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Superhydrophobic rubber-modified polybenzoxazine/SiO2 nanocomposite coating with anti-corrosion, anti-ice, and superoleophilicity properties Eugene B. Caldona, Al Christopher C. de Leon, Patrick G. Thomas, Douglas F. Naylor III, Bryan B. Pajarito, and Rigoberto C Advincula Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04382 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 2, 2017

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Superhydrophobic Rubber-Modified Polybenzoxazine/SiO2 Nanocomposite Coating with Anti-corrosion, Anti-ice, and Superoleophilicity Properties

Eugene B. Caldonaa,b,c, Al Christopher C. De Leona, Patrick G. Thomasa, Douglas F. Naylor IIIa, Bryan B. Pajaritob, and Rigoberto C. Advinculaa*

a

Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA

b

Department of Chemical Engineering, University of the Philippines Diliman, Quezon City 1101, Philippines

c

Department of Chemical Engineering, Saint Louis University, Baguio City 2600, Philippines

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to) *Corresponding author. E-mail: [email protected]; Phone: +1 216-368-4566


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ABSTRACT The integration of nanometer-sized fillers into polymer matrices to create nanocomposite materials has attracted a great deal of interest, not only because these materials can be tailored to specific practical applications, but also can exhibit synergistic combinations of properties that display multifunctionality. Herein, we successfully incorporated silica (SiO2) nanoparticles into the rubbermodified polybenzoxazine (PBZ) by mixing, and applied as a nanocomposite coating that exhibits both superhydrophobicity and superoleophilicity through a facile dipping and spraying technique. We used PBZ, not only because of its near-zero shrinkage upon polymerization, chemical resistance, good dielectric, thermal, and mechanical properties, but also most importantly its low surface free energy and low water absorptivity. With superhydrophobicity coexisting with superoleophilicity in one material, potential anti-corrosion, anti-ice, and organics/water separation applications of the coating were investigated. Results revealed that the rubber-modified PBZ coating with the optimum SiO2 loading was able to display superior anti-wettability and anti-corrosion performance even during prolonged exposure to corrosive environment. The coating also showed promising anti-icing ability by preventing ice/snow from adhering to the surface and delaying icing of water upon striking the surface. Furthermore, when our coating was applied onto a metal mesh, the resulting coated membrane was able to effectively separate dichloromethane (DCM), a nonpolar oil, from water. Combined with good thermal and adhesion properties, the existence of all the aforementioned properties makes the developed nanocomposite a very promising coating material for multifunctional application purposes.

KEYWORDS: polybenzoxazine, silica, nanocomposite, superhydrophobic, superoleophilic, anticorrosion, anti-ice, oil/water separation, multifunctional



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1. INTRODUCTION Considered as one of the most researched areas in nanotechnology, polymer nanocomposites have played an important role in our everyday life for having demonstrated their high-performance and multifunctional materials1. They often exhibit better combinations of mechanical, thermal, optical, and barrier properties than those of conventional composites and the neat polymer2. Their application as surface coatings is becoming increasingly desirable due to their capability of possessing properties that lead to multiple useful applications3,4,5,6. Wettability is one important property that has triggered much attention because of its great advantages in practical applications7,8,9. It has been studied that the wettability of solid surfaces depends upon its surface chemical composition (which determines the surface free energy) and morphology7,8,10,11. Surface free energy can be used as a measure of the degree of water adsorption on the surface, therefore surfaces with low energy tend to be hydrophobic12. Conversely, morphology determines the correct surface geometrical structure that leads to a desired wetting behavior which can be best described by the Wenzel and the Cassie-Baxter models. The first model describes that the water droplet follows the roughness and sticks to the surface13, whereas, the second model explains that the droplet suspends on the spikes of the rough surface and air trapped in the trough minimizes the contact area14. In either case, much higher water contact angles (CA) are observed for rough surfaces than those for flat surfaces. As a result, these models form the guidelines for the study of superhydrophobic surfaces15. Superhydrophobic surfaces are generally defined as super water-repellent surfaces characterized by CA ≥ 150o and sliding angles (SA) < 10o. They are of particular interest because properties such as anti-contamination, anti-sticking

16,17

, and self-cleaning

15,18

have been

demonstrated. Sequentially, these properties are desirable for many industrial and biological applications such as anti-corrosion for metals

19

, anti-fouling for marine applications


20,21

, and self-

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cleaning and anti-icing (anti-sticking of snow) for outdoor surfaces 5,22,23,24,25. The surface of a lotus leaf is naturally well-known to exhibit superhydrophobicity with a CA as large as 161o and SA as small as 2o 7. Such superior wettability is due to the combination of micro- and nanoscale hierarchical structures and low surface energy materials present on the leaf surface26. Since then, numerous methods have been reported to mimic the lotus leaf surface for the fabrication of artificial superhydrophobic surfaces27,28,29,30,31. Essentially, these methods are divided into two approaches: creating roughness on hydrophobic surfaces, or modifying rough surfaces with low surface energy materials26,32,33. Amongst these methods, the utilization of costly and low surface energy fluorinated polymeric materials, has been mostly reported10,34,35. However, it was found that such materials are harmful because of their potential risks for health and environmental hazards during the lifetime of the functionalized surface36,37,38,39. Therefore, such concerns have resulted to difficulty in scale-up for industrial applications36,40, which then have led to the development of several approaches particularly involving the use of fluorine-free polymeric materials as alternatives for superhydrophobicity purposes24,41,42,43,44. In the past years, the role of polybenzoxazine (PBZ) in the field of super water-repellency has been observed and studied45,46,47. PBZ is a new class of fluorine- and silicon-free, thermosetting polymeric material that exhibits low surface energy46,48,49,50 that is even lower than that of pure polytetrafluoroethylene (PTFE)

48

. Also, PBZ materials possess excellent properties such as near-

zero shrinkage upon polymerization 49,50, low water absorption49,50, resistance to chemicals51 and UV light52, dielectric49,50,53,54, thermal, and mechanical properties49,50. However, PBZ suffers from brittleness55,56 which unfortunately limits their application. As a result, approaches to toughen cured PBZ have been designed55. One of the most promising is the incorporation with rubber57. It has been



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found that rubber addition has improved fracture toughness

58,59

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without undue sacrifices to the

desirable properties 60. Herein, we report the use of a rubber-modified PBZ in fabricating a surface coating. The reaction path for the synthesis of the BZ monomer and its thermal curing leading to PBZ are shown in Figure 1. We selected hydroxyl-terminated, and epoxidized polybutadiene (HTBD) as the toughening modifier because it can react with the hydroxyl groups produced upon benzoxazine ring opening and thus can crosslink the PBZ matrix

50

. Additionally, we incorporated silica (SiO2)

nanoparticles to induce surface roughness and produce a superhydrophobic coating. It is worth noting that chemical modification with low surface energy materials (e.g. expensive fluorochemicals) is not required to realize superhydrophobicity. Moreover, SiO2 has excellent properties such as high hardness, low refractive index, and reasonable price that make it as one of the most promising nanoparticles employed in protective coatings61,62.

Figure 1. Synthesis of the linear diamine-based benzoxazine monomer and the ring opened polybenzoxazine

Application wise, we demonstrate that the fabricated PBZ/Rubber/SiO2 (PRS) nanocomposite is able to provide a superior corrosion-resistant coating for metals. Corrosion is the destructive attack of a metal by reaction with its environment63, and the costs it brings amount to several percent of the gross domestic product of an industrialized country

64

. Therefore, the protection of metals is of


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utmost importance. In addition, we aim to show that the coating possesses anti-icing capability. Similar to corrosion, adhesion of ice and snow to outdoor surfaces is well known to cause serious problems in cold-climate regions and may hinder the operational performance of many industrial equipment, machines and vehicles65.

Although there is no material that completely prevents

ice/snow accretion, some coatings are believed to offer reduced adhesion or delayed frosting66. This is due to a good correlation found between wettability and ice adhesion67, in which the surface energies of water and ice are comparable68. This relationship is often presented as ice adhesion strength as a function of CA66,67,69,70,71,72. However, this relationship does not always follow a common trend: some studies showed that ice adhesion decreases with increasing CA66,69, whereas others found no relationship between the two

70,71,72

. It was suggested that incorporation of

topographical features on hydrophobic surfaces leading to superhydrophobicity can reduce ice adhesion strength67. In general, it is still not clear whether a correlation exists, and if superhydrophobic surfaces really demonstrate anti-icing. This study is an effort to supplement this issue. Lastly, we aim to show that our superhydrophobic coating demonstrates superoleophilicity. The phenomenon of superoleophilicity is possible if a liquid (e.g., oil), has a lower surface energy than that of the solid surface73. As a result, when oil touches the surface, it spreads, resulting in a CA ≈ 0o. Therefore, if the energy of the surface lies between those of water and oil, it might show both hydrophobicity

and

oleophilicity10.

Correspondingly,

superhydrophobic

surfaces

show

superoleophilicity because of their high roughness and low surface energy74. In addition, the surface tension difference between water and oil (≈ 43 mN m−1) helps illustrate why superhydrophobic surfaces are usually superoleophilic 75. Therefore, when superhydrophobicity and superoleophilicity coexist, separation of oil from water can be realized. Such coatings with unusual but special



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wettability have attracted great interest in research because of their enriched practical applications5,8,9,22,23,24. Hence, the ultimate goal of this study is to fabricate a multifunctional PRS nanocomposite coating and for the first time, demonstrate its capability to resist corrosion attack, reduce ice adhesion/formation, and separate organics from water.

2. EXPERIMENTAL SECTION 2.1. Materials Impact-resistant carbon steel (CS) sheets (Type A516) were obtained from McMaster Carr and was used as the substrates for anti-corrosion and anti-ice tests. The CS sheets had thickness of 0.0625 in. and composition of 0.27-0.31% C, 0.79-1.30% Mn, 0.13-0.45% Si, 0.035% P, 0.035% S, and Fe making up the remaining percentage by weight. Pro grade advanced sandpapers, used in polishing the CS substrates, were purchased from 3M. Glass micro slides (GS) from VWR, 1 mm thick, were also used as substrates for anti-ice tests. Moreover, type 316 stainless steel (SS) wire cloth (120-mesh) from Grainger was the substrate used in the organics/water separation experiment. Hydrophobic fumed SiO2 (7 nm diameter) was obtained from Degussa Corporation. 99%purity phenol was purchased from EMD Chemicals, Inc, while paraformaldehyde (reagent grade), and HTBD were both purchased from Sigma-Aldrich. 1,12-diaminododecane with 98% purity was bought from TCI. Chloroform (99.8% purity) was obtained from Acros, whereas, 99.9% HPLC grade tetrahydrofuran (THF), 99.9%-purity dichloromethane (DCM) and acetone (99.7% purity) were all obtained from Fisher-Scientific. 1 N NaOH solution was prepared by dissolving NaOH pellets (Fisher-Scientific, 99.4%) in Milli-Q water (18.2 MΩ cm resistivity). 99.2%-purity crystalline NaCl (Fisher-Scientific) was used to prepare 0.5 M NaCl aqueous solution. All chemicals involved were used as received.


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2.2. Substrate Preparation The CS sheets were first cut into 1.5 x 2.0 cm pieces and polished with increasing grades (400, 600, and 1200) of sandpapers. The GS, on the other hand, were cut into 1.5 x 2.5 cm pieces, whereas the SS mesh into desired dimensions. All substrates, after cutting, were sonicated in acetone for 15-20 min, dried and kept under vacuum prior to use. 2.3. Synthesis of Benzoxazine Monomer The difunctional linear diamine-based benzoxazine monomer was synthesized (via a modification of the solventless method76), through the Mannich reaction of stoichiometric quantities (2:4:1) of phenol, paraformaldehyde, and 1,12-diaminododecane. The reactants were dry mixed and refluxed in chloroform for 16 h at a concentration of 5 mL solvent per gram of reactants. After cooling to room temperature, the crude monomer solution product was washed several times with 1 N NaOH solution and then rinsed with Milli-Q water until neutral. The washed products were dried over anhydrous sodium sulfate overnight and filtered. Chloroform was then removed by rotary evaporation, and the obtained benzoxazine (BZ) monomer was refrigerated until use. 2.4. Preparation of Coatings Preparation of the PRS coatings was performed by following the subsequent steps. Firstly, HTBD/BZ solution was prepared by melt mixing 20% by weight of the polybutadiene rubber with the BZ monomer under vigorous mechanical stirring at 100 oC for 2 h. Secondly, HTBD/BZ solution was mixed with varying amounts of SiO2 (0%, 25%, 30%, 35%, and 50% by weight, with corresponding nomenclatures of PR, PRS25, PRS30, PRS35, and PRS50, respectively) dissolved in THF solvent: specifically, this is done by first dispersing SiO2 in THF assisted by ultrasonication; HTBD/BZ solution was added afterwards, and mechanically stirred and ultrasonically mixed for 30 min each before use. Thirdly, the GS substrates were dip-coated into the different coatings for



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uninterrupted five times with a withdraw speed of 100 mm per minute. The substrates remained in the solution for 1 minute every time. Finally, the THF solvent was evaporated by drying in an oven at 60oC for 1 h followed by curing of the coated substrates at 200oC for 2 h. To ensure full protection during anti-corrosion tests, the CS substrate was first dip-coated with an inner layer (i.e., PR), followed by an outer layer (i.e., PRS). With the presence of the nanosilica particles in the topcoat and upon full evaporation of the THF solvent, it is clear that the surface should be able to exhibit roughness in nano- and microscale. The inner layer was partially cured prior to forming the outer layer to ensure good coating adhesion. For the SS mesh, the PRS coating was applied by means of a spray-coater. Note that only the PRS coating that gave the superhydrophobicity characteristic was used for this purpose. 2.5. Instrumentation The FT-IR spectroscopy was done with Cary 600 Series FTIR Spectrometer (Agilent Technologies) and scanned between 4000 and 400 cm−1. All spectra were recorded with nominal spectral resolution of 2 cm−1 and 128 scans were collected and averaged for each spectrum. Thermal stability was studied using a TGA 2050 Thermogravimetric Analyzer (TA Instruments) under a continuous nitrogen purge (110 mL/min). Weight loss was monitored as 5-10 mg samples were heated at a rate of 10oC/min to a maximum temperature of 800oC. Static water contact angle was measured by using a CAM 200 Optical Contact Angle Meter (KSV Instruments Ltd.). Adhesion test measurement was done based on the ASTM standard test method D-3359 for measuring the adhesion strength by tape test. Scanning electron microscopy (SEM) analysis was performed via a JEOL JSM-6510LV SEM whereas atomic force microscopy (AFM) was done using PicoScan 2500 AFM from Agilent Technologies.


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Electrochemical measurements for the evaluation of anti-corrosion property were done using Autolab PGSTAT 12 Potentiostat (MetroOhm, Inc.), with platinum as the counter electrode, Ag/Ag+ in 0.5 M NaCl as the reference electrode, and the carbon steel substrates as the working electrodes. Potentiodynamic polarization scan (PPS) was performed by scanning from −0.025 to +0.025 V vs Ag/Ag+ reference electrode (0.5 M NaCl) about the open circuit potential (OCP) while electrochemical impedance spectroscopy (EIS) was done for seven frequency decades from 10 mHz to 100 kHz with a 10 mV amplitude with respect to the OCP. Anti-icing experiment was performed in naturally occurring snow that took place on the morning of April 15, 2014 in Cleveland, OH, United States. Both the bare and PRS50-coated GS, with the assistance of metals clips, were vertically exposed outdoors between 7:30-9:00 am during which it was snowing hard with a freezing temperature of -3 oC. Laboratory-scale anti-icing experiment was also done using liquid nitrogen (N2) to back up the first experiment. The organics/water separation capability, on the other hand, was demonstrated by performing a liquid separation experiment involving the liquids water and DCM. The coated SS mesh was used as the separation membrane such that it was positioned between a glassware adapter and a test tube. The adapter, which was placed above, had a small opening (facing up) through which the liquids were introduced. The test tube, on the other hand, was placed at the bottom to collect any liquid that passes thru. The two were sealed with a metal clamp to ensure no leakage takes place.

3. RESULTS AND DISCUSSION 3.1. Fourier-Transform Infrared Spectroscopy Fourier-Transform Infrared Spectroscopy (FT-IR) was done to investigate and confirm the structures of the synthesized BZ, PBZ, PR and the PRS nanocomposite coating. The spectra are shown in



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Figure 2a. For the spectrum of the diamine-based BZ monomer, the presence of the BZ ring aromatic ether is verified by the presence of the peak at 1220 cm-1 due to the asymmetric stretching of C-OC76,77 while the characteristic mode of benzene with an attached oxazine ring is found at 921 cm-1 78. Upon curing, these bands have dramatically decreased to negligible intensities (PBZ spectrum), and thus confirming the polymerization of the BZ monomer. The broad band centered at 3287 cm-1 also confirms the formation of the hydroxyl groups in PBZ. On the other hand, the bands of C-N-C in both spectra of BZ and PBZ are seen in the regions from 1150 to 1030 cm-1 and from 84 to 750 cm-1 for the asymmetric and symmetric stretching modes, respectively77 while the two peaks found from 2920 to 2850 cm-1 (stretching of C-H) and the peak at 720 cm-1 (CH2 rocking) for the four spectra in Figure 2a evidently confirms the presence of the linear alkane chain.

Figure 2. (a) Infrared spectra of BZ monomer, PBZ, PR, and PRS50; and, (b) TGA of neat PBZ, neat PR, and PRS50

The spectrum of the neat PR is almost identical to that of the pure PBZ, except for the bands at 1673 cm-1 and 918 cm-1 which are due to the stretching of C=C and the bending of C-H in alkene chain of the polybutadiene. Also, the disappearance of the peak at 3287 cm-1, which is the characteristic band of the hydroxyls in PBZ, demonstrates that the rubber has been successfully grafted into the PBZ matrix. Though the rubber is hydroxyl terminated, still the O-H group could not


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be seen in the spectrum owing to the fact that the rubber is just 20% of the total weight of the polymer matrix. While still showing the peaks for the PR but with reduced intensities, the spectrum of the PRS50 further revealed three new peaks79,80 that verify the incorporation of SiO2 in the PR coating. First and foremost, at 1063 cm-1, the largest peak observed, is dominated by the asymmetric motion of silicon atoms in the siloxane bond, also known as the Si-O-Si stretching. Secondly, the peak at 452 cm-1 is due to the rocking motion of oxygen in the Si-O-Si bond. And lastly, the peak near 800 cm-1 indicates the symmetric vibrations of silicon atoms in the siloxane bond (or this is also known as the Si-O-Si bending). 3.2. Thermogravimetric Analysis The results of the thermogravimetric analysis (TGA) of the neat PBZ, the PR and the PRS nanocomposite are shown in Figure 2b. It can be seen that the neat PBZ degrades in a two-stage major weight-loss process55: the first onset weight loss, observed at 250 oC and continued with around 20% total weight loss at 450 oC, is associated with the degradation at the Mannich bridge, an associated loss of amine-related compounds, and some weight loss due to the substituted phenols in the diamine chain; the second and further succeeding onset higher temperature weight losses are dominated by the degradation of phenol, mainly the presence of free hydroxyl groups, and aliphaticrelated compounds. A similar thermal behavior is found in the neat PR, but with lower thermal stability owing to the presence of the HTBD. Incorporating nanometer-sized inorganic particles by acting as superior insulators and mass transport barriers to the volatile products generated during decomposition81, however, can enhance this thermal stability. Thus, as seen in the thermogram of the PRS50, the addition of SiO2 has greatly increased the thermal stability of the resulting nanocomposite, in which the decrease of line noticeably fell at a much higher temperature.



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3.3. Wetting Behavior of the Coatings With the aim of exploring the wetting behavior of our nanocomposite coatings, the GS substrates were dip-coated with the PR and the PRS and CA measurements were performed afterwards. In Figure 3a, the PR coating showed a hydrophobic surface with a CA of 101o±1. This is due to the existence of strong intramolecular hydrogen bonding between the hydroxyl and the amino groups in the Mannich bridges which is responsible for the low surface free energy property of PBZ materials 82

. With the addition of SiO2 nanoparticles, even higher CA values were achieved. Increasing SiO2

contents showed CA values in an increasing fashion that evidences rougher surface which according to the Wenzel or the Cassie-Baxter model, introduces a more hydrophobic surface. As a result, PRS25, PRS30, and PRS35 showed better hydrophobicity with CA values of 110o±1, 116o±1, and 132o±1, respectively. However, these CAs are low that they could not be adequately considered superhydrophobic.

Further

addition

of

SiO2

increases

the

likelihood

of

achieving

superhydrophobicity. Accordingly, the PRS50 ended up with a CA = 158o±1 (Figure 3b), sufficient enough to be classified as superhydrophobic. This can be attributed to the roughness brought about by the presence of the densely packed SiO2 nanoparticles, as evidenced by the 2-D AFM image in Figure 3c. There are also many nano-depressions detected which are created when the THF solvent evaporated and the polymeric film was formed with roughness upon thermal curing. Therefore, PRS50 is considered to carry out a detailed investigation throughout the study. It was further noticed that the surface of the PRS50-coated GS exhibited high droplet adhesion (see Figures S1a-c in the Supporting Information); hence, it is said to possess the rose-petal effect83. Rose petals possess grooves in microscale and further nanoscale depressions on these grooves in which droplets of water only infiltrate into the microscale folds but not into the nanoscale ones to form a partial Wenzel state. Therefore, it is hypothetical that there are still air pockets trapped


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between the surface beneath the droplet and the surface of the nanoscale trenches. This was evidenced when a lower meniscus of water appeared upon immersing the PRS50-coated GS into the water (see Figure S1d in the Supporting Information). Thus, the existence of such trapped air suggests potential anti-corrosion property of the coating. The spray-coated SS mesh, on the other hand, formed a Cassie-Baxter state and is said to be exhibiting the lotus effect 16, in which water droplets roll off easily and simultaneously take away any dirt present on the surface. Thus, it possesses a self-cleaning property16,22,26 (see Figures S1e and f, and V1). Additionally, the bar graph in Figure 3d compares the CA of neutral water with those of pH values of 1 and 14 measured on the surface of the PRS50-coated mesh. Interestingly, the coated mesh displayed superhydrophobicity not only for neutral water but also for water under extremely acidic and basic conditions, in which the CA values are 156o±1 and 157o±1, respectively. Furthermore, the acidic and the basic water droplets were observed to have rolled off easily when the surface was moved or slightly tilted. Therefore, such results suggest potential use of the coating under harsh environments.

Figure 3. (a) Static water contact angles of PR, PRS25, PRS30, PRS35, and PRS50; (b) Water droplet on PRS50 surface (inset: CA); (c) Atomic force microscopy (AFM) 2-D image of PRS50; (d)



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Static water contact angles of PRS50 at pH values of 1, 7, and 14; and, Optical images of PR- and PR2S50-coated CS before and after adhesion test with ASTM ratings

3.4. Adhesion Measurement The adhesion of the PR and the PR2S50 (i.e., PR film with PRS50 as topcoat) film coatings to the CS substrates was studied and evaluated based on the ASTM standard test method D-3359. The test mainly involves the application and the removal of pressure-sensitive tape over cross-cuts made in the coating films, and checks whether the adhesion of the coating is at a generally adequate level. Optical images before and after application of the tape and results are shown in Figure 3e. The classification chart for adhesion test results as per ASTM is shown in Figure S2 (see the Supporting Information). After removing the scotch tape from the PR film, the edges of the cuts were still seen to be completely intact and smooth, and not a single square of the lattice was detached. Thus, the poor adhesion of PBZ films84 was demonstrated to have been improved by the incorporation of the HTBD as supported by the 5B rating of the PR coating. On the contrary, even though it is known that the presence of a large amount of SiO2 in a coating can minimize the adhesion45,46, it is interesting to discover that PR2S50 coating still managed to display an acceptable rating of 4B, in which a detachment of a small flake of the coating was spotted at one intersection. In addition, there was no major detachment detected and good adhesion was observed between the CS and the PR2S50 film. This satisfactory result can be credited to the PR inner layer which was partially cured prior to forming the PRS50 on top. Such strategy is believed to have given way for the PRS50 film to further strengthen its adhesion by attaching itself to the uncured PR sites. In this manner, the risk of detachment of the resultant coating film upon complete curing can be reduced, thus explaining the ASTM rating so-obtained for the PR2S50.


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3.5. Anti-corrosion Property 3.5.1. Potentiodynamic Polarization Scans Having demonstrated its anti-wettability, the anti-corrosion property of the PR2S50 coating was investigated at two different immersion times by means of a series of electrochemical measurements in 0.5 M NaCl solution. For comparison purposes, the bare and the PR-coated CS were used as the control substrates. Open-circuit potential (OCP) measurements were first performed during which the samples were immersed in the NaCl solution for 1 day to ensure a steady-state system. This was immediately followed by variation of the potential about the OCP and plotting the logarithm of the resulting current against the applied potential to quantify the anti-corrosion performance of the samples. Corrosion currents (Icorr) and corrosion potentials (Ecorr) were then determined by numerically fitting the resulting plots, called the Tafel polarization curves, to the Butler-Volmer Equation. Icorr and Ecorr are extracted via a computer routine by specifying the cathodic and anodic branches and using non-linear least square fitting method of Levenberg/Marquardt85.

Figure 4. Tafel plots of the uncoated and coated CS after (a) 1 day of immersion; (b) 7 days of immersion in 0.5 M NaCl solution; and, (c) Schematic diagram of the protection mechanism of the PR2S50 coating



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The Tafel plots for the two different immersion times are shown in Figures 4a and b. It was observed that after 1 day of immersion, the PR-coated CS showed reduced cathodic current compared to the bare CS. This reduction is simply due to the hydrophobic nature of PBZ resulting to a barrier property that effectively prevents the corrosive NaCl solution from entering into the CS surface. A significant drop in the current for the PR sample, as shown by its anodic polarization curve, was also observed, implying that the anodic dissolution process of the substrate is delayed by the PR coating86. Relative to the Tafel plot of the bare CS, the Ecorr and the Icorr of the PR coating shifted to more positive and more negative directions, respectively. Smaller Icorr values indicate lower corrosion rate and larger Ecorr values denote lower corrosion thermodynamical tendency87. Even more, the PR2S50 coating, showed greater shifts of these parameters and cathodic and anodic currents are shown to have lower magnitudes than those of the PR coating. For easy understanding of such comparison, the numerical values involved are tabulated in Table 1. Note that the protection efficiency was calculated using the following formula (eq 1): PE =

୍ౙ౥౨౨,ౘ౗౨౛ ି୍ౙ౥౨౨,ౙ౥౗౪౛ౚ ୍ౙ౥౨౨,ౘ౗౨౛

× 100%

(1)

where Icorr,bare and Icorr,coated stand for the corrosion current density for uncoated and coated carbon steels, respectively. The bare and the PR-coated CS, after 1 day of immersion, had the following: Icorr of 17.4 and 1.83 µA/cm2, respectively, and Ecorr of -737 and -583 mV, respectively. Apparently, comparison of these values implies that the PR demonstrated some anti-corrosion property. Even greater, the PR2S50 film resulted to a very significant anti-corrosion property improvement, as evidenced by a more positive Ecorr and a very low Icorr of -549 mV and 0.03 µA/cm2, respectively. However, after 7 days of continuous immersion in NaCl solution, the Ecorr and the Icorr of the bare CS


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were almost unchanged and the protection efficiency was 1.1%. The protective action of the PR coating, on the other hand, was reduced from 89.5% to 86.9%, as evidenced by a decreased in Ecorr value to -594 mV and a rise in Icorr value to 2.38 µA/cm2. Only the PR2S50 managed to maintain a high Ecorr value and a very low Icorr that corresponded to a very high protection efficiency of 98.1% compared to the original 1-day protection of 99.8%. This excellent protection property is highly attributable to the formation of a very rough surface brought about by the presence of the SiO2 nanoparticles on the outer layer of the coating. As depicted in Figure 4c, mainly, the presence of trapped air in the nanoscale trenches of the rough surface is responsible for minimizing the direct contact between the aggressive species and the coating itself. Thus, this indicates that the dissolution of the CS and the aggressive attack of chlorides are strongly restrained by the PR2S50 surface. Table 1.Tafel parameters for uncoated and coated substrates after immersion in 0.5 M NaCl solution for 1 and 7 days Ecorr, mV vs Ag/AgCl Bare CS PR PR2S50

-737 -583 -549

After 1 day of immersion Icorr, Corrosion Protection µA cm-2 rate, efficiency, mm/year % 17.4 1.83 0.03

8.00 x 10-2 8.37 x 10-3 1.39 x 10-4

89.5 99.8

Ecorr, mV vs Ag/Ag Cl -738 -594 -552

After 7 days of immersion Icorr, Corrosion Protection µA cm-2 rate, efficiency, mm/year % 17.2 2.38 0.33

7.90 x 10-2 1.09 x 10-2 1.51 x 10-3

1.1 86.3 98.1

3.5.2. Optical Images and Morphologies of the Samples Shown in Figure 5a are the optical images of the uncoated and coated CS substrates before and after 1 and 7 days of immersion in 0.5 M NaCl solution. It is clear that the PR coating manifested some corrosion protective property to a certain degree, compared to the uncoated CS. On the other hand, it was observed that the PR2S50 coating, visually displayed excellent corrosion resistance. This is further backed up by the final color of the NaCl solutions used during the immersion test. It



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can be seen from Figure S3 (see the Supporting Information) that the NaCl solution used for the PR2S50 looked perfectly colorless and further maintained its colorlessness even after 7 days. These results are attributable to the presence of the SiO2 nanoparticles inducing roughness to the surface. As a result, air pockets present tend to minimize contact of the coating with water. Thus, corrosive and harsh environments will have less or even no impact to the PR2S50 surface even during prolonged exposure. The SEM images of the bare, the PR-, and the PR2S50 coated CS before and after 7 days of immersion are shown in Figure 5b. Considering the SEM image of the bare CS prior to immersion, it can be seen that it had a smooth morphologic appearance, which suggests that it was excellently polished. That of the PR-coated CS, on the other hand, revealed a compact coating film which was formed homogeneously over its surface. For the PR2S50 coating, the nano-SiO2 particles were easily spotted because of their grainy appearance all over the surface. Morphologic changes were observed after immersing these samples for 7 days. Clearly, the resulting morphology showed that the bare CS was very badly damaged. This can be evidenced by the presence of high density of corrosion products, which were formed uniformly across the surface. Likewise, majority of the surface of the PR-coated CS was shown to have been mildly destroyed by the corrosive medium, as evidenced by the formation of microscale rusts. On the other hand, the PR2S50 showed no apparent defects, and the coating remained almost unchanged after immersion for 7 days. These observations confirm the superior barrier properties of the superhydrophobic PR2S50 which are also consistent with the optical images and the results obtained via Tafel analysis.


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Figure 5. (a) Test panels of bare, PR-, and PR2S50-coated CS before and after 1 and 7 days of immersion in NaCl solution; (b) SEM images of bare CS, PR, and PR2S50 before and after 7 days immersion in 0.5 M NaCl solution (100 µm scale bar)

3.5.3. Electrochemical Impedance Spectroscopy The coatings were also examined using the EIS technology in which the corresponding plots, namely the Bode and the Nyquist, are obtained. This electrochemical measurement involves the application of a small sinusoidal perturbation to a sample under examination and the impedance modulus (z) is recorded as a function of the frequency (f)88. The analysis of the frequency behavior of the impedance allows the determination of the corrosion mechanism and the robustness of the coating89. The Bode magnitude plots (log |z| vs log f) of the samples after 1 and 7 days of immersion are displayed in Figure 6a and b. These plots can be divided into three regions90: at frequencies > 10 kHz (log f = 4), only the electrolyte resistance is measured; in the high frequency region (about 10 kHz to 10 Hz or 1 ≤ log f ≤ 4), the frequency time constant can be explained with the adsorption of the coating; and in the low frequency region (10 Hz to 0.05 Hz or -1.30 ≤ log f ≤ 1), the frequency time constant is related to the oxide layer. While the PR2S50 coating showed a 1-day overall impedance



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value of 5 x 105 ohm cm2, the bare CS and the PR coating showed way much lower impedance values of 186 and 7925 ohm cm2, respectively. After a 7-day exposure time, the impedance values visibly decreased for the bare CS and the PR coating. Such a decrease corresponds to a reduction in resistance capability of the samples to corrosion attack. However, the PR2S50 coating continued to display a very high impedance value, hence, demonstrating superior anti-corrosion performance. To further investigate the electrochemical behavior of the samples, the Nyquist plot of each sample was also considered and studied. In this plot, the imaginary component (Z”) is plotted as a function of the real component (Z’) of the impedance. For all the samples tested: the curves had a semicircle shape indicating that the systems were activation-controlled (i.e., capacitive+resistive); and the diameter of the semicircle gives the charge-transfer resistance at the electrode/electrolyte interface from which the double-layer capacitance can be calculated91. As shown in Figure 6c, after 1 day of immersion, it was observed that the PR had a larger semicircle diameter than that of the bare CS. The PR2S50 showed even a way larger semicircle diameter than the PR coating, implying an increase in the charge-transfer resistance92. Extended immersion of the samples to 7 days showed that the PR2S50 stood out to be the superior coating that could offer the greatest protection property from corrosive environments. This is clearly evidenced by the fact that the coating still had the largest charge-transfer resistance (Figure 6d) even after 7 days of exposure to NaCl solution. Therefore, it is confirmed that the PR2S50 can offer superior protection and barrier against corrosive chloride attack.


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Figure 6. Bode plots of the samples after (a) 1day of immersion, and (b) 7 days of immersion; Nyquist plots after (c) 1 day of immersion; and (d) 7 days of immersion in 0.5 M NaCl solution

3.6. Anti-ice Property The anti-icing properties of our coatings were investigated by performing a series of experiments. The first experiment involved exposure of both the bare and PRS50-coated GS in naturally occurring snow under a freezing weather temperature of -3 oC. Figures 7a and b show the surfaces of both the bare and the coated GS, respectively, after 1.5 h exposure time. Clearly, the bare GS had some snow accretion, whereas the PRS50 surface was observed to show no traces of snow accumulation. This can be verified by looking at the uncoated part of the PRS50 surface, in which deposits of snow were clearly seen. Such result suggests potential application of the nanocomposite as practical anti-icing coating particularly for walls or anything vertical.



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The second and third experiments were carried out in the laboratory with the assistance of liquid N2. In the second experiment, PRS50 surface with a Milli-Q water droplet on top was placed just above a freezing liquid N2 until the water droplet froze. Air was blown afterwards to the surface (Figure 7c), and surprisingly after 2-3 seconds (Figure 7d), the frozen droplet was carried away by the air. This experiment shows that ice built up on the PRS50 surface had poor adhesion, thereby, can be easily removed by any means. The full video clip of this experiment can be watched from V2 in the Supporting Information. The results obtained from these two anti-icing experiments can be explained mainly in terms of the superhydrophobicity nature of the coating surface. As discussed previously, PRS50 coating forms a partial Wenzel state in which water only penetrates into the microscale grooves of the rough surface but not into the nanoscale folds. Hence, there exist trapped air pockets inside the cavities of these nanoscale folds. To a greater extent, it is believed that these air pockets would remain trapped even under a condensing weather condition, such that, these trapped air pockets would considerably minimize the contact of ice/snow with the substrate surface and therefore significantly reducing the adhesion strength of the ice93. Again, the existence of these trapped air pockets was confirmed by simple immersion of our coated substrate, which was shown in Figure S1d.


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Figure 7. Test results for anti-icing properties of (a) bare and (b) PRS50-coated GS in naturally occurring snow; Ice-adhesion test by air-blow (c) with ice droplet before blowing; (d) ice detaches after blowing; Screenshots of frost formation experiments using liquid N2: (e) bare GS; (f) PRS50coated GS; (g) bare CS; and, (h) PR2S50-coated CS

The third experiment demonstrates the ability of our coatings to delay frost formation. The following subsequent steps involved are: immersion of the substrates into a freezing liquid N2, taking the substrates out, and immediately dropping Milli-Q water on their extremely resulting cold surfaces. PRS50-coated GS and PR2S50-coated CS were used as the substrates in this experiment. The uncoated forms were also tested and used as controls. As expected, water immediately dropped on the inclined surfaces of the bare GS (Figures 7e) and CS substrates (Figures 7g) began freezing and sticking to the surface before they could slide. This can be explained by the fact that because such uncoated substrates do not possess low surface energy property, water droplets will spread and maximize their contact area and adhesion with the surface. Therefore, movement of the water



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droplets on such surfaces will be difficult and slower such that they would quickly freeze before they could slide off the surface. On the other hand, water droplets upon immediate contact with the cold surfaces of the PRS50 and the PR2S50 (screenshots of which are shown in Figures 7f and Figures 7h, respectively) flowed and slid off the surface before they could freeze. This is because the cushion of air existing in the cavities of the nanoscale trenches limits to some extent the contact of water droplets with the surface. Such existence of air cushion coupled with the influence of gravity will make it hard for water droplets to rest on the inclined surface94; hence, they easily slide off the surface before they could freeze. Moreover, such frost formation delay is due to the belief that some superhydrophobic surfaces demonstrate anti-icing ability – ours is one of these. The full video clip of Figures 7e and f can be watch from V3 and V4 in the Supporting Information.

3.7. Organic/Water Separation Experiment The surface morphology of the SS mesh was also investigated via SEM technique. As shown in Figure 8a, significant differences were observed between the bare and the PRS50-spray-coated mesh. The bare mesh, whose pores have an opening of about 150 µm and an initially smooth surface structure, was visibly knitted by SS wires. The SS mesh, after it was coated with PRS50, seemed to have retained its original shape with no further observed damages. No PRS50 coating was seen to be blocking the openings of the mesh, hence, free passage of air through these openings was ensured. Moreover, the wires of the coated mesh showed formation of very small peak-, ball-, and crater-like structures on the surface. The peaks and the craters are the results of aggregated balls which stuck to and embedded in each other. Such presence of assemblies brings roughness, with both nano- and microscale structures, to the surface, and thus is responsible for the self-cleaning property of the coated mesh.


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Figure 8. (a) SEM images of bare and PRS50-coated SS mesh; Series of screenshots illustrating (b) water being blocked by PRS50-coated mesh from penetrating through; (c) DCM penetrating through the coated mesh; and (d) separation between water and DCM as they are simultaneously introduced onto the coated mesh

Prior to performing the separation experiment, water and DCM, dyed with bromothymol blue and nile red, respectively, were separately introduced onto the PRS50-spray-coated SS mesh to demonstrate both superhydrophobicity and superoleophilicity of the coating. As shown in a sequence of screenshots of Figure 8b, the coated mesh hindered the water from permeating through the openings of the mesh, thereby confirming superior superhydrophobicity. Conversely, the coated mesh (Figure 8c) displayed outstanding superoleophilicity such that it allowed DCM to screen via



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gravity through the openings (See video clips V5 and V6 in the Supporting Information). These observations can be explained by the Wenzel model in terms of the CA for a liquid on a rough surface13: increase in roughness of a surface decreases the CA of hydrophilic materials and increases for hydrophobic ones. Since PBZ is a known low surface energy material, the CA for water will increase and that for a liquid with a similar surface tension as oil (e.g., DCM) will decrease upon the introduction and further increasing of surface roughness. Therefore, with the roughness brought about by the SiO2 nanoparticles, both superhydrophobicity and superoleophilicity could be induced. With these two properties together in one material, oil/water separation is potentially possible. This is demonstrated in sequential video frames shown in Figure 8d (full video clip in V7 in the Supporting Information), in which both the water and the DCM were introduced simultaneously onto the PRS50coated mesh. Interestingly, only the DCM passed through while the water was held and collected on the surface. Again, water stayed behind and DCM flowed through because of the superhydrophobicity and superoleophilicity of the coated mesh. Such outstanding capability of the material can provide opportunities in many practical applications such as oil contamination and spill problems.

4. CONCLUSION In conclusion, we successfully fabricated a nanocomposite coating composed of rubber-modified PBZ and SiO2 nanoparticles that is able to demonstrate both superhydrophobicity and superoleophilicity properties. With the low surface energy property of the PBZ and the roughness features provided by an optimum amount of SiO2, we were able to tune the wettability of the resultant coating and make it superhydrophobic. Having achieved this level of anti-wettability, we investigated the potential anti-corrosion application of the coating. Based on the results obtained via


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electrochemical measurements in a chloride solution, PR2S50 coating showed excellent protection property against corrosion attack even after prolonged exposure. This superb protection is mainly due to the very thin layer of air remaining trapped in the troughs of the nanoscale regions of the rough coating film. Such trapped air acts as a protective barrier boundary that prevents the invasion of aggressive chlorides into the substrate. Note that PR2S50 surface was fabricated by formation of an inner layer of the PR followed by an outer layer of the PRS50. Such a film surface is formed since good adhesion of the coating is desired and essential if it were to have effective anti-corrosion properties. This is evidenced by the good adhesion result obtained via ASTM adhesion test. Also, with the theory that some superhydrophobic materials induce icephobicity, we explored the possibility of the potential anti-icing capability of the coating. Results showed that the PRS50 coating is able to resist cold weather conditions that when exposed outside during a snowfall, it prevents snow from accumulating on its surface. Likewise, results from qualitative laboratory scale anti-icing experiments further revealed that the coating is able to retard ice adhesion and formation of frost on its surface by allowing the condensed water droplets to quickly slide off before they could freeze. Such anti-icing property is believed to be attributable to the air cushion present in the rough nanoscale regions of the coating surface which prevents frost formation from water droplets and minimizes the contact of snow/ice with the film surface, thereby lessens the adhesion. Lastly, with the realization that surfaces with superhydrophobicity could also exhibit superoleophilicity, we prepared a separation membrane to demonstrate whether the coating can possibly separate DCM from water. Remarkably promising results were obtained as the coating indeed was able to separate the two liquids by allowing DCM to penetrate through the membrane while the water was seized on top. Given that DCM has nearly the same surface tension as oil, the coating can be potentially



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utilized in real-life situations that deal with oil spill and contamination problems. Thus, we were able to show that the fabricated nanocomposite coating demonstrates multifunctional properties.

AUTHOR INFORMATION Corresponding Author *Phone: +1 216-368-4566 Email: [email protected] Notes The authors declare no competing financial interest. SUPPORTING INFORMATION Photos showing that water droplet sticks to the PRS50-coated GS PRS50-coated CS with water droplet tilted at 90o and 180o Photos evidencing the existence of trapped air in the immersed PRS50-coated GS as demonstrated by the presence of a lower meniscus of water in the middle photo Video frames showing the behavior of water droplet as it fell to the PRS50-spray-coated mesh Photo showing the self-cleaning property of the PRS50-coated mesh (inset: shape of water droplet on unmoved coated mesh surface) Classification of adhesion test results as per ASTM Photos of the NaCl solutions used for the different samples after 1 and 7 days of immersion Video clips showing the following: PRS self-cleaning property; Ice droplet adhesion strength; frost formation on uncoated GS; frost formation delay on PRS; PRS-coated mesh hindering water permeation; PRS-coated mesh allowing DCM to pass through; water/DCM separation.

FUNDING SOURCES This work was supported in part by the National Science Foundation (CMMI NM 1333651 to NFS and RCA and STC-0423914 to RCA) and the DOST-ERDT program of the Philippines.


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