equivalent electrical circuit utilized to fit the EIS data. In this circuit, Rs,Rcoat and Rcorr are the solution resistance, the coating resistance, and charge transfer resistance, respectively. Table3 represents the impedance data extracted from the equivalent electrical circuit of the samples

Figure 10. Employed electrical equivalent circuit for the fitting of the Nyquist and bode curves.

Table 3. Electrochemical impedance spectroscopy (EIS) parameters for the scratched coatings and immersed in the 3.5 wt.% NaCl.

Rs CPEcoat-T Rcoat CPEdl-T Rcorr Sample Code CPEcoat-p CPEdl-P (Ω cm2) (sn Ω−1 cm−2) (Ω cm2) (sn Ω−1 cm−2) (Ω cm2) A 129.60 1.05×10-4 5.44×10-1 276.2 1.03×10-4 5.64×10-1 6.62×103 B 353.60 6.89×10-6 7.92×10-1 611 1.94×10-5 7.38×10-1 4.19×104 C 847.30 1.20×10-9 9.77×10-1 3.77×104 4.44×10-7 6.32×10-1 6.97×105 D 875.40 1.03×10-7 5.92×10-1 2.69×103 1.75×10-6 6.97×10-1 1.05×105

Table 4. The EIS parameters for the scratched coatings and immersed in the 0.5 M HCl.

Rs CPEcoat-T Rcoat CPEdl-T Rcorr Sample Code CPEcoat-p CPEdl-P (Ω cm2) (sn Ω−1 cm−2) (Ω cm2) (sn Ω−1 cm−2) (Ω cm2) A 2.43×102 1.14×10-9 9.04×10-1 234.10 6.38×10-5 5.15×10-1 2.05×103 B 421.20 1.63×10-9 9.85×10-1 476.60 2.00×10-5 7.51×10-1 4.07×104 C 4.00×103 1.45×10-9 9.90×10-1 2.21×103 1.51×10-5 6.90×10-1 1.01×104 D 25.61 2.95×10-10 9.90×10-1 227.80 6.41×10-5 4.97×10-1 2.38×103

In addition, Figures 8b and 9b demonstrate the bode plots of the coatings immersed in NaCl, and HCl solution, respectively. It should be noted in bode plots that the impedance modulus |Z| at lower frequencies shows the total corrosion resistance of the utilized system and consequently the efficiency of the self-healing performance. These results are in good agreement with the Nyquist plots, according to the fact that samples C and B have the highest total impedance for NaCl and HCl immersion solutions, respectively. According to these results, it can be concluded that the released CAR was not enough to protect the scratched area for sample B in NaCl solution. However, for the HCl media, the corrosion resistance and barrier property of the coating are more sensitive to particle agglomeration and the adhesion of the coatings. In fact, the increase in the nanocapsules content Appl. Sci. 2020, 10, 3171 13 of 17 Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 17 immersedFigure in8. ( thea) Nyquist 3.5 wt.% and NaCl(b) bode solution. diagrams In of addition, the scratched Table samples4 summarizes immersed thosein the NaCl acquired solution for the samplesfor 8 immerseddays. in the 0.5 M HCl solution. In Nyquist plots, the high-frequency time constant is attributed to the capacitive behavior of the epoxy coating containing nanocapsules. Whereas the low-frequency one can be related to the charge transfer at the interface of the metallic substrate and self-healing coating. According to Figure8a, in the Nyquist data of sample C, there are two capacitive loops that are attributed to the adequately released healing agent for the self-healing process. In fact, there is a significant difference between the semi-circle radii of sample C and the other samples, indicating the effectiveness of the corrosion protection of the self-healing coating. When nanocapsules in the epoxy coating increased from 2 to 4 wt.%, the corrosion resistance clearly decreased due to smaller dimensions of the capacitive loop. This is due to the agglomeration of the nanocapsules in the epoxy coating leading to localized corrosion and penetration of the corrosive NaCl solution into the surface of the self-healing coated substrates. Generally, the agglomerated area provides a localized form of corrosion through some defects, such as holes, cavities, and cracks [45]. It can be seen that the radius of the capacitive loop increased when the self-healing nanocapsule was incorporated in the epoxy coating, proving the self-healing agent’s role in the protection of the metallic surface. Comparing the corrosion resistance, the coatings with various amounts of nanocapsules show that the corrosion resistance of the samples in the NaCl solution was significantly higher than those acquired for the samplesFigure immersed 9. (a) Nyquist in the and HCl (b) solution.bode diagrams For of example, the scratched Rcorr samplesfor the sampleimmersed C in in the NaCl HCl solutionsolution was 697,000for 8Ω days.cm2 , which reduced to 10,100 Ω cm2 when it was immersed in the HCl solution.

FigureFigure 10. 10. EmployedEmployed electrical electrical equivalent equivalent circuit circuit for for the the fitting fitting of the Nyquist and bode curves. Table 3. Electrochemical impedance spectroscopy (EIS) parameters for the scratched coatings and immersedTable 3. Electrochemical in the 3.5 wt.% NaCl.impedance spectroscopy (EIS) parameters for the scratched coatings and immersed in the 3.5 wt.% NaCl. n n Sample Rs CPEcoat-T (s Rcoat CPEdl-T (s Rcorr 2 Rs 1 CPE2 coat-TCPE coat-p Rcoat2 CPE1 dl-T2 CPEdl-P Rcorr 2 CodeSample Code(Ω cm ) Ω− cm− ) CPEcoat-p (Ω cm ) Ω− cm− ) CPEdl-P (Ω cm ) (Ω cm2) (sn Ω−1 cm−2) (Ω cm2) (sn Ω−1 cm−2) (Ω cm2) A 129.60 1.05 10 4 5.44 10 1 276.2 1.03 10 4 5.64 10 1 6.62 103 A 129.60 × 1.05×10− -4 ×5.44×10− -1 276.2 1.03×10× −-4 5.64×10× -1− 6.62×10×3 B 353.60 6.89 10 6 7.92 10 1 611 1.94 10 5 7.38 10 1 4.19 104 B 353.60 × 6.89×10− -6 ×7.92×10− -1 611 1.94×10× −-5 7.38×10× -1− 4.19×10×4 C 847.30 9 1 4 7 1 5 1.20 10− -99.77 10− -13.77 10 4 4.44 10−-7 6.32 10-1− 6.97 510 C 847.30 × 1.20×107 ×9.77×101 3.77×10× 3 4.44×10× 6 6.32×10× 1 6.97×10× 5 D 875.40 1.03 10− 5.92 10− 2.69 10 1.75 10− 6.97 10− 1.05 10 D 875.40 × 1.03×10-7 ×5.92×10-1 2.69×10× 3 1.75×10× -6 6.97×10× -1 1.05×10×5

Table 4. The EIS parameters for the scratched coatings and immersed in the 0.5 M HCl. Table 4. The EIS parameters for the scratched coatings and immersed in the 0.5 M HCl. n n Sample Rs CPEcoat-T (s Rcoat CPEdl-T (s Rcorr Rs CPEcoat-TCPE coat-p Rcoat CPEdl-T CPE Rcorr CodeSample Code(Ω cm 2) Ω 1 cm 2) CPEcoat-p( Ω cm2) Ω 1 cm 2) CPEdl-Pdl-P (Ω cm2) (Ω cm2−) (sn− Ω−1 cm−2) (Ω cm2) (sn− Ω−1 cm− −2) (Ω cm2) 2 9 1 5 1 3 A A 2.43 102.43×101.142 101.14×10− -99.04 109.04×10− -1 234.10234.10 6.38 6.38×1010−-5 5.15×105.15 10-1− 2.05×102.05 310 × × 9 × 1 × 5 × 1 × 4 BB 421.20 421.201.63 101.63×10− -99.85 109.85×10− -1 476.60476.60 2.00 2.00×1010−-5 7.51×107.51 10-1− 4.07×104.07 410 3 × 9 × 1 3 × 5 × 1 × 4 C C 4.00 104.00×101.453 101.45×10− -99.90 109.90×10− -12.21 2.21×1010 3 1.511.51×1010−-5 6.90×106.90 10-1− 1.01×101.01 410 × × 10 × 1 × × 5 × 1 × 3 D 25.61 2.95 10 -109.90 10 -1 227.80 6.41 10 -5 4.97 10-1 2.38 310 D 25.61 × 2.95×10− ×9.90×10− 227.80 6.41×10× − 4.97×10× − 2.38×10×

InIn addition, FiguresFigures8 b8b and and9b 9b demonstrate demonstrate the the bode bode plots plots of the of coatingsthe coatings immersed immersed in NaCl, in NaCl, and andHCl HCl solution, solution, respectively. respectively. It should It should be noted be noted in bode in bode plots plots that thethat impedance the impedance modulus modulus|Z| at |Z| lower at lowerfrequencies frequencies shows shows the total the corrosion total corrosion resistance resistan of thece utilized of the systemutilized and system consequently and consequently the efficiency the efficiencyof the self-healing of the self-healing performance. performance. These results These are in results good agreement are in good with agreement the Nyquist with plots, the according Nyquist plots,to the according fact that samplesto the fact C that and samples B have C the and highest B have total the highest impedance total for impedance NaCl and for HCl NaCl immersion and HCl immersionsolutions, respectively. solutions, respectively. According toAccording these results, to these it can results, be concluded it can be that concluded the released that CAR the released was not CARenough was to not protect enough the scratchedto protect areathe scratched for sample area B in for NaCl sample solution. B in NaCl However, solution. for the However, HCl media, for the HCl media, the corrosion resistance and barrier property of the coating are more sensitive to particle agglomeration and the adhesion of the coatings. In fact, the increase in the nanocapsules content Appl. Sci. 2020, 10, 3171 14 of 17 corrosion resistance and barrier property of the coating are more sensitive to particle agglomeration and the adhesion of the coatings. In fact, the increase in the nanocapsules content decreased the adhesion of the coating and made it more susceptible to the penetration of the corrosive ions to the metallic substrate in the acidic media. On the other hand, the hydrogen ion possibly catalyzed the healing reaction between epoxy and CAR functional groups at the initial release of CAR. Since the healing reaction and healing solidification is accelerated, fewer healing agents were allowed to release. Consequently, the healing reaction in the acidic media is less sensitive to the available healing agent and is mostly controlled by the adhesion of the coatings to the metallic substrate. Therefore, for the scratched samples immersed in the HCl solution, the highest self-healing performance was observed for sample B. The healing reaction efficiency was measured to be 99% and 95% for the samples C (in NaCl solution) and B (in HCl solution), respectively, according to Equation 5) [46,47]:

%HE = (1 Rcorr /Rcorr) 100 (5) − 0 × where Rcorr0 and Rcorr are the charge transfer resistances of the control and self-healing coatings, respectively.

4. Conclusions In the present study, a CAR was encapsulated in SAN nanocapsules through the coaxial electrospraying for self-healing purposes. Morphological studies revealed that the uniform core–shell nanocapsules with a mean diameter of 708 252 were prepared, while the core content and encapsulation ± yield were measured to be 32% and 69%, respectively. In addition, morphological studies revealed the formation of the rough outer surface, which made them suitable for better dispersion and enhanced self-healing performance, and a soft inner surface of the nanocapsules, confirming the successful encapsulation process without any reaction between shell and core materials. The FTIR and TGA test results confirmed the presence of both SAN and CAR in the prepared nanocapsules and the thermal stability of the capsules up to 270 ◦C. Furthermore, the DSC test results confirmed the reactivity of released healing agent from nanocapsules. The evaluation of the mechanical properties of the coatings showed that the incorporation of the nanocapsules decreased both the adhesion of the coating to the substrate and bending elongation of the coatings. The evaluation of the self-healing performance revealed that the coatings containing nanocapsules have the self-healing ability during the salt spray test. In addition, the EIS test results showed that the coating has the autonomous ability to heal in both NaCl and HCl media. The best self-healing performance occurred for the coating containing 2 wt.% nanocapsules in the NaCl solution with 99% healing efficiency. In contrast, for HCl media, the best self-healing performance was observed for the coating containing 1 wt.% of the nanocapsules with 95% healing efficiency.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/10/9/3171/s1, Figure S1: FESEM images of the prepared capsules at 22 kV, 0.6 and 0.05 mL/h, and 15 cm of applied voltage, flow rates of the shell and core solutions, and the distance of needle to the collector, respectively at the shell solution concentration of (a) 4 w/v %; (b) 6 w/v %. Figure S2: FESEM images of the prepared capsules at 0.6 and 0.05 mL/h, 15 cm, and 5 w/v % of flow rates of the shell and core solutions, the distance of needle to the collector, and concentration of the shell solution, respectively at the applied voltage of (a) 20 kV (dripping mode) and (b) 24 kV (multi-jet mode). Figure S3: FESEM images of the prepared capsules at 22 kV, 15 cm, and 5 w/v % of the applied voltage, the distance of needle to the collector, and concentration of the shell solution, respectively. The flow rate of the core solution is 0.05 mL/h while the flow rate of shell solution is; (a) 0.5 mL/h; (b) 0.7 mL/h and. The flow rate of shell solution is 0.6 mL/h and the flow rate of core solution is; (c) 0.04 mL/h; (d) 0.06 mL/h. Figure S4: FESEM images of the prepared capsules at 22 kV, 0.6 and 0.05 mL/h, and 5 w/v % of the applied voltage, flow rates of the shell and core solutions, and concentration of the shell solution, respectively at the distance between needle to the collector of (a) 13 cm, and (b) 17 cm. Figure S5: FESEM images of the prepared nanocapsules at 22 kV, 0.6 and 0.05 mL/h, 15 cm, and 5 w/v % of the applied voltage, flow rates of the shell and core solutions, the distance of needle and collector, and concentration of the shell solution, respectively while the solvent was (a) DCM, and (b) DMF (wet collector and incapability of the process for encapsulating the healing agent). Author Contributions: Conceptualization, R.M., S.N.K. and R.E.N.; methodology R.M., S.N.K., R.E.N., and M.S.K.; software, R.M., R.E.N., M.S.K.; validation, R.E.N., M.S.K., and O.D.; formal analysis, R.M., R.T.; investigation, Appl. Sci. 2020, 10, 3171 15 of 17

R.M., and R.T.; resources, S.N.K.; data curation, R.E.N., M.S.K., R.T., and O.D.; writing—original draft preparation, R.M.; R.E.N., R.T. M.S.K., writing—review and editing, S.N.K., R.E.N., M.S.K., and O.D.; supervision, S.N.K., and R.E.N.; project administration, S.N.K.; funding acquisition, S.N.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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