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Preparation and Anti-scaling performance of Superhydrophobic PPS/PTFE composite coating Huijuan Qian, Yanji Zhu, Huaiyuan Wang, Hua Song, ChiJia Wang, Zhanjian Liu, and Hongwei Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03975 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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Industrial & Engineering Chemistry Research

Preparation and Anti-scaling Performance of Superhydrophobic PPS/PTFE Composite Coating Huijuan Qian†, ‡, Yanji Zhu†, Huaiyuan Wang*,†, Hua Song*,†, Chijia Wang†, Zhanjian Liu†, Hongwei Li† † College of Chemistry and Chemical Engineering, Northeast Petroleum University, Daqing, 163318, Republic of China

‡ College of Chemical Engineering, Daqing Normal University, Daqing 163712, Republic of China

KEYWORDS: superhydrophobic coating; PPS/PTFE composite coating; anti-scaling; epoxy-silicone resin coating

ABSTRACT: In this paper, the superhydrophobic polyphenylenesulfide (PPS)/polytetrafluoroethylene (PTFE) composite coating was fabricated and applied to evaluate the anti-scaling performance. Compared with the commercial hydrophobic epoxy-silicone resin coating, the superhydrophobic PPS/PTFE composite coating exhibited unique anti-scaling performance. The deposition rate of CaCO3 scaling on the superhydrophobic PPS/PTFE coating was only 38.6% of that on the epoxy-silicone resin coating. The surface morphology, size and crystal type of the scaling were analyzed. The results indicated that the formation and growth of CaCO3 scaling were significantly influenced by the cooperative effect of topography and low surface energy of

*Corresponding authors. Fax: +86 459 6503083. E-mail addresses: [email protected]

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the superhydrophobic PPS/PTFE composite coating. There were few nucleation sites at the surface of superhydrophobic coating owing to the relatively low surface energy and absorbed bubbles. Together with the space constraint of the topography, the nucleation and growth of CaCO3 scaling was inhibited on the surface of superhydrophobic PPS/PTFE coating. KEYWORDS: Superhydrophobicity, PPS/PTFE, Composite Coating, Anti-scaling, CaCO3 scaling

1. INTRODUCTION Wastewater from many factories, especially in petroleum and chemical industry, contains abundant Cl-, Ca2+, Mg2+, Ba2+, carbonate ion, bicarbonate ion and sulfate ion. As we know, once the salification ions (a metal cation or the ammonium ion, and the oxide anion or nonmetal ions) meet each other and the amount of produced salts exceed their solubility in water under certain temperature and pressure, the above ions coexisting in metallic pipelines or pumps will lead to scaling. Scaling gathered on the inner wall of pipelines is often unavoidable in many fields, such as heat exchangers, cooling towers, steam turbines. In the process of fluid transportation and oilgas gathering-transferring, the scaling problem decreases the service life of equipments and fluid transportation efficiency. The scaling deposition also increases the flow resistance and narrows the area of cross-section conduits, and results in higher pressure drop and energy consumption. It can also cause significant technical and economic problems.1,2 Usually, scaling types of oilfield produced fluid include CaCO3, CaSO4, MgCO3 and BaCO3. Among them, calcium carbonate (CaCO3) is the most common type of scales in oil development. Anti-scaling plays a paramount part in the petroleum and chemical industry.


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The utilization of polymer coatings is an effective and inexpensive option for protecting metal surfaces against scaling. According to literature, it is a favorable way to spray hydrophobic coatings (the static water contact angle is greater than 90°) on the interior surface of pipelines. Sugama and co-workers3 had examined the availability of polyphenylenesulfide (PPS) /polytetrafluoroethylene (PTFE) with a static water contact angle (WCA) of 117° in silica scale prevention, and the results demonstrated that the silica deposition reduced obviously. Liu4 reported that the CaCO3 nucleation rate on the hydrophobic fluorosilane surface was only 45% of that on the hydrophilic copper surface. Ning5 stated that the adhesive force between the CaCO3 scaling layer and the hydrophobic coating was relatively weak, which is the main reason for the coating with antiscaling performance. Oldani et al

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prepared ZrO2/TiO2-perfluoropolyethers

(PFPE) composite coatings and found that the adhesion of CaSO4 on the coated surfaces was 7090% lower than that of the bare stainless steel surface owing to the highly hydrophobic performance. Dowling et al7 stated that both the 2,4,6,8-tetramethylcyclotetrasiloxane (TC) and TC/fluorosiloxane hydrophobic surfaces had anti-fouling of CaCO3. Cheng et al8 reported that the electroless Ni-Cu-P-PTFE coatings (WCAs=92-105°) had better anti-scaling performance than the bare steel in boiling water. Compared with the mild steel surface of the heat exchangers, the adhesion of fouling at the surfaces of Ni–Cu–P-PTFE reduced obviously. Malayeri et al9 and Yang et al10 conducted CaSO4 and CaCO3 scaling experiments on the hydrophobic coatings, respectively. They revealed that the hydrophobic coatings can significantly prolong the induction period of fouling and reduce the scaling rate. Cai et al11 informed that the hydrophobic TiO2fluoroalkylsilane (FPS) coating exhibited favorable antifouling performance in CaCO3 solution. Consequently, some researchers had emphasized the anti-scaling properties at different hydrophobic coating.


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A superhydrophobic surface with high water contact angle (CA, larger than 150°) and low water sliding angle (SA, lower than 10°), has attracted much attention from academia and industry in recent years.12,13 In nature, many creatures have superhydrophobic properties, such as lotus leaf,14 shark skin,15 desert beetle’s back,16 water strider,17 and cicada’s wings.18 Superhydrophobicity has wide applications in practice, such as self-cleaning,19 drag reduction,20 anti-scaling.21 To the best of our knowledge, as compared to hydrophobic coatings for antiscaling, there are few researches on scale inhibition of superhydrophobic coatings. Li et al22 prepared a metal alloy superhydrophobic coating (CA=157°) with efficient self-cleaning properties, and the anti-scaling property was also investigated. Results indicated there were only aragonite crystals on the superhydrophobic coating, while almost rhombohedral CaCO3 crystals deposited on the bare steel substrate. Jiang et al23 observed that the weight of CaCO3 scaling on superhydrophobic anodized CuO nanowire surface (CA=154.1°) was only 64.3% with respect to hydrophobic modified pure Cu surface (CA=102.5°). Up to now, however, antiscaling of superhydrophobic polymer composite coatings has not been reported. This article is intended to prepare the superhydrophobic PPS/PTFE composite coating (polymer based composite coating with high mechanical performance, wear resistance and corrosion resistance) and investigate its scale inhibition performance in CaCO3 supersaturated solution. The Q235 carbon steel was chosen as the substrate which is widely used for pipeline material in the oil field. In order to compare the effect of scale inhibition on different material coatings, the commercial hydrophobic epoxy-silicone resin coating (with high temperature resistance, anti-corrosion and weather ability) was used as a reference. Compared with hydrophobic epoxy-silicone resin coating, the superhydrophobic PPS/PTFE composite coating manifested excellent scale-inhibiting performance, which will be discussed in detail later. It is


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desirable to open a new window for antiscaling studies of polymer based superhydrophobic coatings. The fabricated superhydrophobic polymer composite coating provided the feasible solution for industrial scaling inhibition. 2. EXPERIMENTS 2.1. Materials. The Q235 (GB/T700-2006) carbon steel and stainless steel 304, 11.5 mm in width, 72.4 mm in length and 2 mm in thickness were used as the metallic substrate. Commercial W304 epoxy-silicone resin was purchased from Yuanen composition material Co., Ltd. The PPS resin powder was supplied by Yuyao Degao Plastic Technology Co. Ltd. (China). Commercial PTFE was bought from DuPont, USA. The nanometer silica (SiO2) was supplied by Hi-Tech Reagent Factory of Nanjing (China). The dehydrated alcohol, ethyl acetate, the ethylene glycol (EG) and calcium nitrate terahydrate were bought from Huadong Reagent Factory, Shenyang, China. Sodium bicarbonate was provided by Hengxing Chemical Reagent Factory of Tianjin (China). In the process of experiment, all chemical agents used were analytically pure. 2.2. Preparation of epoxy-silicone coatings. The Q235 plates were degreased into pure acetone and then polished with 1000 mesh sandpapers to the mean roughness of 0.15-0.30 µm. Then, the samples were cleaned with dehydrated alcohol. Thoroughly mixed 33.3 vol.% epoxysilicone resin and 66.7 vol.% ethyl acetate. After that, sprayed the above solution on the astreated Q235 plates by using an air spray gun under pressure of 0.6 MPa. And finally, the samples were cured at 200 °C for 1.5~2.0 h. The hydrophobic epoxy-silicone resin coating with smooth surface was obtained. 2.3. Preparation of PPS/PTFE composite coatings. The initial material PPS/1.0wt% silicon dioxide nano-particles were sprayed on the treated Q235 carbon steel and 304 stainless steel substrate for the premier, and followed by annealed at 300-350 °C. Then, the various weight


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ratio PPS/PTFE (15%, 31%, and 45wt%, as follow) powders as surface coating material were spread around in dehydrated alcohol by stirring-ultrasonic dispersion for 30 min. The wet coatings on carbon steel and 304 stainless steel substrates were prepared by spraying under the pressure of 0.4-0.6 MPa, dried at 60 °C for 30 min, then solidified at 300-350 °C for 1.5 h,24 and cooled to room temperature. Thus the superhydrophobic PPS/PTFE composite coating with special micro- and nano scale structure was gained. Superhydrophobic PPS/31%PTFE coatings at stainless steel substrate with different roughness surface also were fabricated by adjusting the curing temperature and time. 2.4. CaCO3 scale formation on the epoxy-silicone and PPS/PTFE coatings. Calcium carbonate scale solution was prepared (equation 1) with 250 mL 14.20 g/L Ca(NO3)2·4H2O, and 250 mL 10.08 g/L NaHCO3 solutions in a 500 mL screw-topped glass bottle. The mixed solution was then heated to 60°C using a thermostatic water bath. The CaCO3 scale will precipitate through the crystallization from a static supersaturated solution. C a (NO 3 )2 ⋅ 4 H 2 O+ 2 NaHCO 3 → CaCO 3 + 2 NaNO3 + 5 H 2 O+ CO 2 ↑

(1)

The concentration of calcium ion in the supersaturated solution was quantitatively analyzed by EDTA titration (GB/T 7476-1987). The coated specimens were immersed in supersaturated CaCO3 solution at 60 °C and tested up to 360 h. Specific scaling steps are as follows: first, the epoxy-silicone resin or PPS/PTFE coated Q235 carbon steel and 304 stainless steel specimens were vertically immersed into different screw-topped glass bottle containing 250 mL 14.20 g/L of Ca(NO3)·4H2O solution. Second, 250 mL 10.08 g/L of NaHCO3 solution was added slowly into calcium nitrate terahydrate solution. At last, above screw-topped glass bottles were placed into thermostatic water bath of 60 °C. After a certain time interval, the sample specimens were gently taken out from the solution, and then dried at the temperature of 100-105 °C for 1.0 h in an electric constant temperature drying oven.


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2.5. Characterization. The static contact angles were measured with a contact angle meter (JGW-360A, Chengde city Chenghui testing machine co., LTD) at indoor temperature. The surface morphology of coatings before and after deposition of calcium carbonate was characterized by scanning electron microscopy (SEM, Quanta 200). The structure of the prepared coatings was characterized by Fourier transform infrared (FT-IR) spectroscopy (Tensor27). The crystal structure of CaCO3 scaling on specimens was examined by X-ray diffraction powder diffractometer (XRD) (PW3040/60, PANalytical). 3. RESULTS and DISCUSSION 3.1 Preparation of the superhydrophobic PPS/PTFE composite coating. Generally speaking, the chemical compositions have obvious effects on the WCA and scaling behaviors. As shown in table 1, the WCA increases with the content of PTFE increase. It reaches superhydrophobicity of 151.3° at the PTFE content of 31%, and 160.5° at the PTFE content of 45%. On the contrary, the scaling weight decreasing with PTFE content increase. We can find that the PPS/PTFE superhydrophobic coatings showing much better scale inhibition. The XRD analysis of scaling on the PPS/PTFE coating (Fig. S2) with different contents of PTFE shows clearly the formation of CaCO3 scaling, with the diffraction peaks at 21.04, 23.24, 25.10, 27.60, 29.45, 32.39, 37.30, 39.69, 41.77, 47.73, 48.95, 52.13, 53.94, 57.30, 58.40, 66.60, respectively. The corresponding morphology analysis in Figure S1 also verifies the scaling formation on the superhydrophobic surfaces. The polyhedrons CaCO3 are the main structure of the scale at the surface of PPS/15%PTFE, and prism of PPS/31%PTFE, cuboid of PPS/45%PTFE superhydrophobic coating, respectively. When the content of PTFE is high, the scaling deposition on the superhydrophobic PPS/31%PTFE composite coating is similar to that of PPS/45%PTFE coating. Considering the decreasing of adhesion strength to substrate with


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high content of 45% PTFE. Together with the economy of superhydrophobic coating, the PPS/31%PTFE is chosen as the best option for later scaling studies. In addition, the CaCO3 scaling experiments on stainless steel substrate with and without superhydrophobic PPS/PTFE coating have been carried out. It can be seen from table S1 that the scaling weight at the stainless steel substrate is influenced by the roughness. When the roughness of stainless steel surface increases, the scaling also increases. However, after the stainless steel is coated with PPS/31%PTFE superhydrophobic coating, surface roughness has small effect in the formation of CaCO3 scaling. The superhydrophobicity of PPS/PTFE coating plays an important role in preventing CaCO3 scaling. Table 1. Water contact angle (°) and scaling weight at the surface of superhydrophobic PPS/PTFE coating Samples

WCA (°)

Scaling weight (mg/cm2)

PPS/15%PTFE

135.2

0.58

PPS/31%PTFE

151.3

0.42

PPS/45%PTFE

160.5

0.40

3.2 Characterization of different Coatings. To obtain the topography information of coatings, the SEM measurement is carried out. Figure S3 shows the surface images of hydrophobic epoxy-silicone resin coating and superhydrophobic PPS/PTFE coating. It could be seen in Figure S3a that the surface of hydrophobic epoxy-silicone resin coating is relatively smooth and there is no obvious rough structure. The static water contact angle is 98.9°, which indicates that the epoxy-silicone resin coating has high hydrophobic property. Whereas there are abundant micro- and nano-scale structures on the surface of superhydrophobic PPS/PTFE coating as shown in Figure S3b. The partial enlarged detail in Figure S3b1 confirms that the surface is made up of quantityies of papillaes with average diameter of about 20 µm. On the surface of papillas is covered with a large amount of nanobuds. The surface morphology of


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superhydrophobic PPS/PTFE coating is extremely similar to that of lotus leaf (Figure 2c). The coating structure is closely related to the priming nano-SiO2, PPS and PTFE. It might arise from the dispersed nano-SiO2 form nipples on PPS and PTFE. The static WCA of superhydrophobic PPS/PTFE composite coating is 151.3° (Figure S3b), which is much higher than that of epoxysilicone coating. The PPS/PTFE composite coating has higher superhydrophobicity than the epoxy-silicone coating because of the presence of fluorine group (-CF2-) in PTFE.25 Figure 1 shows the FTIR spectrum of the superhydrophobic PPS/31%PTFE composite coating. Significant absorption bands at 3064 and 1572 cm-1 are related to C-H and C=C stretching vibration in the benzene ring of the PPS matrix. The band at 1472 cm-1 is framework vibration of the benzene ring. The peaks at 1179 and 810 cm-1 correspond to the C-S stretching and bending vibration in the aromatic ring, respectively. The bands that appeared at 1300-1200 cm-1 and 640-500 cm-1 are the signals of -CF2- stretching vibration and -CF- bending vibration, those are originated from PTFE.

Figure 1. FTIR spectrum of the superhydrophobic PPS/31%PTFE composite coating.

3.3. Anti-scaling property of the superhydrophobic coating. In order to illustrate the antiscaling performance of coatings, the experiments of CaCO3 scale formation had been carried out. As shown in Figure 2a, the weight of calcium carbonate scaling on different coatings increases


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with prolongation of immersion time. The scaling behavior shows a shorter induction time about 0.5 h, which is the time of delay before scaling particles are deposited on the hydrophobic epoxysilicone resin surface. Soon after, the scaling rate increases to a certain degree. The whole growth of CaCO3 scaling can be divided into two stages. In the first stage, the mass of CaCO3 increases from 0.1mg/cm2 to 2.3 mg/cm2, with an average scaling rate of 0.0153 mg/(cm2·h). In the second stage, it has a very slow scaling rate of 0.0104 mg/ (cm2·h), which is attributed to the reduction of CaCO3 nucleation sites on the hydrophobic surface. Interestingly, there is a dynamic balance between the scaling rate and the fall off rate, and the two values are very close. Compared with the commercial hydrophobic epoxy-silicone coating, the superhydrophobic PPS/PTFE surface has a longer scaling induction time about 8.0 h, which is prolonged nearly 16 times. In the first stage, the scaling rate of CaCO3 elevated from 0.07 mg/cm2 to 0.88 mg/cm2 (Figure 2b). The average scaling rate of calcium carbonate is 0.0044 mg/(cm2·h), which is equivalent to 28.8% of hydrophobic epoxy-silicone surface. In the second stage, the average scaling rate is almost unchanged with 0.0047 mg/(cm2·h), which is far less than that of the hydrophobic epoxy-silicone resin coating. The total growth rate of CaCO3 scaling is still small. Therefore, with respect to the hydrophobic epoxy-silicone resin surface, the superhydrophobic PPS/PTFE composite coating has superior CaCO3 antiscaling performance.


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Figure 2. The weight of CaCO3 scaling versus immersion time on different coatings: hydrophobic epoxysilicone (a), and superhydrophobic PPS/PTFE composite coating (b).

Besides, the morphology and dimension of calcium carbonate crystals are analyzed by SEM. As shown in Figure 3a, there are cube, cuboid, six-sided, trapezoid column and five arris terrace shaped crystals on the hydrophobic epoxy-silicone resin coating after scaling formation for 24h, which are typical crystal shape of calcite. The grain size ranged from 1.9 to 7.2 µm and the dimension of most crystals is 6-7 µm. Beyond that, there are a few small crystal particles with diameter of 0.1-0.2 µm. When the immersion time is extended to 96 h, the scaling layer became highly dense (Figure 3b) that resulting in a full coverage of the entire surface. The CaCO3 crystals exhibit the tightly packed structure. The crystal morphology includes cube, cuboid and a large number of unusually shaped blocks with a diagonal of 2.0-6.0 µm. In the meantime, there are a lot of small grains and the thin willow clusters in the growth stage, which is the vaterite26 according to the results of XRD in Figure 5a. However, the grain size of the CaCO3 deposit is mostly 2.0-4.0 µm, which is smaller than that of 24 h’s (Figure 3a). The above results indicate that the morphology and dimension of the CaCO3 crystals on the superhydrophobic PPS/PTFE composite coating have a significant difference on the surface of hydrophobic epoxy-silicone coating with the same immersion time. There are several reasons for this difference. First, the


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crystal density is relatively small when immersion time is closed to 24 h. So the crystal could grow in a larger space by the widthwise growth of individual nucleation positions.10,

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Consequently, it is apt to form a larger scale crystal. The size and density of crystals increase, resulting in a complete coverage of the coating surface (Figure 3b). Second, the crystal particles would continue to grow in the void among the CaCO3 crystals or grow on top of the initial formation CaCO3 crystal layer26 (Figure 3b). To some degree, the crystal shape of calcium carbonate is restricted. Last, because of the existence of the initial formation CaCO3 crystal layer, the surface roughness of the epoxy-silicone resin coating increases, which provides more nucleation sites. As a result, the nucleation rate and the number of crystals are larger than that of 24 h.

Figure 3. Serial SEM images of CaCO3 scaling on hydrophobic epoxy-silicone resin coating with different soaking time: 24 h (a), and 96 h (b).

Compared with hydrophobic epoxy-silicone coating, the dimension and morphology of CaCO3 crystals on the surface of superhydrophobic PPS/PTFE composite coating (Figure 4) has a fundamental difference, and the later one showing unique scaling inhibition property. There are three types of scaling on the surface of superhydrophobic PPS/PTFE composite coating, including flaky rhombic crystal with a length of 0.4-1.2 µm, globular villous clusters with a


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diameter of 1-4 µm and nano-scaled needle cluster (Figure 4a). When the scaling time prolongs from 24h to 96h, the size of CaCO3 crystals has slightly increase, flaky rhombic crystals with length in the range of 2-3 µm, globular villous clusters with a diameter of 3-4 µm (Figure 4b). According to XRD results in Figure 7b, it could be concluded that the flaky rhombic crystal, globular villous cluster and nano-scaled needle cluster are calcite, aragonite and vaterite, respectively. Flaky rhombic crystals and fluffy clusters have a tendency to form rosette crystals with a diameter of 6-10 µm. The average dimension of CaCO3 crystals on the surface of superhydrophobic PPS/PTFE composite coating is much smaller than that of on the hydrophobic epoxy-silicone. There are few scaling crystals with a loose structure on the surface of superhydrophobic PPS/PTFE composite coating. In addition, the density of the crystals is markedly scanty, which is consistent with the results of Figure 2.

Figure 4. SEM images of CaCO3 scaling on the superhydrophobic PPS/PTFE composite coating with different soaking time: 24 h (a), 96 h (b).

In order to understand the crystal form of the precipitate on the coating surface, the XRD analysis of the deposits on the hydrophobic epoxy-silicone coating in supersaturated calcium carbonate solution at 60 °C for 0 h, 24 h and 96 h is shown in Figure 5a. The diffraction peaks at 4.6° and 64.9° are the characteristic of ferrum from the Q235 carbon steel substance. The


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characteristic diffraction peaks of calcite and vaterite are detected. Among them, the main peaks of calcite appeared at 23.04°, 29.44°, 31.42°, 35.95°, 39.40°, 43.15°, 47.49°, 48.49°, 57.40° and 65.6°, indexing to (012), (104), (006), (110), (113), (202), (018), (116), (211), and (012) respectively. The diffraction peak of vaterite appeares at 61.38°, indexing to (226). However, the crystal of CaCO3 scale on the superhydrophobic PPS/PTFE composite coating is shown in Figure 5b, it is much different from that of epoxy-silicone coating. The characteristic peaks of calcite occurred at 2θ of 23.04°, 29.44°, 35.95°, 39.40°, 43.15°, 47.49°, 48.49°, and 57.40° indexing to (012), (104), (110), (113), (202), (018), (116), and (122) respectively, in both spectra of 24 h and 96 h. When the immersion time reaches 24 hours, the peak of vaterite appeared at 61.38° (226) and the peak of aragonite appeared at 31.22° (002) and 62.95° (321). After testing the superhydrophobic PPS/PTFE composite coating to 96 h, the peaks of vaterite at 61.38° and aragonite at 31.22° are obtained. Compared with results in Figure 5a, the differences of crystal types of CaCO3 scale on the hydrophobic epoxy-silicone coating and superhydrophobic PPS/PTFE composite coating are mostly reflected in following three aspects. Firstly, the crystal form of calcium carbonate on different coatings is different. There are three crystalline polymorphs including calcite, aragonite and vaterite at the surface of superhydrophobic PPS/PTFE coating, while calcite is the main crystal for epoxy-silicone coating. Secondly, at the same soaking time, the intensity of the calcite diffraction peak at 29.44° on the superhydrophobic PPS/PTFE composite coating is much lower than that on the hydrophobic epoxy-silicone coating. Thirdly, with the increase of immersion time, the increment of calcite peak height (29.44°) on PPS/PTFE is less than that on epoxy-silicone resin. The results also show that the amount of calcium carbonate scaling at the superhydrophobic PPS/PTFE


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composite coating is much less than that of hydrophobic epoxy-silicone coating, which is also in agreement with the anti-scaling results in Figure 2.

Figure 5. XRD patterns of the CaCO3 scale deposited on the surface of different coatings against immersion time: on the hydrophobic epoxy-silicone resin coating (a), and on the superhydrophobic PPS/PTFE composite coating (b).

3.4. The Influence of Topography and Surface Free Energy of Coatings on Scaling. The molar fractions of calcite, aragonite and vaterite can be calculated from the X-ray diffraction data by the following relationships:28 XC =

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(2) 3 .9 × I A 2 .9 × I V 1+ + IC IC I XA = 3 .9 × A × X C (3) IC I XV = 2 .9 × V × X C (4) IC where, the subscripts C, A and V indicate calcite, aragonite and vaterite, respectively. IC, IA and IV represent the intensity of the selected XRD diffraction peak, calcite (29.44°), aragonite (31.22°) and vaterite (61.38°), respectively. X is the molar fraction of different polymorphs. The calculated results are presented in Figure 6. It could be observed that the molar fractions of


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CaCO3 polymorphs are quite different with types of coatings. The molar content of calcite and vaterite has remained approximately constant on epoxy-silicone coating. When the immersion time from 24 h to 96 h, as shown in Figure 6a & 6b, the content of calcite slightly increases from 75.2% to 75.5%, and the content of vaterite slightly decreases from 24.8% to 24.5%. However, the molar fractions of calcite, vaterite and aragonite on the superhydrophobic PPS/PTFE composite coating with 24 hours immersion (Figure 6c) are 25.7%, 34.3% and 40.0%, respectively. When the immersion time increase to 96 hours (Figure 6d), the content of CaCO3 polymorphs changes accordingly. The fraction of vaterite and aragonite decreases to 32.3% and 34.2%, respectively. At the same time, the fraction of calcite increases distinctly to 33.5%. Although, the molar content of calcite increased, the value is still far less than that on the hydrophobic epoxy-silicone coating. Results show that the major crystal of CaCO3 scaling on hydrophobic epoxy-silicone coating is calcite. However, there is no significant different content for the three kinds of CaCO3 polymorphs on the superhydrophobic PPS/PTFE composite coating.

Figure 6. The molar fractions of CaCO3 polymorphs deposited on different coatings: on the hydrophobic epoxy-silicone coating (a, b), and on the superhydrophobic PPS/PTFE composite coating (c, d) with different deposition time: (a, c) 24h, and (b, d) 96h.


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Figure S3 shows the different surface topographies of the hydrophobic epoxy-silicone coating and the superhydrophobic PPS/PTFE composite coating. On the flat surface of hydrophobic epoxy-silicone coating, the CaCO3 crystals could nucleate and grow easily, resulted in a short induction time of scaling. However, there are abundant papillaes at the surface of superhydrophobic PPS/PTFE composite coating, which is covered with incalculable nano-villi (Figure S3b). These unique abundant nano-villi and micro-papillaes structure provides absorbing sites for trapping large quantities of air bubbles. The massive air pocket in the superhydrophobic coating has an important role in prolonging the scaling induction time. The detailed mechanism diagram is exhibited in Figure 7. An air gap is formed at the interface of the solid and liquid. Initially, the wetting behavior is in a state of the Cassie-Baxter, in Figure 7a. The air layer in the superhydrophobic coating could hinder the nucleation of CaCO3 and extend the scaling induction period.29 After the induction period, with the extension of immersion time, the local position of the air gap would be destroyed. Then, the water-drop containing calcium ions and carbonate ions would enter into the nano/micro structures, increasing the liquid-solid contact area. The deposited CaCO3 from the bulk solution or nucleation would grow on the superhydrophobic coating surface.30,31 The grain of nano-CaCO3 crystal has a tendency to grow at peaks of papillae, as shown in Figure 7a. With the time increase, the water containing CaCO3 deposition will extend at the surface, and the wetting behavior is in a transition state as shown in Figure 7b. Lastly, the nucleation of CaCO3 spreads to the valley between two papillaes, the wetting behavior changes from Cassie-Baxter state to Wenzel state, as shown in Figure 9c.32 Although, the CaCO3 is more likely to nucleate and grow on mastoid peaks of the superhydrophobic PPS/PTFE composite coating, the special topography could effectively reduce the scalling due to the small nominal contact area of the mastoid peaks. Generally speaking, the


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smaller the contact area, the lower adhesion of scaling on the coating surface.33 In the stage of crystal growth, the calcium carbonate crystals can grow without any limit on the surface of the epoxy-silicone coating, and the morphology of the crystal is the typical geometry shape of calcite (Figure 3). However, the growth of calcium carbonate at the superhydrophobic PPS/PTFE composite surface is restricted owing to its space constraint, and the morphology of rosette crystal appeares in Figure 4 (b1, b2). The results are in according with the study of Loste et al,34 they found that the crystal morphology could be affected by the environment. When crystals grow on surface with special structure and properties, different morphologies could be obtained.

Figure 7. The transformation of wetting model between Cassie-Baxter and Wenzel on the surface of superhydrophobic PPS/PTFE composite coating: Cassie-Baxter state (a), transition state (b), Wenzel state (c).

Surface energy is another key parameter that can influence the adhesion of precipitate on solid surfaces. It offers a direct measure of the interfacial attractive forces. Surface free energy of different coatings is calculated by a series of equations according to static contact angles of deionized water and ethylene glycol. The static contact angle (θ) of pure liquid on the solid surface is associated with the solid–liquid interfacial energy ( γSL ). The surface free energy of solid ( γS ) and the surface tension of liquid–vapour ( γL ) can be expressed in Young’s equation:35

γL COS θ = γS - γSL

(5)


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Owens and Wendt36 integrated the nonpolar Lifshitz-van der Waals (LW) component γLW and the polar Lewis acid-base (AB) component γAB, the following equations can be obtained:

γS = γSLW + γSAB

(6)

γLLW + γLAB

(7)

γL

=

γL (1 + COS θ) = 2( γSLW γLLW )0.5 + 2( γSAB γLAB )0.5

(8)

In order to get the values of γ SLW and γ SAB , the contact angles of at least two liquids on a solid surface have to be determined, in which the surface tension components are known. Surface free energy parameters of deionized water and EG are listed in Table 2. The measured CAs and the calculated surface free energy of epoxy-silicone and superhydrophobic PPS/PTFE composite coating are presented in Table S2. The average CAs of water and glycol at the PPS/PTFE composite coating are higher than that of the epoxy-silicone coating. Table 2. Surface Free Energy Parameters of H2O and EG Surface free energy parameters (mJ/m2 ) Standard liquid

The type of polarity

γLAB

γLLW

γL

H 2O

51.0

21.8

72.8

polarity

(HOCH2)2

19.0

29.3

48.3

nonpolar

The relation between the calculated surface free energy ( γS γ SLW and γ SAB ) and the weight of , CaCO3 scaling on different coatings for 360h are shown in Figure 8. It can be seen that the surface free energy ( γS ) of hydrophobic epoxy-silicone resin coating reaches a higher value of 41.3 mJ/m2, which suggests that the hydrophobic epoxy-silicone resin coating has a stronger polarity. Compared with the hydrophobic epoxy-silicone resin, the surface free energy of the obtained superhydrophobic PPS/PTFE composite coating is very small, only 0.9 mJ/m2, which indicates that the later one has much lower surface energy. The γ SLW component has a great


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contribution to the surface free energy γS .The larger γ SLW component, the smaller γS . It is reported that the surface energy of calcite and vaterite is 38.62 and 37.30 mJ/m2, repectively.37,38 The surface energy value of the epoxy-silicone coating in this article is intensely close to that of calcite and vaterite. It is well known that the adhesive strength between the crystalline phase and the surface reaches a maximum when surface free energy of the crystalline phase is equivalent to that of the surface.39 The data obtained appear to be similar to those reported by Li et al.28,40 Therefore, it is observed that there is 2.2 mg/cm2 CaCO3 scaling deposited on hydrophobic epoxy-silicone resin coating after 360 h. However, there is only 0.85 mg/cm2 deposits on superhydrophobic PPS/PTFE coating. The latter is only 38.6% of the former, indicating that the superhydrophobic coating showing unique scale inhibition, which is attributable to the attractive decrease of surface free energy of superhydrophobic PPS/PTFE coating. The low surface energy of the coating makes ions or molecules can be easily escaped from the surface. Therefore, crystals with low surface free energy are less likely to adhere on the surface, which leads to a lower rate of scaling and a longer induction period. In addition, low surface tension would derive a harmful effect on the growth process, the morphology and the crystalline form of calcium carbonate crystals.23 Owing to the existence of low surface energy material of PTFE, on the superhydrophobic PPS/PTFE composite coating, the induction time is prolonged in scaling tests.26,41 The intensity of main calcite peak decreases obviously, illustrating that the crystallinity of calcium carbonate reduces as well as much less nucleation sites on the superhydrophobic surface. All these indicate that the scaling of CaCO3 decreased on the superhydrophobic PPS/PTFE coating.


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Figure 8. The relationship between the surface energy and CaCO3 scaling weight at the hydrophobic epoxysilicone coating and the superhydrophobic PPS/PTFE coating after testing with 360h at 60 °C.

Consequently, this study reveals that the low surface free energy and topography of superhydrophobic PPS/PTFE composite coating could affect the scaling formation of calcium carbonate and lead to the diversification of CaCO3 scaling deposition behaviours. Not only the morphology, dimension and weight of CaCO3 scaling at the superhydrophobic PPS/PTFE composite coating have changed. But also the crystal forms and molar fractions of CaCO3 polymorphs are different from that of hydrophobic epoxy-silicone coating. The schematic growth of calcium carbonate on different coatings is shown in Figure 9. At the surface of hydrophobic epoxy-silicone coating (Figure 9a), the CaCO3 solute could nucleate at any location with a short induction period due to their similarity of surface energy. The shapes of the crystals were mainly cubic, cuboid, trapezoidal, and so on. Combined with XRD analysis, we can believe that calcite is the mainly typical morphology. At the same time, there are a lot of thin willow clusters that proved to be vaterite (Figure 9a-i). With the increasing of immersion time, the number of crystals increases, and the surface is almost covered with the tightly packed structure. Then, the crystal


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particles would continue to grow in the void among the calcium carbonate crystals or on top of the initial formation calcium carbonate crystal layer (Figure 9a-ii).

Figure 9. Schematic diagram of CaCO3 crystallization process: on the hydrophobic epoxy-silicone coating (a), and on the superhydrophobic PPS/PTFE composite coating (b).

By contrast, on the super-hydrophobic PPS/PTFE composite coating (Figure 9b), the nanoCaCO3 crystal grain initially grows at peaks of papillae and then spreads to the valley between the papillaes. Due to the relatively low surface energy and special acicular topography, there are few nucleating points, existing with three different morphologies of CaCO3 crystals grown on super-hydrophobic PPS/PTFE composite coating (Figure 9b-iii). The flaky rhombic crystals, globular villous clusters and nano-scaled needle clusters are calcite, aragonite and vaterite, respectively. Beyond that, because of the space constraint, the morphology of CaCO3 crystal tends to form rosette structures, as shown in Figure 9b-iv.

4. CONCLUSIONS In this work, the superhydrophobic PPS/PTFE composite coating was prepared and used for CaCO3 scaling inhibition tests. The superhydrophobic PPS/PTFE composite coating exhibited promising scale-inhibiting performance, possessing few nucleation sites and space constraint


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effect. The scaling rate of CaCO3 on the superhydrophobic PPS/PTFE composite coating was only 38.6% of that on the hydrophobic epoxy-silicone resin coating. In addition, the morphology, dimension and crystal form of calcium carbonate on superhydrophobic PPS/PTFE composite coating were different from that of the hydrophobic epoxy-silicone surface, which could be assigned to the synergistic of topography and low surface energy of the superhydrophobic PPS/PTFE composite coating (0.9 mJ/m2). We believe that the superhydrophobic PPS/PTFE coating is a good candidate for antiscaling with a broad prospect of application in the field of oil, petrochemical and marine transports.

Supporting Information. CaCO3 Scaling at the surface of superhydrophobic PPS/PTFE coating with stainless steel substrate (Table S1), the average CA of water and glycol at epoxy-silicone resin and PPS/PTFE coating (Table S2), morphology of calcite scaling (Figure S1) and XRD analysis of CaCO3 scaling (Figure S2) at the PPS/PTFE composite coating with different content of PTFE.

Acknowledgements The research is financially supported by the National Young Top Talents Plan of China (2013042), National Science Foundation of China (21676052, 21606042), and the China Postdoctoral Science Foundation Funded Project (2016M600406).

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