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In this paper, the superhydrophobic poly(vinylidene fluoride)/fluorinated ethylene propylene/SiO /CNTs-EDTA (PFSC- EDTA) composite coating was successfully fabricated and applied for anti-scaling performance. The deposition of CaC O on the surface of the superhydrophobic PFSC-EDTA composite coating reached 0.0444 mg/cm for 192-h immersion into the supersaturated CaC O solution, which was only 11.4% that of the superhydrophobic PFSC composite coating. At the 2+ interface between the CaC O solution and the PFSC-EDTA coating, the Ca could be firstly chelated by EDTA that was benefit for improving the anti-scaling performance of the superhydrophobic PFSC-EDTA composite coating. In another hand, the addition of EDTA to the CNTs played an important role in fabricating the SiO -centric and CNTs-EDTA-surrounded multilevel micro–nanostructure in the superhydrophobic PFSC-EDTA composite coating, in favor of maintaining the air film under the water and the stability of the superhydrophobic surface. The research supplies a new way of improving anti- scaling performance of superhydrophobic coating by incorporating the organic chelating agent at the interface and changing the traditional way of scale prevention. Keywords Anti-scaling · Superhydrophobic coating · EDTA · Poly(vinylidene fluoride) · Carbon nanotubes 1 Introduction transportation efficiency (Vazirian et al. 2016; Wang et al. 2019a; Liu et al. 2019). The pipeline will be completely Scaling is very common in the petroleum industry, heat blocked in serious cases, which may affect the normal exchangers, high-power heat sinks and electronics and production of the oilfield (Wang et al. 2019b). Therefore, the corresponded industries especially in the process of people pay more attention to the anti-scaling technology oil exploitation. The formation of scale is due to the pre- and materials. The chemical method for scale prevention cipitation of inorganic salts such as calcium carbonate, is widely used in petroleum industry, that the chemi- calcium sulfate and magnesium hydroxide. The scale in cal anti-scaling agent was poured into the liquid to be 2+ the pipeline will narrow the area of cross section con- treated by binding with the divalent cations such as Ca , 2+ 2+ duits, increase the flow resistance of fluid and reduce the Mg, Ba and so on. At present, the commonly used commercial-scale inhibitor includes ethylenediaminetet- raacetic acid (EDTA), diethylenetriamine pentaacetic acid Edited by Xiu-Qiu Peng (DTPA), ethylenediaminetetramethylene phosphonic acid (EDTMP), aminotrimethylene phosphonic acid (ATMP) Handling Editor: Zhen-Hua Rui and 1-hydroxyethylidene-1, 1-diphosphonic acid (HEDP) (Khormali and Petrakov 2016). These scale inhibitors * Huai-Yuan Wang 2+ 2+ can not only form complexes with Ca /Mg and reduce email@example.com supersaturation level, but also weak their adhesion to the College of Chemistry and Chemical Engineering, Northeast surface due to their morphological dissymmetry (Mpelwa Petroleum University, Daqing 163318, China and Tang 2019). Though the addition of the anti-scaling College of Chemical Engineering, Daqing Normal agent could inhibit the formation of scale particles in solu- University, Daqing 163712, China tion, the high cost and low efficiency restricted its appli- Beijing Smart-Chip Microelectronics Technology Co., Ltd, cation, especially in pipeline flow system. As a typical Beijing 102200, China Vol.:(0123456789) 1 3 952 Petroleum Science (2021) 18:951–961 chelant, EDTA is a kind of chelating ligand with high- 2 Experimental affinity constant, which can form metal–EDTA complex (Mpelwa and Tang 2019; Kan et al. 2020). Because the 2.1 Materials and reagents reaction is stoichiometric, for example, one mole of EDTA 2+ can chelate two moles of Ca . Therefore, a large number Commercial epoxy resin (EP, E-44) was supplied by Nanjing of chelators are needed for effective prevention (Mpelwa Huntsman Advanced Materials Co. (China). PVDF powders and Tang 2019). Recently, an effective and promising strat- were bought from Shanghai 3F Co. Ltd. (China). FEP pow- egy to reduce the formation of inorganic scales has been ders were purchased from DuPont, USA. The nanometer the fabrication of anti-scaling surface by coating; among silica (SiO , Nanjing Hydratight Nanomaterials Co. Ltd, them, the pre-stored of scale inhibitors to the functional China) and CNTs (Beijing Boyu New Material Technol- coating is one innovative idea (Zhu et al. 2021). ogy Co., Ltd, China) were used as received. Other chemical Due to the low surface free energy and the rough micro- agents were all analytically pure. EDTA was bought from structure, polymer-based superhydrophobic coating with Tianjin No. 1 Chemical Reagent Factory. The absolute ethyl water contact angle (WCA) greater than 150° has shown alcohol, ethyl acetate, calcium nitrate terahydrate, sodium its unique advantages in self-cleaning, anti-corrosion, fluid bicarbonate, ethanediamine and ethylene glycol (EG) were drag reduction, anti-icing and antifouling; it has become all bought from Huadong Reagent Factory, Shenyang, China. a research hot spot in many fields (Latthe et al. 2019). In our previous research, it was found that the polymer-based superhydrophobic composite coating owes excellent anti- 2.2 Preparation of the superhydrophobic PFSC scaling properties (Qian et al. 2017; 2020). Inspired by the composite coating anti-scaling of the polymer superhydrophobic composite coating and the chemical scale inhibitor, it is worthwhile It is well known that the Q235 carbon steel and aluminum to investigate the combination of these two ways for scale materials are widely used as tubing materials or substrate prevention. Nevertheless, there are few reports on the appli- materials in the petroleum industry. The schematic dia- cation of organic chelating agents to the superhydrophobic gram of the preparation process for superhydrophobic polyvinylidene fluoride (PVDF) coating and the study of PFSC composite coating is shown in Fig. 1. The alu- CaCO scale inhibition. The scale prevention performance minum plate (80 mm × 80 mm × 1 mm) was firstly polished of superhydrophobic PVDF composite coating was studied with 600-mesh silicon carbide sand grain paper for about preliminarily (Qian et al. 2020). It is necessary to further 20 min; then, the sample was polished for 20 min by 800- study the deposition of CaCO on the polymer-based super- mesh abrasive paper and finally polished in the same way hydrophobic composite coating, coupling with the effect of with a 1000-mesh sand paper for another 20 min to obtain organic chelating agent. the average roughness of 0.15–0.30 μm. After that, the In this article, the superhydrophobic PVDF/FEP/SiO / polished aluminum plate was cleaned with deionized water CNTs-EDTA (PFSC-EDTA) composite coating was fab- and then put into anhydrous ethanol solution for ultrasonic ricated by incorporating PVDF, fluorinated ethylene pro- cleaning for 5 min to remove the dirt and grease attached pylene (FEP), nano-SiO and EDTA-modified CNTs at the aluminum plate surface. (CNTs-EDTA). The deposition of CaC O scaling at the 2.0 g epoxy resin and 0.1 g PVDF powders were ultra- PFSC-EDTA coating surface was carried out at the tem- sonically dispersed in 10 mL ethyl acetate to form a uni- perature of 60 °C under the static state. The role of EDTA form solution. Then, 0.2 g ethylenediamine was added into on the scale inhibition of the superhydrophobic PFSC-EDTA the above solution and ultrasonically dispersed for 15 min. composite coating was investigated and discussed in detail. The resulting solution was sprayed on the as-treated alu- In order to understand the effect of EDTA for CaCO scaling minum plates as the basement layer, followed by drying at the surface of superhydrophobic PFSC-EDTA composite at 180 °C for 20 min. The spraying pressure is 0.6 MPa. coating, the superhydrophobic PFSC composite coating was The distance between the aluminum plate and the spray used as a reference. Owing to the coupled effect between gun is 18 cm. After that, 0.7 g PVDF powders, 0.3 g FEP the organic chelating agents (EDTA) and polymer-based powders, 0.05 g nano-SiO particles and 0.05 g carbon superhydrophobic composite coating, the PFSC-EDTA nanofillers (CNTs) were ultrasonically dispersed in 10 mL composite coating exhibits unique anti-scaling performance. of absolute ethyl alcohol for 30 min. Next, the above solu- This research on the organic chelating agent-enhanced anti- tion as a surface functional coating material was erupted scaling performance of the superhydrophobic coating will on aluminum plate under a pressure of 0.6 MPa. In this play an important role in industrial application. We believe way, the superhydrophobic PFSC composite coatings were that this work will pave a new way for the anti-scaling of obtained after drying at 180 °C for 90 min. functional coating in petroleum industry. 1 3 Petroleum Science (2021) 18:951–961 953 Polishing Ultrasonic treatment cleaning Aluminum plate 0.1 g PVDF 0.2 g ethylenediamine 2 g epoxy resin Ultrasonic Ultrasonic treatment treatment 15 min 180 °C Drying 20 min Ethyl acetate Absolute ethyl alcohol 0.05 g CNTs 0.7 g PVDF 0.05 g SiO 0.3 g FEP Epoxy resin coating Ultrasonic treatment Drying 80 °C 30 min 180 °C 90 min Superhydrophobic Absolute ethyl alcohol PFSC coating Fig. 1 Schematic diagram of the preparation process for superhydrophobic PFSC composite coating 2.3 Preparation of the superhydrophobic 2.4 CaCO scale formation on PFSC and PFSC‑EDTA PFSC‑EDTA composite coating composite coating The basement layer of the PFSC-EDTA composite coating The scaling experiments of different coatings were carried was the same as that of the PFSC composite coating. More out in a static supersaturated calcium carbonate solution, concretely, 0.1 g CNTs were added to an organic chelating which was prepared by the reaction of calcium nitrate and agent EDTA solution at a concentration of 0.1009 mol/L sodium bicarbonate by Eq. (1) for ultrasonic impregnation. The time for EDTA and Ca NO ⋅ 4H O + 2NaHCO → CaCO + 2NaNO 3 2 3 3 3 CNTs impregnation was 12 h. Then, the reaction suspen- 2 sion was filtered and separated. CNTs impregnated with + 5H O + CO ↑ 2 2 EDTA (CNTs-EDTA) were dried in a constant tempera- (1) ture blast drying oven at 100–105 °C for 3 h to constant The specific experimental steps were described as fol- weight. Besides, 0.7 g PVDF powders, 0.3 g FEP powders, lows: Firstly, sodium bicarbonate and calcium nitrate ter- 0.05 g nano-SiO particles and 0.05 g CNTs-EDTA were 2 ahydrate were filtered through 0.45-µm membranes, respec- ultrasonically dispersed in 10 mL of absolute ethyl alcohol tively, and then heated to 60 °C in a constant temperature for 30 min. Next, as the surface functional coating mate- water bath for standby. Secondly, the coated samples (PFSC rial, the above solution was sprayed on the surface of the or PFSC-EDTA) were then immersed vertically into differ - aluminum plate at a pressure of 0.6 MPa. In the end, the ent screw-capped glass bottles containing a supersaturated superhydrophobic PFSC-EDTA composite coatings were solution of calcium carbonate. After a certain time interval, fabricated after drying at 180 °C for 90 min. 1 3 954 Petroleum Science (2021) 18:951–961 the coated samples were gently removed from the solution, PW3040/60, PANalytical) patterns of the superhydrophobic washed with deionized water, and then dried and weighed. PFSC and PFSC-EDTA composite coating were obtained in At least three samples were tested for each coating, and the air conditions at 2θ from 10° to 70°. The crystal structure average value of the test results was adopted as the scaling of CaCO on different coating surfaces was also studied by amount of the coating sample. XRD. The surface roughness of the coating was measured by a portable surface roughness tester (SJ-210; Mitutoyo). 2.5 Characterization In order to ensure the measurement accuracy, five different positions of the same coating were selected, and the average The static contact angles (CAs) of the prepared coat- value of the measurement results was calculated. ings were measured by contact angle meter (JGW-360A, Chengde City Chenghui Testing Machine Co., Ltd.) with a droplet (5 μL) of deionized water at room temperature. 3 Results and discussion The average CA was obtained from five measurements on different positions of the same specimen. The surface mor - 3.1 Surface morphology and chemical composition phology of the coatings before and after scaling were char- of coatings acterized by scanning electron microscopy (SEM; Quanta 200). The functional groups at the prepared coating’s surface The morphology of the as-prepared superhydrophobic PFSC were detected by Fourier transform infrared (FTIR) spectro- and PFSC-EDTA composite coatings was analyzed by the scope (Spectrum 2000; PerkinElmer) over the wave number scanning electron microscope (SEM). As shown in Fig. 2a, −1 domain from 400 to 4000 cm . The X-ray diffraction (XRD; the CNTs dispersed uniformly in the grains and grain Fig. 2 SEM image of PFSC (a, a1) and PFSC-EDTA (b, b1) superhydrophobic composite coating 1 3 Petroleum Science (2021) 18:951–961 955 boundaries of the PFSC composite coating and formed a of PVDF and is associated with (131) reflection (Fig. 3b) special network structure. After incorporation with PVDF (You et al. 2012). and FEP, the static water contact angle (WCA) of the PFSC Compared with the XRD patterns of the pure CNTs and composite coating reached 152.30°. Moreover, the CNTs the pure PVDF, there were new diffraction peaks at 17.8°, agglomerated in some regions (Fig. 2a1) owing to the strong 44.7° and 65.1° (Fig. 3c and d) of the superhydrophobic van der Waals force, entangled with polymer and nano-SiO PFSC and PFSC-EDTA composite coating, respectively. The grains and formed a special microscopic rough structure weak peak at 17.8° can be assigned to (100), the α phase (Baghbanzadeh et al. 2016). In addition, there was much of reflection of the PVDF. Two strong peaks at 2θ values of porous, forming a staggered nanonetwork among the CNTs, 44.7° and 65.1° (100) belong to the aluminum substrate. As polymer particles and nano-SiO (Yan et al. 2018), which a kind of semicrystalline polymer, PVDF exhibits at least was benefit for wrapping air in the nanopores structure. four crystalline phases including α, β, γ, and δ, which differ For a superhydrophobic coating, the increase in gas–liquid both in physical and in electrical properties. Among them, interface proportion (Jagdheesh et al. 2019) will enhance the the α phase PVDF is a monoclinic lattice conformation, protection of interface by the air film and further prevent the which is also the most stable and common crystal structure. invasion of corrosive species (Zhang et al. 2018). The β phase PVDF is the most important crystal structure Compared with the CNTs in superhydrophobic PFSC with an orthorhombic structure. The γ phase PVDF has an composite coating, CNTs-EDTA dispersed more evenly in orthorhombic lattice, which is usually obtained by annealing PFSC-EDTA composite coating (Fig. 2b). Local enlarged α phase crystal at high temperature or isothermal crystal- image (Fig. 2b1) showed a rough structure centered on SiO lization. The XRD analysis in Fig. 3 showed the PVDF in and surrounded by CNTs-EDTA formed in the coating. After superhydrophobic PFSC and PFSC-EDTA composite coat- incorporation with PVDF and FEP, the WCA of PFSC- ing including α, β and γ crystalline phases. The EDTA is EDTA composite coating was 150.63° and still showed illegible in XRD for tiny amounts. superhydrophobicity. The FTIR spectra of pure CNTs, CNTs-EDTA, super- The XRD patterns of the pure CNTs, the pure PVDF, the hydrophobic PFSC and PFSC-EDTA composite coating superhydrophobic PFSC composite coating and the PFSC- were performed (Fig. 4) for analyzing the change of sur- EDTA composite coating are shown in Fig. 3. The diffrac- face functional groups. For the pure CNTs (Fig. 4a), only −1 tion peaks at 2θ value of 25.7° (002) and 42.8° (100) are a flat band can be seen at 3426 cm that corresponds to the characteristic of CNTs (Fig. 3a) (Gull et al. 2016). The the stretching vibration of –OH (Lu et al. 2016). Com- diffraction peaks at 18.3°, 26.5° and 33.0° are attributed to pared with pure CNTs, there were three more diffraction the (020), (021) and (130) α-phase of PVDF, respectively bands in the spectrum of CNTs-EDTA (Fig. 4b). Among −1 (Fig. 3b). The sharp and strong peak at 19.9° (110) cor- them, the band at 1624 cm is attributed to the vibration responds to β-phase of PVDF (Rajabzadeh et al. 2009). In of carboxyl–COOH. The small and sharp absorption band −1 addition, the diffraction peak at 38.2° is attributed to γ -phase at 1394 cm corresponds to the in-plane bending vibra- tion of –CH from EDTA. The weak absorption band at Fig. 4 FTIR spectra of CNTs (a), CNTs-EDTA (b), PFSC composite Fig. 3 XRD patterns of the CNTs (a), pure PVDF (b), as-prepared coating (c) and PFSC-EDTA composite coating (d) PFSC coating (c) and the PFSC-EDTA coating (d) 1 3 956 Petroleum Science (2021) 18:951–961 −1 1289 cm is assigned to the stretching vibration of –CN (Rončević et al. 2019). Combined with the SEM of com- posite coating (Fig. 2), it can be determined that the EDTA has been successfully loaded on the CNTs. When it comes to the superhydrophobic PFSC (Fig. 4c) and PFSC-EDTA composite coating (Fig. 4d), the absorption bands at 3026 −1 and 2984 cm are attributed to the asymmetric stretching vibration of C–H (Ibara et al. 2013), which is independent of the crystalline phase of PVDF. The significant absorp- −1 tion band at 1403 cm is related to the bending vibration −1 of –CH . The strong band at 1186 cm is attributed to the bending vibration of –CF and –CF from the PVDF and the 2 3 −1 FEP, respectively. The bands at 1070 cm of CF bending −1 −1 vibration, 877 cm of C–C vibrations, 763 cm of –CH −1 rocking and 614 cm of C F bending imply the existence −1 Fig. 5 Deposition rate of CaC O scaling at the surface of the superhy- of α phase PVDF (Rafiee et al. 2010). The band at 841 cm drophobic PFSC composite coating (a) and superhydrophobic PFSC- of –CH bending is correlated with the β phase of PVDF. EDTA composite coating (b) −1 The specific characteristic band at 483 cm represents the CH bending vibration of γ phase PVDF. The FTIR results superhydrophobic PFSC composite coating. In the whole show that there are three crystal phases PVDF in the supe- rhydrophobic PFSC and PFSC-EDTA composite coating, stage of crystal growth from 8 to 192 h, the average scal- ing rate of CaCO on the PFSC composite coating was which is consistent with that of XRD (Fig. 3). The band at −1 2 about 796 cm in PFSC-EDTA composite coating is caused 0.0019 mg/(cm ·h). However, the superhydrophobic PFSC-EDTA composite by the symmetric stretching of Si–O from nano-SiO . The −1 bands at 1624 and 1289 cm can be assigned to the sym- coating experienced a long CaC O scale induction period of about 20 h (Fig. 5b), which was 2.5 times that of superhy- metric stretching vibration of –COOH and –CN that come from EDTA, which is the direct evidence for success inter- drophobic PFSC composite coating. In the first stage from the 20 to the 96th h, the weight of CaCO scaling raised calating of EDTA into the superhydrophobic PFSC-EDTA composite coating (Shi et al. 2018). from 0.0158 to 0.0387 mg/cm with the average rate of 0.0003 mg/(cm ·h), which was equivalent to 11.7% of supe- rhydrophobic PFSC composite coating. In the second stage from the 96th h to the 192th h, the average rate of CaCO 3.2 Anti‑scaling of the superhydrophobic PFSC and PFSC‑EDTA composite coating scaling was further reduced to 0.0001 mg/(cm ·h), which was only 25% of the superhydrophobic PFSC composite In order to illustrate the anti-scaling performance of the coating. In the whole stage of crystal growth from the 20 h to the 192th h, the average scaling rate was 0.0002 mg/(cm ·h), superhydrophobic coating, the deposition testing of the CaCO scaling at the superhydrophobic PFSC and PFSC- which was much lower than that of the superhydrophobic PFSC composite coating. The mass of calcium carbonate EDTA composite coating surface was carried out. As shown in Fig. 5, the amount of calcium carbonate scaling on the on the superhydrophobic PFSC-EDTA composite coating surface increased little, which was only 11.4% that of the surface of superhydrophobic PFSC and PFSC-EDTA com- posite coating increased with the immersion time. In com- superhydrophobic PFSC composite coating. The main rea- son is that the EDTA at the superhydrophobic PFSC-EDTA parison with the superhydrophobic PFSC-EDTA composite 2+ coating, the superhydrophobic PFSC composite coating had composite coating interface can chelate the Ca in the solu- tion, coupling the coatings’ superhydrophobicity, and further a higher scaling tendency (Fig. 5a). Meanwhile, it experi- enced a short induction period of scaling for about 8.0 h. prevented the deposition of CaCO at the surface. In addition, the crystal morphology of CaCO at different After that, the scaling rate of CaC O increased sharply. The 3 3 growth process of CaCO scaling consists of two stages: superhydrophobic composite coating surface was character- ized by SEM. As shown in Fig. 6a, the cuboid, needlelike, In the first early stage that is from 8 to 96 h, the amount of CaCO scaling increased from 0.0393 to 0.3465 mg/cm , trapezoid prism and irregular block crystals were seen on the surface of superhydrophobic PFSC composite coat- with an average scaling rate of 0.0035 mg/(cm ·h). In the second stage from 96 to 192 h, the scaling rate decreased ing for immersion time of 24 h, while for the PFSC-EDTA composite coating in Fig. 6b, the CaC O scale at the supe- to 0.0004 mg/(cm ·h), which was probably caused by the reduction in nucleation sites of CaCO scaling on the rhydrophobic surface mainly includes the cube, cuboid and 1 3 Petroleum Science (2021) 18:951–961 957 Fig. 6 SEM images of CaC O crystallization scaling at the surface of PFSC (a) and PFSC-EDTA (b) composite coating for immersion time of 24 h crystals at the surface of the superhydrophobic PFSC and superhydrophobic PFSC-EDTA composite coatings are the aragonite CaCO . XRD results of calcium carbonate scale at the surface of superhydrophobic PFSC and PFSC-EDTA composite coat- ing after 24-h CaCO deposition are shown in Fig. 7. The characteristic peaks at 44.6 and 65.1° belonged to the alu- minum substrate. The diffraction peaks of calcite appeared at 23.0 (012), 29.5 (104), 36.0 (110), 39.4 (113), 43.2 (202), 47.5 (024), 48.5 (116), 57.4 (122) and 58.2 (1010). In addi- tion, the aragonite also existed at the surface of superhy- drophobic PFSC and PFSC-EDTA composite coating. The characteristic peaks of aragonite appeared at 26.2 and 38.5° indexing to (111) and (130), respectively. Results showed that the calcite and aragonite CaC O crystals were all formed Fig. 7 XRD patterns of CaC O scaling on superhydrophobic PFSC 3 (a) and superhydrophobic PFSC-EDTA (b) composite coating for at the surface of the superhydrophobic PFSC and PFSC- immersion time of 24 h EDTA composite coating and with different peak intensity. For the superhydrophobic PFSC composite coating (Fig. 7a), the peak intensity of calcite at 29.5° and aragonite at 26.2° rodlike. According to the relevant literature, cube/cuboid, was all weak. When it comes to the PFSC-EDTA composite spherical and needle/rodlike structures are typical of calcite, coating (Fig. 7b), the strong peak at 38.5° of aragonite can vaterite and aragonite, respectively (Gopi and Subramanian be seen clearly and the weakness peak at 29.5° may be a sign 2012). Therefore, the cubic, cuboid, trapezoidal prism and of relative low content of calcite CaC O crystals. The results irregular block crystals are typical crystal forms of calcite 3 of XRD (Fig. 7) and SEM (Fig. 6) analysis of CaCO scale (Niu et al. 2014). In the meantime, the calcite is the most 3 showed that the addition of organic chelating agent EDTA stable phase in calcium carbonate crystals. Compared with in the superhydrophobic PFSC-EDTA composite coating the crystal morphology of calcite, the needlelike/rodlike 1 3 958 Petroleum Science (2021) 18:951–961 water and ethylene glycol, the surface free energy of differ - Table 1 Molar fraction of CaCO polymorphs deposited on the sur- face of different coatings ent superhydrophobic coating can be calculated by a series of equations (Young 1805): Superhydrophobic composite Molar fraction, % coatings cos = − Calcite Aragonite (4) L L SL PFSC 69.3 30.7 LW AB = − (5) PFSC-EDTA 4.4 95.6 S S S LW AB = − (6) L L induced the formation of aragonite crystal and significantly changed the morphology of CaCO crystal. 0.5 0.5 LW LW AB AB The molar fraction of aragonite and calcite can be cal- (1 + cos)= 2 + 2 (7) S L S L culated from X-ray diffraction data by Eqs. (2 ) and (3) (Liu where θ is the static CA of pure liquid on solid surface, γ et al. 2011), and γ are the surface free energy of solid and the surface tension of liquid–vapor, respectively. γ is the solid–liquid SL X = (2) I + 0.41I A C interface energy. The homologous surface tension compo- nents of deionized water and ethylene glycol are shown in Table S1. The results obtained from measurement and cal- X = (3) culation of CAs and surface free energy of different super - 2.44I + I A C hydrophobic coatings are shown in Table S2. In addition, the where X is the molar fraction of CaCO polymorphs, the surface roughness of superhydrophobic composite coatings subscripts “A” and “C” represent aragonite and calcite, was measured before CaCO scaling test (Table S3). R is the 3 a respectively. I and I indicate the intensity of the main peak arithmetic average deviation (Rahman et al. 2011): A C of aragonite at 38.5° and calcite at 29.5°, respectively. As shown in Table 1, the molar fraction of calcite and aragonite R = Z(x)dx (8) in CaCO scaling that deposited at the surface of the supe- rhydrophobic PFSC composite coating is 69.3 and 30.7%, where Z(x) and l represent the sampling height and the that is, calcite is the principal constituent of the CaC O length of the assessed profile at an arbitrary point x , respec- scaling. For the superhydrophobic PFSC-EDTA composite tively. During the CaCO scaling test, the scanning length coating, the aragonite becomes the main part with content −1 and the scanning speed were 5.0 mm and 0.25 mm·s , of 95.6% and calcite of 4.4%. As we know, the calcite is the respectively. most stable one in the three crystals: CaCO of calcite, arag- As shown in Fig. 8, the surface free energy of the supe- onite and vaterite (Gopi and Subramanian 2012). Results rhydrophobic PFSC and PFSC-EDTA composite coating of the PFSC-EDTA composite coating show that the exist- is 0.40 (Fig. 8a) and 0.95 mJ/m (Fig. 8b), respectively, ence of organic chelating agent EDTA induced the forma- that is consistent with the rule of the lower the surface free tion of aragonite crystals (Gopi et al. 2015); the more the aragonite or vaterite in the scale, the better the anti-scaling performance (Li et al. 2015; Ge et al. 2016), which is ben- efit for preventing the deposition of CaCO at the surface. Therefore, the addition of organic chelating agent EDTA has a significant effect on the anti-scaling performance of the superhydrophobic PFSC-EDTA composite coating. 3.3 Eec ff t of the EDTA on anti‑scaling of superhydrophobic composite coating The adhesion of CaC O scaling on the superhydrophobic composite coating can be analyzed by the surface energy, which is the direct measurement of the interfacial attrac- tive forces. For the superhydrophobic coating, the surface Fig. 8 Surface free energy, surface roughness and CaC O weight for roughness is also another important parameter for anti-scal- 192-h immersion of superhydrophobic PFSC (a) and PFSC-EDTA ing performance. According to the static CAs of deionized (b) composite coating 1 3 Petroleum Science (2021) 18:951–961 959 energy, the better the anti-scaling performance of the coat- calcium carbonate scale at the surface of superhydrophobic ing (Azim et al. 2014; Förster and Bohnet 1999; Muller- PFSC (Fig. 6a) and superhydrophobic PFSC-EDTA (Fig. 6b) Steinhagen and Zhao 1997). On the one hand, the addition of composite coating identie fi d the obvious inu fl ence of EDTA EDTA in the fabrication of superhydrophobic PFSC-EDTA to the crystal morphology of CaC O , which is also consistent composite coating would increase the surface free energy with others’ result (Gopi and Subramanian 2012). and decrease the scaling rate of calcium carbonate only to 11.4% (0.0444 mg/cm , Fig. 8b) that of the superhydropho-3.4 Anti‑scaling mechanistic bic PFSC composite coating (0.3893 mg/cm , Fig. 8a). On of the superhydrophobic PFSC‑EDTA composite the other hand, the surface roughness of the superhydropho- coating bic PFSC-EDTA composite coating is 18.494 μm, which is a little higher than 17.568 μm that of the superhydro- As discussed, the deposition of CaCO at the surface of the phobic PFSC composite coating owing to the rough surface superhydrophobic coating was influenced by the air film that of SiO -centric and CNTs-EDTA-surrounded multilevel retains between the solution and the coating. For the super- micro–nanostructure as shown in SEM (Fig. 2). That is to hydrophobic PFSC composite coating, although the air film say both the surface free energy and roughness are respon- can be retained at the interface of supersaturated C aCO sible for the anti-scaling of the superhydrophobic coating, solution and coating (Fig. 9a), still a number of CaC O which is also consistent with the previous study (Qian et al. scales can be formed owing to the poor superhydrophobic 2020). Furthermore, in terms of the chemical composition stability under the water (Fig. 9b). As for the superhydro- of the coating, the biggest difference between the superhy - phobic PFSC-EDTA composite coating, the most important drophobic PFSC and PFSC-EDTA composite coating is factor influencing the anti-scaling behavior is the organic whether there is the organic chelating agent EDTA. As a chelating agent EDTA, followed by the surface roughness, 2+ good complexing agent of Ca , EDTA could help reducing and finally the surface free energy. For the superhydrophobic the number of free calcium ions in the solution, which also PFSC and PFSC-EDTA composite coating, a porous and decreases the nucleation driving force of calcium carbonate rough structure was formed among CNTs, nano-SiO and formation (Meng and Park 2014). Consequently, the EDTA PVDF (Fig. 2a) (Latthe et al. 2019), which is also the prereq- at the interface of coating and solution can still play a certain uisite condition for the formation of superhydrophobic coat- 2+ role on anti-scaling by chelating the free Ca and prompting ing (Tretheway and Meinhart 2004). Together with the high good CaCO deposition prevention performance of super- strength of CNTs, the porous structure can be maintained hydrophobic PFSC-EDTA composite coating. The SEM of well and contributed for the stability of the air film (Fig. 9a). Fig. 9 Schematic diagram of CaC O scaling formation at the surface of superhydrophobic PFSC (a, b) and PFSC-EDTA (c, d) composite coat- 2+ ing, and the interfacial chelation between Ca and EDTA of superhydrophobic PFSC-EDTA composite coating (e) 1 3 960 Petroleum Science (2021) 18:951–961 In addition, the low surface energy and tackling effect of superhydrophobicity, which is benefit for maintaining the CNTs can effectively prevent the adhesion of CaCO on the air film under the water and the stability of the superhy - coating surface (Fig. 9b) (Qian et al. 2020). drophobic surfaces. The most important of all is that the 2+ After the CNTs were impregnated with EDTA, the anti- interfacial chelation of EDTA and Ca at the supersatu- scaling of the superhydrophobic PFSC-EDTA composite rated CaCO solution and superhydrophobic PFSC-EDTA coating has been greatly improved. As shown in Fig. 9c, the coating interface further enhanced the coating’s anti-scaling. superhydrophobicity of the PFSC-EDTA composite coating Results showed that the amount of CaCO scaling on the assures the stable existence of air film at the interface of the superhydrophobic PFSC-EDTA composite coating after CaCO solution and coating. Owing to the enhancements 192-h immersion into the supersaturated CaC O solution 3 3 of CNTs at the interface, the EDTA dispersed in the gaps was 0.0444 mg/cm , which is only 11.4% that of the supe- among the fillers of CNTs, nano-SiO and the polymer of rhydrophobic PFSC coating. This research provides a new PVDF. The SiO -centric and CNTs-EDTA-surrounded rough method of fabrication for the anti-scaling surface by intro- structure would help to wrap air and form a thick air film and ducing the scale inhibition at the interface of superhydro- increase the proportion of gas–liquid interface (Jagdheesh phobic coating, which is the development and optimization et al. 2019), which could reduce the contact area between of traditional way of scale prevention in petroleum industry. solid and liquid and hinder the nucleation and growth of cal- Supplementary Information The online version contains supplemen- cium carbonate on the coating surface (Fig. 9c). In addition, tary material available at (https ://doi.org/10.1007/s1218 2-021-00558 the rough structure of the PFSC-EDTA composite coating -x). surface is good for wrapping the air and maintaining the air film stability (Fig. 9c) (Qian et al. 2020). The most important Acknowledgements The research was financially supported by the 2+ point is the Ca ions that from the supersaturated CaC O National Science Foundation for Distinguished Young Scholars of China (Grant No. 51925403), the Major Research Plan of National solution can be chelated by the EDTA at the solution/coat- Natural Science Foundation of China (Grant No. 91934302) and the ing interface, which would reduce the contact chances of National Science Foundation of China (21676052, 21606042). 2+ 2− Ca ions with C O to form calcium carbonate (Fig. 9d). 2+ The interfacial chelation between EDTA and C a ions at Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- the interface is shown in Fig. 9e; it can be seen that the tion, distribution and reproduction in any medium or format, as long two amino nitrogen and four carboxyl oxygen in the EDTA as you give appropriate credit to the original author(s) and the source, molecular could coordinate with calcium ion and form the provide a link to the Creative Commons licence, and indicate if changes 2+ intermediated EDTA-Ca chelate that also had inhibition were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated of calcite scaling (Zhu et al. 2021). All in all, the synergis- otherwise in a credit line to the material. If material is not included in tic effect of coatings’ superhydrophobicity that depending the article’s Creative Commons licence and your intended use is not on the surface roughness and surface free energy and the permitted by statutory regulation or exceeds the permitted use, you will 2+ interfacial chelation of EDTA and Ca at the supersaturated need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/. CaCO solution/coating interface tremendously enhance the anti-scaling of the superhydrophobic PFSC-EDTA compos- ite coating. 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Petroleum Science – Springer Journals
Published: Feb 18, 2021
Keywords: Anti-scaling; Superhydrophobic coating; EDTA; Poly(vinylidene fluoride); Carbon nanotubes
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