polymers

Article Fabrication and Properties of Superhydrophobic Waterborne Polyurethane Composites with Micro-Rough Surface Structure Using Electrostatic Spraying

Fangfang Wang 1, Lajun Feng 1,2,*, Guangzhao Li 1, Zhe Zhai 1, Huini Ma 1, Bo Deng 1 and Shengchao Zhang 3

1 School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China; wff[email protected] (F.W.); [email protected] (G.L.); [email protected] (Z.Z.); [email protected] (H.M.); [email protected] (B.D.) 2 Key Laboratory of Corrosion and Protection of Shaanxi Province, Xi’an 710048, China 3 Faculty of Printing, Packing Engineering and Digital Media Technology, Xi’an University of Technology, Xi’an 710048, China; [email protected] * Correspondence: [email protected]; Tel.: +86-029-8231-2733



Received: 14 September 2019; Accepted: 21 October 2019; Published: 24 October 2019


Abstract: Waterborne polyurethane (WPU) coatings hold advantages of good toughness, low cost and environmental protection. However, the low water contact angle (WCA), poor wear and corrosion resistance make them unsuitable for application in the superhydrophobic coatings such as antipollution flashover coatings for transmission lines, self-cleaning coatings for outdoor equipment and waterproof textiles. A series of superhydrophobic WPU composites (SHWPUCs) with micro-rough surface structure was prepared by electrostatic spraying nano-SiO2 particles on WPU composites with low surface energy. It showed that as the hydrophobic system content rose the WCAs of the composites first increased and then remained stationary; however, the adhesion and corrosion resistance first increased and then decreased. An appropriate addition of the hydrophobic system content would lead to a dense coating structure, but an excessive addition could increase the interfaces in the coating and then reduce the coating performance. When the mass ratio of the WPU dispersion, polytetrafluoroethylene (PTFE) particles and modified polydimethylsiloxane was 8:0.3:0.4, 10 g/m2 nano-SiO2 particles were sprayed on the uncured coating surface to construct the SHWPUC with a WCA of 156◦. Compared with pure WPU coating, its adhesion and corrosion resistance increased by 12.5% and one order of magnitude, respectively; its wear rate decreased by 88.8%.

Keywords: superhydrophobic WPU composites; electrostatic spraying; nano-SiO2 particles; adhesion; corrosion resistance; wear rate

1. Introduction Protective coatings with superhydrophobicity have a widespread application in the fields of antipollution flashover coatings applied in transmission lines, self-cleaning coatings applied in outdoor equipment and waterproof cloth, etc. [1–3]. However, the most widely used hydrophobic coatings are solvent-borne coatings and their volatile organic solvent (VOC) contents are commonly over 40 wt %. During the processes of production, storage and using, solvent-borne coatings are prone to emit a lot of organic solvents which can not only pollute the environment but also have some potential safety problems [4]. Additionally, these organic solvents are generally flammable and high cost. With the formulation of environmental laws and regulations around the world and the increasing

Polymers 2019, 11, 1748; doi:10.3390/polym11111748 www.mdpi.com/journal/polymers Polymers 2019, 11, 1748 2 of 13 environmental protection awareness of human nowadays, it would be inevitable for us to research and apply waterborne coatings holding advantages of low VOC emission, low toxicity and environmental protection. Waterborne polyurethane (WPU) coatings with water as the dispersion medium are basically low cost, solvent-free, non-toxic and non-flammable, in addition, they will not pollute the environment and can avoid safety problems during production and application compared with solvent-based PU coatings [5]. Furthermore, polar groups such as –COOH and –OH in the molecular chains of WPU can produce crosslinking polymerization reactions under certain conditions [6–8], thus, it could strengthen the coating structure and enhance the adhesion to the substrates. Due to these excellent properties WPU coatings are extensively applied in many industrial areas such as coatings, adhesives, synthetic leathers, packaging films, membranes, biomaterials and waterproof textiles [9–12]. However, the traditional WPU coatings have some drawbacks such as low water contact angle (WCA), poor wear resistance and corrosion resistance, so that their applications in the superhydrophobic coating are restricted [13]. In order to solve these problems, it is necessary to modify the WPU coating to improve its properties such as hydrophobicity and wear resistance and thus expand its application range [14]. Previous reports have shown that the surface hydrophobicity can be improved by the addition of low surface energy materials on rough surfaces or the construction of rough structures on low surface energy surfaces [15–18]. In recent years, researchers have improved the performances of WPU resins by adding materials containing elements such as fluorine and silicon. Polytetrafluoroethylene (PTFE) material holds excellent properties of high fluoride content, thermochemical stability, high hydrophobicity and low surface friction [19]. Additionally, the WCA of the untreated PTFE coating is up to 120◦ and the addition of PTFE materials to the resin matrix can reduce the surface free energy of the coating [20,21]. Moreover, the incorporation of nano-SiO2 particles to the coating with low surface energy can not only construct the micro-rough surface structure, increasing the strength of polymer materials, but also improve the wear resistance and corrosion resistance of the coating [22–24]. Shin et al. [25] synthesized a series of waterborne fluorinated acrylate-based PUs for application in antifouling coatings. Krol et al. [26] prepared a kind of hydrophobic WPU coating by the incorporation of fluorine. Serkis et al. [27] prepared a new type of water-based PU/silica nanocomposite by the addition of nano-SiO2 particles. Zhang et al. [28] used modified nano-SiO2 particles to synthesize UV-curable water-based transparent coating. Generally, the performance of the WPU coating has been improved to some extent by modification, but the process is complex and time-consuming, moreover, the dispersion of nano-fillers has not been well dissolved. Therefore, the method of electrostatic spraying nano-SiO2 particles was used in this work to construct micro-rough surface structure on the coating with low surface energy and to improve the properties such as hydrophobicity and wear resistance. Electrostatic spraying is pollution-free and has excellent atomization capability and operability. In addition, nano-SiO2 particles can be well dispersed under the action of a high-voltage electrostatic field and most importantly, the driving force arising from the compressed air may make the nanoparticles greatly adhere to the coating surface [29]. The modified PTFE particles with different contents were added to the WPU dispersions to prepare WPU composites with low surface energy (LSWPUCs). Then, nano-SiO2 particles were sprayed using an electrostatic spraying device on the surfaces of the uncured LSWPUCs to form a micro-rough surface in this work. Thus, the superhydrophobic WPU composites (SHWPUCs) were formed owing to the adhesion of nano-SiO2 particles in the coating during curing. The adhesion, corrosion resistance and wear resistance of the obtained SHWPUCs were measured to evaluate their durability. Additionally, the effects of the micro-rough surface structure and the hydrophobic system content on the properties of WPU composites were studied. This may provide a useful reference for the design, preparation and application of superhydrophobic waterborne coatings. Polymers 2019, 11, 1748 3 of 13

2. Materials and Methods

2.1. Experimental Materials WPU dispersion was supplied by Jining Huakai Resin Co. Ltd. (Jining, China). Its solid content, viscosity and VOC concentration were 35%, 75 cps and 253 g/L, respectively. Polydimethylsiloxane 1 (PDMS) emulsion (average Mw = 115,000 g mol− ) was supplied by Shanghai Yuanye Biotechnology Co. Ltd. (Shanghai, China). Ethyl silicate (TEOS, analytically pure) was supplied by Tianjin Kermio 1 Chemical Reagent Co. Ltd. (Tianjin, China). Dioctyldilauryltin (DOTDL, Mw = 743.7 g mol− ), 98%, was supplied by Shanghai Yuanye Biotechnology Co. Ltd. (Shanghai, China). Sodium chloride (NaCl, analytically pure) was supplied by Tianjin Kemio Chemical Reagent Co. Ltd. (Tianjin, China). Anhydrous ethanol (analytically pure) was supplied by Tian in Fuyu Fine Chemical Co. Ltd. (Tianjin, China). PTFE particles (MP1200) particles with average particle size 3 µm were purchased from ≤ Dupont (Wilmington, DE, US). Nano-SiO2 (N-100) particles were supplied by Jining Huakai Resin Co. Ltd. (Jining, China). The average diameter, specific surface area, and silica content of nano-SiO2 were respectively 40–60 nm, 300 25 m2/g and 99.8%, respectively. ± Q235 steel (50 mm 20 mm 3 mm) was used as the metal substrate and was roughened by a × × YX-6050A sand blasting device (Anbangruiyuxin Machine Technology Development Co. Ltd., Wuhan, China). The air pressure was 0.6–0.8 MPa. The distance between the Q235 steel substrate and the spray gun was 110–150 mm. The spray time was 30–40 s.

2.2. Preparation of the LSWPUCs PTFE particles should be modified to improve the dispersibility before adding. At room temperature, PDMS, TEOS (5 wt % vs. PDMS) and DOTDL (1.5 wt % vs. PDMS) were mixed by an 85–2 magnetic stirring device (Hangzhou Instrument Motor Co., Ltd., Hangzhou, China) at a speed of 100–150 r/min for 30 min. Then, the mixture was treated by a KQ-50B ultrasonic dispersion device (Kunshan Ultrasonic Instrument Co., Ltd., Kunshan, China) for 10 min to prepare the modified PDMS (M-PDMS) emulsion (shown in Figure1A). Subsequently, the M-PDMS emulsion and PTFE particles were added to WPU by magnetic stirring at a speed of 250–300 r/min for 40 min. The obtained M-PDMS/PTFE/WPU dispersions (shown in Table1) were first placed at room temperature for 20–30 min and then coated on the Q235 steel substrates. The thickness of the coatings (110–120 mm) was controlled by weighting during coating. After coating, the samples were cured first at room temperature for 1 day and then at 150 ◦C for 1 h in an oven (Zhejiang Yuyao Yuandong CNC Instrument Factory, Yuyao, China). Thus, a layer of the LSWPUC was prepared on the Q235 steel substrate. The samples were named as #1 LSWPUC, #2 LSWPUC, #3 LSWPUC and #4 LSWPUC. In addition, the pure WPU coating (named as #0) was also cured in the same way described previously for reference.

Table 1. System of the modified polydimethylsiloxane (M-PDMS)/polytetrafluoroethylene (PTFE)/waterborne polyurethane (WPU) dispersions.

Samples #1 #2 #3 #4 WPU (g) 6 8 8 6 PTFE (g) 0.1 0.2 0.3 0.3 M-PDMS (g) 0.2 0.2 0.4 0.6

2.3. Preparation of the SHWPUCs The M-PDMS/PTFE/WPU dispersions were prepared and then coated on the Q235 steel substrates in the same way described previously. After coating, the samples were placed at room temperature for 2–3 h. In this case, the obtained coatings were under semi-dry and nonflowing conditions and their surfaces were slightly tacky. In order to enhance the dispersion and adhesion of nano-SiO2 particles, a NEW KCI-CU801 electrostatic spraying device (Shenzhen Honghaida Instrument Co., Polymers 2019, 11, 1748 4 of 13

Polymers 2018, 10, x FOR PEER REVIEW 4 of 13 Ltd., Shenzhen, China) was used to spray SiO2 nanoparticles on the uncured coating surfaces to 2 surfacesconstruct to the construct micro-rough the micro-rough surface structure. surface The structure. sprayed The content sprayed of nano-SiO content 2ofparticles nano-SiO was2 particles 10 g/m . wasThe 10 voltage g/m2.of The electrostatic voltage of spraying electrostatic was 50–60spraying kV. was The pressure50–60 kV. of The the compressedpressure of the air wascompressed 0.6–0.7 MPa. air wasThe 0.6–0.7 spraying MPa. time The was spraying 20–30 s. time The was distance 20–30 between s. The distance the spray between gun and the the spray sample gun was and 110–150 the sample mm. wasAfter 110–150 spraying, mm. the After samples spraying, were the cured samples first at were room cured temperature first at room for 1 temperature day and then for at 1 150 day◦C and for then1 h in at an150 oven. °C for Thus, 1 h in a an layer oven. of theThus, SHWPUC a layer of was the completelySHWPUC was prepared completely on the prepared Q235 steel on substrate.the Q235 steelThe schematicsubstrate. illustrationThe schematic of the illustration LSWPUC of is the shown LSWPUC in Figure is shown1. The in obtained Figure 1. samples The obtained were named samples as were#1 SHWPUC, named as #2 #1 SHWPUC, SHWPUC, #3 #2 SHWPUC SHWPUC, and #3 #4SHWPUC SHWPUC. and #4 SHWPUC.

Figure 1. (A) modification of the PDMS emulsion and (B) Schematic illustration of the superhydrophobic Figure 1. (A) modification of the PDMS emulsion and (B) Schematic illustration of the WPU composite (SHWPUC). superhydrophobic WPU composite (SHWPUC). 2.4. Measurements 2.4. Measurements 2.4.1. WCA Test 2.4.1. WCA Test The WCAs of the coatings were measured by an SDC-200 contact angle testing machine (Dongguan ShengdingThe WCAs Precision of the Instrument coatings Co.,were Ltd., measured Guangzhou, by an China)SDC-200 to characterizecontact angle the testing hydrophobicity. machine (DongguanA 20 µL pure Shengding water droplet Precision was injected Instrument with a microCo., Ltd., syringe Guangzhou, for each test. China) Each ofto the characterize two samples the for hydrophobicity.each coating was A measured 20 µL pure six water times droplet and the was measurements injected with were a micro averaged syringe to determinefor each test. the Each WCA. of the two samples for each coating was measured six times and the measurements were averaged to determine2.4.2. Adhesion the WCA. Test The adhesion test was performed according to ISO 4624:1978 at room temperature by a 2.4.2. Adhesion Test D2-5DL universal mechanical testing machine (Changchun Mechanical Institute, Changchun, China). TheThe coating adhesion was totally test was peeled performed off from according the steel substrate to ISO 4624:1978 during test. at room Each temperature coating was measuredby a D2-5DL ten universaltimes and mechanical the measurements testing weremachine averaged (Changchun to calculate Mechanical the adhesion Institute, by Equation Changchun, (1): China). The coating was totally peeled off from the steel substrate during test. Each coating was measured ten times and the measurements were averaged toσ calculate= P/A, the adhesion by Equation (1): (1) where σ is the adhesion (MP), P is the maximumσ = loadP/A, (N) and A is the coating area (mm2). (1) where σ is the adhesion (MP), P is the maximum load (N) and A is the coating area (mm2).

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2.4.3. Corrosion Resistance Test A 3.5 wt % NaCl solution was used as the corrosion medium to test the corrosion resistance of the obtained coatings under harsh conditions. The polarization curve and electrochemical impedance spectroscopy of the coating were measured at room temperature by a Ver4.2corr Test System (Wuhan Corr Test Co., Ltd., Wuhan, China) with a three-electrode cell. The sample was used as the working electrode, the Pt electrode was used as the auxiliary electrode, and the saturated calomel electrode was used as the reference electrode. The test area was 0.785 cm2. The samples were immersed in 3.5 wt % NaCl solution at 40 ◦C for 30 days before testing. The testing was started when the open circuit potential of the system was stable.

2.4.4. Wear Resistance Test The wear resistance test of the coating was carried out at room temperature in accordance with ASTM G99-05 by an HT-1000 high temperature scratch testing machine (Lanzhou Zhongke Kaihua Development Co., Ltd., Lanzhou, China), with the coating against a steel bearing ball (Ø2.5 mm) with a hardness level of HRC62. The applied load was 5 N, the rotation speed of the steel ball was 400 r/min, the sliding radius was 5 mm, and the wear time was 10 min. The wear rate and friction coefficient were used to evaluate the wear resistance of the coating. The wear rate was calculated by Equation (2).

I = ∆m/2πrntF%, (2) where I is the specific wear rate (cm3/mm N), ∆m is the loss weight (g), r is the sliding radius (mm), n is the rotation speed of the steel ball (r/min), t is the wear time (min), F is the applied load (N) and % is the density of the WPU coating (g/cm3).

2.4.5. Morphology Analysis The 3D micromorphologies of the coating surfaces were observed by an LEXT OLS4000 laser confocal scanning microscope (OLYMPUS, Tokyo, Japan) to characterize the effect of the micro-rough surface structure on the hydrophobicity of the coatings. The magnification was 108–17,280 ; the field × of view was 16 µm to 2.56 mm. After the wear test, the surface morphologies of the wear tracks were observed by a VEGA3 XMU Scanning Electron Microscope (SEM) (TESCAN, Brno, Czech).

3. Results and Discussion

3.1. Hydrophobicity of WPU Composites Figure2 shows the WCAs of di fferent WPU composites. The WCAs of #0 (pure WPU coating), #1 LSWPUC, #2 LSWPUC, #3 LSWPUC and #4 LSWPUC were 83◦, 107◦, 110◦, 112◦ and 113◦, respectively, stating that the low surface energy coating was successfully prepared by the incorporation of the hydrophobic system of M-PDMS/PTFE. When the M-PDMS/PTFE system content was below 8.05 wt % (#3 M-PDMS/PTFE system vs. M-PDMS/PTFE/WPU system), the WCAs of the LSWPUCs first increased and then remained stationary as the M-PDMS/PTFE system content rose. The reason may be as follows. PTFE as a kind of symmetrical and non-polar polymer has high fluoride content and low surface energy. PDMS holds low surface energy and the M-PDMS will generate a crosslinked network structure after modification. Then, the M-PDMS as the modifier in this work would adsorb on the surfaces of PTFE particles. Under the action of magnetic stirring, the M-PDMS/PTFE system could be evenly dispersed in the WPU dispersion, leading to the reduction of the surface energy of the WPU composites. When the M-PDMS/PTFE system content reached 8.05 wt %, the hydrophobic groups of the coating surface were under the saturated condition. As the M-PDMS/PTFE system content continued to increase, the change in the WCA of the LSWPUC remained stable. Polymers 2019, 11, 1748 6 of 13 Polymers 2018, 10, x FOR PEER REVIEW 6 of 13

ItIt cancan bebe seenseen fromfrom FigureFigure2 2 that that the the WCAs WCAs of of #1 #1 SHWPUC, SHWPUC, #2 #2 SHWPUC, SHWPUC, #3 #3 SHWPUC SHWPUC and and #4 #4 SHWPUC were 154°, 155°, 156° and 155°, respectively, displaying their superhydrophobic behaviors. SHWPUC were 154◦, 155◦, 156◦ and 155◦, respectively, displaying their superhydrophobic behaviors. This indicated that electrostatic spraying nano-SiO2 particles on the coating surface with low surface This indicated that electrostatic spraying nano-SiO2 particles on the coating surface with low surface energyenergy couldcould constructconstruct thethe micro-roughmicro-rough surfacesurface structurestructure andand thenthen improveimprove thethe hydrophobicityhydrophobicity ofof thethe coating.coating. This may bebe explainedexplained byby thethe following.following. The WPU compositescomposites were allall inin thethe uncureduncured state when nanoparticles were sprayed. Under the action of high-voltage electrostatic field, nano- state when nanoparticles were sprayed. Under the action of high-voltage electrostatic field, nano-SiO2 SiO2 particles would first enter the locations of the coatings where there were some defects or less particles would first enter the locations of the coatings where there were some defects or less nano-SiO2 nano-SiO2 particles and thus result in an even distribution. In addition, nano-SiO2 particles can be particles and thus result in an even distribution. In addition, nano-SiO2 particles can be rushed into therushed uncured into compositesthe uncured due composites to the high airdue pressure to the ofhigh electrostatic air pressure spraying. of electrostatic Subsequently, spraying. a layer Subsequently, a layer of uniform WPU composites could be coated on the surfaces of nano-SiO2 of uniform WPU composites could be coated on the surfaces of nano-SiO2 particles [23]. Therefore, theparticles micro-rough [23]. Therefore, surface structurethe micro-rough was formed surface on structure the surfaces was of formed the WPU on composites,the surfaces asof shownthe WPU in Figurecomposites,1. as shown in Figure 1.

Figure 2. Water contact angles (WCAs) of the WPU composites with different M-PDMS/PTFE Figure 2. Water contact angles (WCAs) of the WPU composites with different M-PDMS/PTFE system system contents. contents. Figure3 shows the 3D micromorphologies of di fferent coating surfaces as observed by laser confocalFigure scanning 3 shows microscope. the 3D micromorphologies The surface micromorphology of different of coating #0 (Figure surfaces3A) was as as observed smooth as by that laser of #3confocal LSWPUC scanning (Figure microscope.3B), indicating The that surface the incorporationmicromorphology of M-PDMS of #0 (Figure/PTFE 3A) system was wouldas smooth not greatlyas that aofff ect#3 LSWPUC the surface (Figure micromorphologies 3B), indicating of that the th preparede incorporation WPU composites. of M-PDMS/PTFE The micro-rough system would surface not structuregreatly affect on the the surface surface of micromorphologies #3 SHWPUC (Figure of3 C)the was prepared similar WPU to that composites. of #4 SHWPUC The (Figuremicro-rough3D). However,surface structure due to theon the facts surface that a of low #3 contentSHWPUC of the(Fig M-PDMSure 3C) was/PTFE similar system to that was of added #4 SHWPUC to #3 SHWPUC (Figure 3D). However, due to the facts that a low content of the M-PDMS/PTFE system was added to #3 and that the number of nano-SiO2 particles embedded in its surface was relatively uniformly compact, theSHWPUC resulting and micro-rough that the number structure of of nano-SiO #3 SHWPUC2 particles was a littleembedded better thanin its that surface of #4 SHWPUC.was relatively This resultuniformly was consistentcompact, the with resulting that of the micro-rough WCA tests. struct Moreover,ure of it #3 demonstrated SHWPUC was that a thelittle hydrophobicity better than that of theof #4 coating SHWPUC. may beThis closely result related was consistent to the micro-rough with that of surface the WCA structure tests. andMoreover, the superhydrophobic it demonstrated coatingthat the can hydrophobicity be prepared byof electrostaticthe coating may spraying be cl [osely30]. related to the micro-rough surface structure and the superhydrophobic coating can be prepared by electrostatic spraying [30].

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Figure 3. MicromorphologiesMicromorphologies of different different WPU composites of (A) #0; ( (B)#3)#3 LSWPUC; LSWPUC; ( (C)#3)#3 SHWPUC; SHWPUC; and (D) #4 SHWPUC, ((µm).µm).

According to the Wenzel equation, cos θω = r cos θ0, where θω is the apparent contact angle, According to the Wenzel equation, cos 𝜃 𝑟 cos 𝜃 , where 𝜃 is the apparent contact angle, θ0 is the eigen contact angle and r is the micro-roughness. When θ0 of the coating was over 90◦, 𝜃 is the eigen contact angle and 𝑟 is the micro-roughness. When 𝜃 of the coating was over 90°, its its hydrophobicity increased as the surface micro-roughness rose. It can be seen that the LSWPUCs hydrophobicity increased as the surface micro-roughness rose. It can be seen that the LSWPUCs prepared by the addition of the M-PDMS/PTFE system to WPU were hydrophobic; their WCAs prepared by the addition of the M-PDMS/PTFE system to WPU were hydrophobic; their WCAs were were over 90◦. In addition, spraying nano-SiO2 particles on the coating surfaces increased the over 90°. In addition, spraying nano-SiO2 particles on the coating surfaces increased the micro- micro-roughness and obtained the SHWPUCs whose WCAs were greater than 150 . roughness and obtained the SHWPUCs whose WCAs were greater than 150°. ◦ In the case of Cassie–Baxter state, the rough surface is regarded as a compound interface composed In the case of Cassie–Baxter state, the rough surface is regarded as a compound interface of air and solid. Water droplets can stand at the top of the rough structure of the superhydrophobic composed of air and solid. Water droplets can stand at the top of the rough structure of the surface which has very low adhesion to water droplets. According to Cassie–Baxter equation, superhydrophobic surface which has very low adhesion to water droplets. According to Cassie– cos θc = f (1 + cos θ0) 1, where θc is the apparent contact angle, θ0 is the eigen contact angle, and f Baxter equation, cos 𝜃− 𝑓1 cos 𝜃 1, where 𝜃 is the apparent contact angle, 𝜃 is the eigen is the solid–liquid contact area percent. θc can increase with the decrease of f . As for #3 SHWPUC, its contact angle, and 𝑓 is the solid–liquid contact area percent. 𝜃 can increase with the decrease of 𝑓. θ0 and θc were 112◦ and 156◦, respectively, and it was calculated that f = 0.13. That is, the solid–gas As for #3 SHWPUC, its 𝜃 and 𝜃 were 112° and 156°, respectively, and it was calculated that 𝑓 = 0.13. That is, the solid–gas contact area percent reached 87%, indicating that a large amount of gas

Polymers 2019, 11, 1748 8 of 13 contact area percent reached 87%, indicating that a large amount of gas was between the micro-rough Polymers 2018, 10, x FOR PEER REVIEW 8 of 13 structures of the coating surface and thus the coating was superhydrophobic. 3.2. Adhesion of the SHWPUCs 3.2. Adhesion of the SHWPUCs The adhesion of the coating to the steel substrate is a main performance evaluation. Figure 4 The adhesion of the coating to the steel substrate is a main performance evaluation. Figure4 shows shows the adhesion of different SHWPUCs to the steel substrates. With an increase in the M- the adhesion of different SHWPUCs to the steel substrates. With an increase in the M-PDMS/PTFE PDMS/PTFE system content, the adhesion of the SHWPUC to the Q235 steel substrate first increased system content, the adhesion of the SHWPUC to the Q235 steel substrate first increased and then and then declined. The adhesions of #0, #1 SHWPUC, #2 SHWPUC, #3 SHWPUC and #4 SHWPUC declined. The adhesions of #0, #1 SHWPUC, #2 SHWPUC, #3 SHWPUC and #4 SHWPUC were 3.26, were 3.26, 3.34, 3.42, 3.67 and 3.11 MPa, respectively. The adhesion of #3 SHWPUC was the best 3.34, 3.42, 3.67 and 3.11 MPa, respectively. The adhesion of #3 SHWPUC was the best among all the among all the obtained coatings, which increased by 12.58% compared with that of #0. The reason obtained coatings, which increased by 12.58% compared with that of #0. The reason may be that the may be that the addition of a small number of PTFE particles to the WPU resins could fill the pores addition of a small number of PTFE particles to the WPU resins could fill the pores arising from the arising from the water evaporation during curing of WPU and then the contact areas between the water evaporation during curing of WPU and then the contact areas between the WPU resin and the WPU resin and the metal substrate could increase. Thus, the adhesion of the coating would be metal substrate could increase. Thus, the adhesion of the coating would be strengthened. Additionally, strengthened. Additionally, the pores of #3 SHWPUC were basically filled when the M-PDMS/PTFE the pores of #3 SHWPUC were basically filled when the M-PDMS/PTFE system content reached the system content reached the critical value of 8.05 wt %, so that the coating structure became dense and critical value of 8.05 wt %, so that the coating structure became dense and the adhesion to the steel the adhesion to the steel substrate was enhanced. As the M-PDMS/PTFE system content went on to substrate was enhanced. As the M-PDMS/PTFE system content went on to rise, the excessive PTFE rise, the excessive PTFE particles would occupy too much contact area between the WPU resins and particles would occupy too much contact area between the WPU resins and the metal substrate, leading the metal substrate, leading to the reduction of the adhesion of #4 SHWPUC. to the reduction of the adhesion of #4 SHWPUC.

Figure 4. Adhesion of different SHWPUCs to the steel substrates. Figure 4. Adhesion of different SHWPUCs to the steel substrates. 3.3. Corrosion Resistance of the SHWPUCs 3.3. Corrosion Resistance of the SHWPUCs Figure5A shows polarization curves of the SHWPUCs with di fferent M-PDMS/PTFE system Figure 5A shows polarization curves of the SHWPUCs with different M-PDMS/PTFE system contents after an immersion in 3.5 wt % NaCl solution at 40 ◦C for 30 days. Figure5B shows corrosioncontents after rates an of immersion different SHWPUCs in 3.5 wt % whichNaCl solution were calculated at 40 °C for by 30 computer days. Figure software. 5B shows The corrosioncorrosion rates of different SHWPUCs which were calculated by computer software.2 The corrosion2 rates of #0,2 rates of #0, #1 SHWPUC, #3 SHWPUC and #4 SHWPUC were 2.21 10− , 1.11 10− , 0.29 10− –2 × –2 × –2 × –2 and#1 SHWPUC,2.06 10 #32 mm SHWPUC/a, respectively. and #4 SHWPUC As the M-PDMS were 2.21/PTFE × 10 system, 1.11 × content 10 , 0.29 increased × 10 and the 2.06 corrosion × 10 × − resistancemm/a, respectively. of the SHWPUC As the firstM-PDMS/PTFE increased and system then declined.content increased The facts the may corrosion be as follows. resistance When of the M-PDMSSHWPUC/PTFE first increased system content and then was declined. low, PTFE The particles facts may could be as fill follows. the pores When generated the M-PDMS/PTFE by the water evaporationsystem content of thewas WPU low, coatingPTFE particles during could curing. fill Then, the pores the coating generated structure by the wouldwater evaporation become relatively of the compactWPU coating to prevent during the curing. diffusion Then, of thethe corrosivecoating struct medium.ure would Thus, become the corrosion relatively resistance compact of the to prevent coating wouldthe diffusion be improved. of the corrosive However, medium. with M-PDMS Thus,/ PTFEthe corrosion system content resistance over of 8.05 the wt coating %, the would increased be interfacesimproved. between However, PTFE with particles M-PDMS/PTFE and WPU system resins could content increase over the8.05 di wtffusion %, th abilitye increased of the interfaces corrosive mediumbetween andPTFE thus particles reduce and the WPU corrosion resins resistance could increa of these the coating. diffusion The ability corrosion of the rate corrosive of #3 SHWPUC medium wasand aboutthus reduce one order the ofcorrosion magnitude resistance lower thanof the that coating. of #0. The coatingcorrosion structure rate of and#3 SHWPUC corrosion was resistance about ofone #3 order SHWPUC of magnitude were the bestlower among than that all the of obtained#0. The coating coatings. structure and corrosion resistance of #3 SHWPUC were the best among all the obtained coatings.

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FigureFigure 5.5. ((AA)) PolarizationPolarization curvescurves andand ( B(B)) corrosion corrosion rates rates of of di differentfferent SHWPUCs SHWPUCs immersed immersed in in 3.5 3.5 wt wt % Figure 5. (A) Polarization curves and (B) corrosion rates of different SHWPUCs immersed in 3.5 wt NaCl% NaCl solution solution at 40at 40◦C °C for for 30 days.30 days. % NaCl solution at 40 °C for 30 days. FigureFigure6 6AA shows shows NyquistNyquist plotsplots ofof thethe SHWPUCsSHWPUCs withwith didifferentfferent M-PDMSM-PDMS/PTFE/PTFE system contents Figure 6A shows Nyquist plots of the SHWPUCs with different M-PDMS/PTFE system contents immersedimmersed inin 3.53.5 wtwt %% NaClNaCl solutionsolution atat 4040 ◦°CC for 30 days. A nearly complete semicircle was observed inimmersedin thethe highhigh in frequencyfrequency 3.5 wt % rangeNaClrange solution onon thethe NyquistNyquist at 40 °C plotsplots for 30 andand days. thethe A semicirclesemicircle nearly complete diameterdiameter semicircle showedshowed thewasthe insulationinsulation observed andinand the shieldingshielding high frequency propertiesproperties range ofof on thethe the coatingcoating Nyquist [[29].29 ].plots As and thethe M-PDMSM-PDMS/PTFEthe semicircle/PTFE diameter systemsystem contentshowedcontent increased,increased,the insulation thethe semicircleandsemicircle shielding diameterdiameter properties in the of high-frequency the coating [29]. range As of the the M-PDMS/PTFE SHWPUC first first systemincreased content and then increased, decreased. the ThesemicircleThe semicirclesemicircle diameter diametersdiameters in the inin high-frequency thethe highhigh frequencyfrequency range rangerange of the ofof SHWPUC #1#1 SHWPUC,SHWPUC, first #3#3increased SHWPUC and andand then #4#4 decreased. SHWPUC wereThewere semicircle allall largerlarger diameters thanthan thatthat ofinof the #0.#0. high Furthermore, frequency the range largest of #1 semicircle SHWPUC, diameter #3 SHWPUC in the and highhigh #4 frequencySHWPUCfrequency regionwereregion all ofof larger #3#3 SHWPUCSHWPUC than that wouldwould of #0. holdhold Furthermore, thethe bestbest shieldingshie thelding largest performanceperformance semicircle againstagainst diameter thethe corrosiveincorrosive the high medium.medium. frequency region of #3 SHWPUC would hold the best shielding performance against the corrosive medium.

FigureFigure 6.6. ((AA)) NyquistNyquist plotsplots and and ( B(B))|Z |Z||0.01Hz0.01Hzvalues values of of di differentfferent SHWPUCs SHWPUCs after after an an immersion immersion in 3.5in wt3.5 %Figurewt NaCl % NaCl 6. solution (A solution) Nyquist at 40 at ◦plots 40C for°C and 30for days. 30(B )days. |Z|0.01Hz values of different SHWPUCs after an immersion in 3.5 wt % NaCl solution at 40 °C for 30 days. AA quarterquarter semicirclesemicircle in in the the low low frequency frequency range range was was related related to theto the corrosion corrosion reaction reaction between between the electrolytethe electrolyteA quarter and and thesemicirclemetal the metal substrate in thesubstrate low and frequency clearlyand clearly stated range st thatated was thethat related NaCl the NaCl solution to the solution corrosion had penetrated had reactionpenetrated to thebetween to Q235 the steeltheQ235 electrolyte substrate steel substrate and after the an after metal immersion an substrate immersion for 30and days. forclearly 30 Figure days.stated6B Figure showsthat the the6B NaCl shows|Z| 0.01Hzsolution thevalues |Z| had0.01Hz ofpenetrated the values SHWPUCs ofto the 0.01Hz withQ235SHWPUCs di steelfferent substratewith M-PDMS different after/PTFE M-PDMS/PTFEan systemimmersion contents. forsystem 30 The days. contents.|Z|0.01Hz Figure valueThe 6B |Z| ofshows the0.01Hz SHWPUC valuethe |Z| of the first SHWPUC increasedvalues of first andthe 0.01Hz thenSHWPUCsincreased decreased and with then with different decreased the increase M-PDMS/PTFE with of thethe M-PDMSincrease system of/ PTFEcontents. the M-PDMS/PTFE system The content. |Z| system Thevalue|Z | 0.01Hzofcontent. the valuesSHWPUC The of|Z| #0, 0.01Hzfirst #1 SHWPUC,increasedvalues of and#0, #3 SHWPUC#1 then SHWPUC, decreased and #4#3 with SHWPUCSHWPUC the increase wereand 6,988.47,#4 of SHWPUC the M-PDMS/PTFE 20,267.16, were 44,888.466,988.47, system and20,267.16, content. 9,618.51, 44,888.46 The respectively. |Z| 0.01Hzand Avalues9,618.51, higher of | respectively.Z#0,|0.01Hz #1 SHWPUC,value A of higher the #3 coating SHWPUC |Z|0.01Hz in the valueand low #4 frequencyof SHWPUCthe coating range were in indicated the 6,988.47, low thatfrequency 20,267.16, the coating range 44,888.46 may indicated have and a 0.01Hz stronger9,618.51,that the coating shieldingrespectively. may eff ecthave A on higher a the stronger external |Z| shielding corrosive value efof medium.fect the on coating the The external in|Z |0.01Hzthe corrosivelowvalue frequency of medium. #3 SHWPUC range The indicated |Z| was0.01Hz the highestthatvalue the of among coating#3 SHWPUCall may the have was obtained thea stronger highest coatings, shielding among which all ef wasthefect obtained75.24% on the higherexternal coatings, than corrosive which that ofwas medium. #0. 75.24% The result The higher |Z| further than0.01Hz indicatedvaluethat of of #0. #3 that SHWPUCThe the result coating was further structurethe highestindicated of among #3 that SHWPUC allthe the coating wasobtained the structure densest coatings, of among which#3 SHWPUC all was coatings. 75.24% was higherthe densest than thatamong of #0.all coatings.The result further indicated that the coating structure of #3 SHWPUC was the densest among all coatings.

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3.4. Wear Resistance of the SHWPUCs ItIt isis evidentevident thatthat the hydrophobicity and corrosioncorrosion resistance of the obtained #3 SHWPUC were thethe bestbest amongamong allall WPUWPU composites.composites. Therefore, th thee wear resistance of #3 SHWPUC was furtherfurther investigatedinvestigated andand itit waswas comparedcompared withwith thatthat ofof thethe purepure WPUWPU coatingcoating underunder samesame conditions.conditions. During thethe wearwear test,test, thethe appliedapplied loadload waswas 55 N,N, thethe rotationrotation speed of the steel ball was 400 rr/min,/min, thethe sliding radiusradius waswas 55 mmmm andand the the wear wear time time was was 10 10 min. min. Figure Figure7A 7A shows shows the the friction friction coe coefficient–timefficient–time curves curves of #0of and#0 and #3 SHWPUC,#3 SHWPUC, and and Figure Figure7B shows 7B shows their weartheir rates.wear Therates. friction The friction coe fficient–time coefficient–time curve of curve #0 was of uniformly#0 was uniformly wavy, however, wavy, however, the friction the coe frictionfficient–time coefficient–time curve of #3curve SHWPUC of #3 SHWPUC first greatly first fluctuated greatly andfluctuated then became and then smooth became after smooth 6 min. after This 6 may min. be This ascribed may be to theascribed low surface to the frictionlow surface of the friction fluorine of 10 3 elementthe fluorine [24]. element The wear [24]. rate The (1.33 wear 10rate− (1.33cm /×mm 10–10 N) cm and3/mm friction N) and coe frictionfficient (0.08)coefficient of #3 (0.08) SHWPUC of #3 × 9 3 declinedSHWPUC by declined 88.82% andby 78.38%,88.82% respectively,and 78.38%, comparedrespectively, with compared those of #0,with1.19 those10 −ofcm #0, /mm1.19 N× and10–9 × 0.37.cm3/mm The N reason and 0.37. for this The would reason be for that this under would the be action that under of a high-voltage the action of electrostatic a high-voltage field electrostatic the surface activityfield the of surface nano-SiO activity2 particles of nano-SiO was probably2 particles enhanced was probably and then enhanced polar groupsand then such polar as –OHgroups groups such wereas –OH absorbed groups on were the surfaces absorbed of nano-SiOon the surfaces2 particles. of nano-SiO Thus, the2 polymerization particles. Thus, reaction the polymerization that occurred betweenreaction that these occurred polar groups between and these some polar polar groups groups and in the some molecular polar groups structure in the of WPUmolecular during structure curing madeof WPU nano-SiO during2 curingfirmly made embed nano-SiO to the composite2 firmly embed coating, to sothe that composite it was di coating,fficult for so nano-SiOthat it was2 particles difficult tofor wear nano-SiO off. In2 particles addition, to thewear high off. hardnessIn addition, and th strengthe high hardness of nano-SiO and strength2 particles of nano-SiO could improve2 particles the strengthcould improve of #3 SHWPUC, the strength whose of #3 wear SHWPUC, resistance whose was wear better resistance than that was of #0. better than that of #0.

Figure 7. (A) Friction coecoefficient–timefficient–time curves and ((B)) wearwear ratesrates ofof #0#0 andand #3#3 SHWPUC.SHWPUC.

Figure8 8 shows shows the the wear wear track track morphologies morphologies of #0of and#0 and #3 SHWPUC #3 SHWPUC after after the wear the test.wear There test. There were awere few a worn few worn pieces pieces on the on wear the wear track track of #0 of (Figure #0 (Figure8A), 8A), which which would would become become abrasive abrasive materials materials to exacerbateto exacerbate the the wear wear of the of coating.the coating. The The wear wear rate andrate frictionand friction coeffi coefficientcient of #0 wereof #0 aggravatedwere aggravated and a wavyand a frictionwavy friction coefficient–time coefficient–time curve was curve generated. was genera It is obviousted. It is that obvious there werethat almostthere were no block almost pieces no onblock the pieces wear trackon the of wear #3 SHWPUC track of #3 and SHWPUC its wear and track its seemed wear track to be seemed shallow to (Figure be shallow8B). EDS(Figure shows 8B). thatEDS thereshows were that athere large were number a large of Si number and F elementsof Si and in F theelements wear trackin the of wear #3 SHWPUC track of #3 (Figure SHWPUC8C), which(Figure may 8C), have which reduced may have its frictionreduced coe itsffi frictioncient and coefficient wear rate and and wear thus rate improved and thus itsimproved wear resistance. its wear Thisresistance. further This indicated further that indicated the superhydrophobic that the superhydrophobic surface of the coatingsurface withof the excellent coating wear withresistance excellent canwear be resistance constructed can by be electrostatic constructed spraying. by electrostatic spraying.

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Figure 8. Wear track morphologies of (A) #0; and (B) #3 SHWPUC. (C) EDS of #3 SHWPUC. Figure 8. Wear track morphologies of (A) #0; and (B) #3 SHWPUC. (C) EDS of #3 SHWPUC. 4. Conclusions 4. Conclusion A series of superhydrophobic WPU composites with micro-rough surface structure were prepared by electrostaticA series of spraying superhydrophobic nano-SiO2 particles WPU oncomposites the obtained with WPU micro-rough composites surface with low structure surface energy.were preparedIt was concluded by electrostatic that the spraying hydrophobic nano-SiO WPU2 composite particles on was the prepared obtained by WPU adding composites the M-PDMS with/PTFE low surfacesystem toenergy. the WPU It was dispersion. concluded As that the M-PDMSthe hydrophobic/PTFE system WPU content composite rose, was the prepared WCAs of theby adding composites the M-PDMS/PTFEfirst increased andsystem then to remained the WPU dispersion. stationary; As however, the M-PDMS/PTFE the adhesion system and corrosion content rose, resistance the WCAs first ofincreased the composites and then first decreased. increased An and appropriate then remained addition stationary; of the hydrophobic however, the system adhesion content and would corrosion lead resistanceto a dense first coating increased structure, and but then an excessivedecreased. addition An appropriate could increase addition the interfacesof the hydrophobic in the coating system and contentthen reduce would the coatinglead to performance.a dense coating When structure, the mass but ratio an of theexcessive WPU dispersion,addition could PTFE particlesincrease andthe 2 interfacesM-PDMS in was the 8:0.3:0.4, coating 10and g/ mthennano-SiO reduce the2 particles coating wereperformance. sprayed onWhen the uncuredthe mass coatingratio of surfacethe WPU to 2 dispersion,construct the PTFE #3 SHWPUC, particles and whose M-PDMS WCA was was 156 8:0.3:0.4,◦. Compared 10 g/m with nano-SiO the pure2 particles WPU coating, were itssprayed adhesion on the(3.67 uncured MPa) increased coating surface by 12.5%, to construct its corrosion the rate#3 SHWPUC, (0.29 10 whose2 mm /WCAa) was was reduced 156°. byCompared almost one with order the × − pureof magnitude, WPU coating, and its moreover, adhesion its (3.67 wear MPa) rate incr (1.33eased10 by10 12.5%,cm3/mm its corrosion N) and friction rate (0.29 coe ffi× 10cient–2 mm/a) (0.08) × − wasdecreased reduced by by 88.8% almost and one 78.3%, order respectively. of magnitude, and moreover, its wear rate (1.33 × 10–10 cm3/mm N) and friction coefficient (0.08) decreased by 88.8% and 78.3%, respectively. Author Contributions: Conceptualization, L.F. and F.W.; methodology, Z.Z., B.D., H.M. and S.Z.; writing—original draft preparation, F.W.; writing—review and editing, L.F. and F.W.; supervision, G.L. Author Contributions: Conceptualization, L.F. and F.W.; methodology, Z.Z., B.D., H.M. and S.Z.; writing— originalFunding: draftThis preparation, work was F.W.; supported writing—review by the Natural and editing, Science L.F. Foundation and F.W.; ofsupervision, Shaanxi Province, G.L. China, grant number 2018JM5053. Funding: This work was supported by the Natural Science Foundation of Shaanxi Province, China, grant Conflicts of Interest: The authors declare no conflict of interest. number 2018JM5053.

ConflictsReferences of Interest: The authors declare no conflict of interest.

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