Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Strong Hydrophobic Coating by Conducting a New Hierarchical Architecture Yong Seok Kim, Mingwei Shang, Shuai Kang, Jacob John Karsseboom, and Junjie Niu* Department of Materials Science and Engineering, University of Wisconsin−Milwaukee, Milwaukee, Wisconsin 53211, United States S Supporting Information *
ABSTRACT: While the hydrophobicity for a self-cleaning surface is feasibly obtained via rational designs of nanostructured materials,1‑2 the longevity of coatings due to the rapid function loss and weak interface bonding and the scalability due to the limited size are not on a solid footing.3 In this article, we report the synthesis of flexible self-cleaning coating with improved mechanical and chemical stability on the basis of a new hierarchical architecture, which comprises functionalized epoxy (EP) resins and industrially available activated carbons. In parallel, a self-cleaning coating with high transparency can be obtained by replacing with oxide particles, which further expands the application fields. The strong bonding force from alkene CH3−C−CH3 and phenyl groups in bisphenol A diglycidyl ether contributes to high rigidity, high toughness, and high-temperature tolerance while the ether linkages lead to high chemical resistance.4 A greatly enhanced adhesion to substrates originates from the preferable interface ring-opening reaction of highly reactive ethylene oxide C2H4O on EP and amine groups on curing agents. Superhydrophobicity is ascribed to the interaction among hydrophobic groups on “grafted” heptadecafluorodecyl acrylate and functionalized particles. The impressive hydrophobic and mechanical properties open an avenue for a reliable self-cleaning coating in commercial products.
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(EP) expresses a high adhesion force to substrates.21 This outstanding adhesion inspires us that if a modified EP in selfcleaning coatings remains more ether or ester groups, a high mechanical stability can be achieved. Recently, researchers have put efforts on creating micro-/nanoconfigurations using polymeric compounds.22 Unfortunately, most of EPs are composed of hydrophilic groups and present a low hydrophobicity that makes them impossible to obtain superhydrophobicity, even though they have a promising bonding force. Here, we report a high mechanical strength, high anticorrosion, and high flexibility self-cleaning coating synthesized using commercial EP and industry-wide activated carbons (ACs) or silica as precursors via a facile chemical method. This hybrid composite coating with scalable production displayed an improved superhydrophobicity with a CA up to 162°. The enhanced mechanical property due to the strong bonding in the EP matrix after a series of polymerizations makes the coating have a lifespan of many years. In addition, our self-cleaning coating demonstrates flexibility on transparency and versatile substrates by applying scalable coating processes.
INTRODUCTION Artificial hydrophobic surface that displays water contact angles (CAs) over 150° inspired from the “lotus leaf” has stimulated extensive studies in interdisciplinary fields.1−6 In particular, a well-engineered architecture with a nano-sized roughness exhibits a capability to trap micropockets of air between the liquid and solid interfaces, thereby leading to hydrophobicity.7,8 On the basis of biomimicry from nature, a number of synthetic surfaces have been recently developed using a series of technologies including chemical vapor/electrophoretic deposition,9 fiber,10 nanowire assemblies,11,12 polymer membrane casting,13 and electrospinning.14 To date, scientists have made a remarkable progress in developing superhydrophobic coatings with a CA greater than 150° even up to 170°.15,16 In addition to hydrophobicity, an ideal self-cleaning coating should also have a strong mechanical and chemical stability. However, this significant property that determines the coating shelf time particularly under destructive/corrosive conditions is less focused.17 Lu et al. created an ethanolic suspension of perfluorosilane-coated titanium dioxide nanoparticles that form a paint that can create a self-cleaning surface that functions upon emersion in oil.18 Tian et al. used particle-filled silicone rubber composites to develop a mechanical robust superhydrophobic surface.19 Another important issue is that a self-cleaning coating should be applicable on a broad range of substrates, regardless of the composition, geometry, and size.8,20 Finally, demands from industries necessitate a lowcost, large-scale manufacturing process. As we know, the epoxy © XXXX American Chemical Society
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EXPERIMENTAL SECTION Materials. Heptadecafluorodecyl acrylate (HDFAA), 12 wt % azobisisobutyronitrile (AIBN) in acetone, sodium hydroxide,
Received: November 10, 2017 Revised: February 1, 2018 Published: February 2, 2018 A
DOI: 10.1021/acs.jpcc.7b11144 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 1. Structural functionalizations of (I) PVDF to form m-PVDF via dehydrofluorination and nucleophilic substitution, subsequently (II) to incorporate with HDFAA via radical polymerization, and then (III) to be grafted onto the EP to form f-EP through dehydration. (IV) AC modification to create the hydrophobic f-AC using PFDA via a refluxing process. (V) Formation of the hybrid composite self-cleaning coating using a dip-coating method.
perfluorodecanoic acid (PFDA), N-N-dimethylformamide (DMF), poly(bisphenol A-co-epichlorohydrin) glycidyl endcapped (Mn: ∼350) as an EP resin, 15 nm silicon dioxide (SiO2, spherical, porous), and 5-amino-1,3,3-trimethylcyclohexane methyl-amine, and a mixture of cis and trans (99%) as a curing agent were purchased from Sigma-Aldrich. Poly(vinylidene fluoride) (PVDF), anhydrous ethanol (94−96%), and AC powder (Norit GSX, steam-activated, acid-washed) were purchased from Alfa Aesar. All chemicals were used without further purifications. Double distilled water was used throughout whole experiments. Synthesis of the Three-Dimensional (3D) Hybrid Composite. The 3D hybrid composite was synthesized through a series of functionalizations of both ACs and EP. We first generated CF3 groups on the AC surface (f-AC) and on the SiO2 surface (f-SiO2) using PFDA. In a typical experiment, 5 g of AC powder was dispersed in 150 mL DMF solution under magnetic stirring for 10 min. Subsequently, 0.3 g of PFDA was added into the solution and was then refluxed at 120 °C for 12 h using a glass reflux system. After reflux, the mixture solution was filtered through a 0.2 μm PVDF membrane filter in a vacuum filtration system. Then, it was rinsed at least 3 times with ethanol to remove excess chemical residuals. Afterward, the sample was peeled off and was dried at 80 °C in an oven for 5 h under atmosphere. For the functionalization of EP, 2 g of PVDF powder was added into 40 mL alkaline solution (5 wt % NaOH) under magnetic stirring for 12 h at room temperature. After filtration and drying, the modified PVDF powder (m-PVDF) was obtained. Subsequently, 0.65 g of m-PVDF and 2 g of EP were mixed in 10 mL DMF solution. In parallel, 0.6 g of HDFAA, 0.016 g of AIBN, and 0.52 g of f-ACs were added into the solution under stirring in an oil bath at 80 °C for 12 h. As a result, the
composite was formed by a one-pot chemical method that includes simultaneous reactions of condensation and freeradical polymerization. The obtained self-cleaning composite solution is ready to be coated on versatile substrates using aforementioned methods along with a 180 °C heating process for 5 h or an air-drying for 24 h. The 3D hybrid composite was also synthesized as a function of f-SiO2 amount through the same procedure. The amount of f-SiO2 was controlled in total polymers (13, 19, 23.8, 27.4, 31, 44.8, 52.4, 59.3, and 64 wt %). Sample Characterization. The surface morphology of the coating was checked on a Hitachi S4800 ultrahigh resolution field emission scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (EDX). The X-ray photoelectron spectroscopy (XPS) was conducted using Thermo Scientific ESCALAB 250Xi which is equipped with an electron flood gun and a scanning ion gun. Diffused Fouriertransform infrared spectroscopy (FTIR) was recorded on a Nicolet 6700 series FTIR spectrometer (Thermo Fisher Scientific, Inc., Madison, WI). Optical grade, random cuttings of KBr powder (International Crystal Laboratories, Garfield, NJ) with 1.0 wt % of the sample were grounded, packed firmly, and leveled off at the upper edge to provide a smooth surface. The FTIR sample chamber was flushed continuously with N2 prior to data acquisition in the range of 4000−400 cm−1 with an offset of ±4 cm−1. A 3D laser confocal microscope (Olympus LEXT OLS4100, Japan) was used to collect the surface roughness information. The sample was coated on a glass slide and was then checked under a cutoff wavelength of λc = 8 μm, a field depth of 257 μm, and a Gaussian filter. The CA was measured using a DataPhysics OCA 15 optical contact angle measuring system, with a dropwise volume from 3 to 10 μL and a speed of injection from 1 to 5 mL/min. B
DOI: 10.1021/acs.jpcc.7b11144 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Binding energy evolution of forming hybrid composites. High-resolution XPS spectra of (I) C 1s and (II) F 1s peaks of the AC, f-AC, PVDF, f-PVDF, and the final composite, respectively.
Mechanical Property. Scratch resistance, Young’s modulus, and hardness measurements of the 3D hybrid composite, commercial Loctite EP resin, and lab-made pure EP resin coated on glass slides were measured using a Nano Indenter (Agilent Technologies, G200) with a Berkovich tip, a XP standard indentation at a depth of 3 μm, and a ramping load scratch from 0 to 500 mN, respectively. Peel adhesion was performed using Scotch Filament Tape 897 Clear under the ASTM standard (D3359-09) of B-cross-cut tape test. Foldability and bendability of the 3D hybrid composite-coated paper towels were also checked. Abrasion test of the hybrid composite coating on the glass substrate was done using a sandpaper under high pressure. Typically, a sandpaper with grit no. 400 was placed facing downward to the coating. Then, 5 kg of the standard weight was added on the top surface of the sandpaper. According to the 18.75 cm2 surface area of the coating on the glass, the added pressure is 26.1 kPa. One abrasion cycle is defined as the sample that was moved forward for 10 cm, then rotated by 90° (faced to the sandpaper), and moved backward for 10 cm along the same route. Chemical Stability. The chemical stability test was completed by immersing the sample into a solution with pH ranging from 2 to 9, respectively.
CC bond, as evidenced by a new C 1s peak at 284.5 eV from the XPS spectra (Figure 2(I), m-PVDF).15,17 The nucleophilic substitution of low-activity F− with high-activity hydroxyl groups was promoted by electron-withdrawing inductive effects, forming OH groups (Figure 1(I)).15 As can be seen from the XPS spectra, two strong C 1s peaks located at 286.4 and 291.0 eV represent the presence of abundant CH/CH2 and CF2 groups in both PVDF and m-PVDF (Figure 2(I)).16 In the following step, the strong hydrophobic−hydrophobic interaction between m-PVDF and HDFAA led to a free-radical generation by breaking the CC bond via an initiator AIBN. Subsequently, HDFAA was successfully connected to m-PVDF after a series of free-radical polymerizations (Figure 1(II)). Consequently, HDFAA was further grafted onto the EP via mPVDF as a bridge by dehydration (Figure 1(III)). Thus, a strong connection between the crosslinking m-PVDF and EP was formed through a C−O−C covalent bonding that was induced by the intermolecular dehydration condensation of hydroxyl groups under high temperatures, forming a new copolymer f-EP (Figure 1(III)). This strong covalent bond further strengthened the whole skeleton. Another benefit from m-PVDF is its amphiphilicity on account of the existing carbon fluoride groups (hydrophobicity) and carbonyl groups (hydrophilicity). In parallel, to further improve the hydrophobicity, we need to create a hierarchical configuration, which contains a micro-/nanoroughness as well as low-energy functional groups having a similarity to lotus leaf. This heteroarchitecture was created through a rational design of AC particles (Figure 1(IV)). The ACs with varying primary sizes ranging from several to tens of micrometers and secondary sizes ranging from tens to hundreds of nanometers provided a hierarchical geometry with a mean surface roughness of 15 μm (Figures S1a and S2b). The original ACs were modified with PFDA through the condensation of COOH and OH groups (Figure 1(IV)). The XPS results clearly depict the appearance of new peaks of f-AC at 688.2, 291.6, and 293.3 eV, which correspond to F (F 1s), CF2 (C 1s), and CF3 (C 1s) groups from PFDA, respectively (Figure 2).20 The EDX mapping data also show the existing F element in the f-AC (Figure S1b). The affiliated
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RESULTS AND DISCUSSION The functional groups such as fluorine, alkane, and siloxane on a hierarchical architecture can lower down the surface energy and trap more air underneath, thereby keeping the spherical shape of the water droplet perpendicularly standing. In our approach, we employed an intermediate mediator, m-PVDF, as a bridge to connect the hydrophobic groups from EP and another HDFAA using a facile “grafting” method (Figure 1).23,24 As a result, we were able to keep both mechanics from the EP and hydrophobicity from the grafted HDFAA via a newly formed copolymer, f-EP. In experiments, the neighboring hydrogen and fluoride atoms from vinylidenefluoride of PVDF were susceptible to form hydrogen fluoride in an alkaline condition (pH > 14.0). This dehydrofluorination generated a C
DOI: 10.1021/acs.jpcc.7b11144 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 3. SEM images of the 3D hybrid composite (AC) with low (a) and high (b,c) magnifications. SEM morphology evolution of the composite on paper towels before (d) and after (e) heating. (f) Elemental mappings of F, C and O from the cross-section of the composite via EDX.
fluorine ligands on f-ACs demonstrated a water-repelling property as same as the groups in HDFAA (Figure 1(IV)). In the final step, both the f-EP and f-ACs constituted a closepacked network through strong reciprocal hydrophobic− hydrophobic interactions (Figure 1(V)), which can be indicated by the enhanced C 1s (CF3) peak at 293.3 eV and O−CO peak at 288.6 eV from both HDFAA and PFDA in the final composite (Figure 2(I), composite) as well as the strong peak of F 1s at 688.2 eV (Figure 2(II)).20 In addition, the two C 1s peaks at 285.3 and 286.8 eV of the composite are ascribed to the CH3−C−CH3 and C−O−C bonds in EP.19 The broadening peak of CC is believed to be due to the number of phenyl groups present in EP. Owing to the wide availability of precursors and the simple approach, the yield of our hybrid composite can reach kilogram scale, enabling broad industrial applications. The functional groups were further confirmed by FTIR measurements (Figure S2a). The CC bond from m-PVDF at 1633 cm−1 which was generated from the dehydrofluorination of PVDF (Figure 1(I)) disappears after grafting HDFAA on fACs. The peak at 1244 cm−1 appears in the resultant coating because of the aromatic ether bond stretching vibration of C− O of EP. Also, the strong peak of symmetric CF3 stretching vibrations from f-AC and HDFAA is observed at 1226 cm−1. The characteristic absorptions of bisphenol A appear at 2964 and 2869 cm−1 in the C−H stretching region from CH3−C− CH3 of EP. The existing strong absorption bands from the phenyl group of EP at 1608, 1512, and 1455 cm−1 ensure a high hardness and a high corrosion resistance. The EDX mapping shows that the composite contains a large ratio of fluorine (Figure 3f). The intensified peaks at 1157 and 874 cm−1 confirm a number of strong stretching vibrations of CF2 and the amorphous phase of m-PVDF that corresponds to mPVDF, HDFAA, and f-ACs in the composite. A band at 1050 cm−1 correlating to the formed C−O−C group evidences the crosslinking reaction through the dehydration of m-PVDF and EP, as illustrated in Figure 1(III). In general, the fluorine
groups on a hierarchical structure deliver a strong waterproof property because they reduce the interface energy between water and trapped air pocket, whereas the phenyl group in EP correlates to a high hardness/rigidity and a high-temperature tolerance. In addition, the existing active C−O−C groups in fEP enable a strong adhesion to substrates along with a high chemical resistance (anticorrosion). Thus, the functional groups on the hierarchical architecture lower the surface energy while providing high mechanical/chemical stability, resulting in a strong superhydrophobicity. The morphology of the hybrid composite coating on the glass was checked by scanning electron microscopy (SEM). Figure 3a,b depicts a hybrid hierarchical structure with a micro-/nanoroughness that comprises f-ACs as the skeleton and the f-EP as the joining, which reinforces the composite with an apparently raised mechanical strength at failure. As can be observed from the enlarged cross section (Figure 3c), the f-ACs are rigorously encapsulated by the f-EP, forming an intersecting network. The morphology evolution of the composite framework on paper towels before and after curing was recorded in Figure 3d,e. The individual carbon clusters with visible boundaries were uniformly distributed among the polymer matrix (Figure 3d). Nonetheless, these clusters were completely embedded into the f-EP matrix by the condensation reaction during the curing process, forming a compacted hybrid composite (Figure 3e). According to the Cassie theory, superhydrophobicity cannot be achieved by constructing a simple secondary papilla structure while a hierarchical architecture is needed instead.25,26 A droplet rests on top of the rough surface on solid−air composite surfaces (Cassie−Baxter wetting regime), where the CA can be described by25 cos θ′ = f cos θ − (1 − f )
(1)
Here, θ′ and θ are the CAs on rough and flat surfaces with the same chemistry, respectively. The f (
