Preparation of Superhydrophobic Cu Mesh and Its Application in

Article pubs.acs.org/IECR

Preparation of Superhydrophobic Cu Mesh and Its Application in Rolling-Spheronization Granulation Wei Jiang,* Jian He, Ming Mao, Shaojun Yuan, Houfang Lu, and Bin Liang Multi-Phases Mass Transfer and Reaction Engineering Laboratory, College of Chemical Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: In this research, an experiment was conducted by introducing superhydrophobic surface into rolling spheronization granulation. The employed superhydrophobic surface was prepared with modification of an anodized Cu mesh with silane FAS-17 and exhibited excellent uniformity, adequate stability, and broad adaptability in the granulation scenario. Completely spherical molecular sieve granules with 99.85% sphericity and 2.53 mm diameter were obtained. The compressive strength of a single granule reached 7.402 N/ea. Only negligible residual mass and slight surface abrasion were observed in the granulation process. Force analysis confirmed that the staged motion behavior of droplets caused by the interaction between the slurry and the superhydrophobic surface benefited to the formation of spherical granules. Successful application of this process for granulation of other substances confirmed its wide suitability. With the advantages of easy fabrication, high-quality granules, and low cost, rolling-spheronization granulation on superhydrophobic surfaces has great potential for scale-up applications.

1. INTRODUCTION Superhydrophobic surfaces have attracted much interest after the explanation of the origin of the “lotus effect” in nature.1−9 With increasing demand for smart surfaces, many studies have focused on the development and applications of water-repellent surfaces, such as self-cleaning,10−16 anti-icing,17,18 antifogging,19,20 antifouling,21,22 drag reduction of liquid and muddy water,23−29 antireflection,30 and anticorrosion.31,32 Among various applications, self-cleaning is one of the most popular applications of superhydrophobic surfaces in industry. Generally, water droplets roll across a superhydrophobic surface, thus removing dirt, because water and dirt have greater affinity for each other than either does for the superhydrophobic surface with a low surface energy.13 However, the studies on this attractive process involved the properties and fabrication of bioinspired surfaces and the motion behavior of water droplets on those surfaces. To the best of our knowledge, solid dart-containing droplets after rolling off the superhydrophobic surface have been rarely studied. Solid-containing droplets usually maintain a spherical shape even after drying on the superhydrophobic surface. Typically in nature, (i) when water droplets roll on a lotus leaf, they collect the dust on the leaf and settle down on the low-lying place of the leaf as spherical slurry balls (Figure 1a−c). (ii) Another typical example in nature is the shape of the raindrops on a spider web after raining. As schematically illustrated in Figure 1d−f, the dust adhered onto the droplets and hung on the spider silk; then the droplets turned into dust granules without any change in the shape after drying. A simple experiment was conducted by dropping water on an artificial superhydrophobic © XXXX American Chemical Society

surface sprinkled with ashes. Thus, a dust-containing dirt droplet was formed on a horizontal plane, as shown in Figure 1g,h. Such adhesion behavior of dust onto water droplets on a superhydrophobic surface is known as the collection effect,14 as schematically illustrated in Figure 1i. The principles behind the above-mentioned two nature facts indicate that superhydrophobic surfaces are potentially useful for the granulation operation in industry. Currently, the granulation technology is widely used in various modern industries including chemical, pharmaceutical, food, coal, mining, metallurgy, ceramic, and agrochemical for reducing dust, thus facilitating handling and enhancing the final utility of the material.33−36 Wet granulation is the most commonly used granulation technology in industry and accounts for ∼80% of the total industrial granulation processes.37−39 Typical wet granulation processes include high shear mixer granulation, fluid bed granulation, spay drying, extrusion, and rolling-spheronization.40,41 Because of good flow property, better compression characteristics, and uniformity of spheres,33,42 the spherical shape of granules is highly desired in granulation for various applications. Since completely spherical dust-containing slurries can be achieved on natural and artificial superhydrophobic surfaces by rolling water droplets on a dusty surface, we envisioned that rollingReceived: December 8, 2015 Revised: March 28, 2016 Accepted: April 27, 2016

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DOI: 10.1021/acs.iecr.5b04685 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. Self-cleaning process and granulation results on hydrophobic surface. a: Superhydrophobicity of lotus leaf; b: self-cleaning effect of lotus leaf; c: dust ball on lotus leaf; d: Superhydrophobicity of spider web; e: Ash collecting effect of spider web; f: Dried ash particle on spider web; g and h: Self-cleaning process of hydrophobic Cu foil with water; i: Schematic graph of self-cleaning process.

Figure 2. Schematic diagram of preparation of superhydrophobic Cu mesh.

Figure 3. Diagram of the device for electrochemical anodization process.

2. EXPERIMENTAL SECTION 2.1. Materials. A 99.9% Cu wire woven square-mesh screen with a mesh number per inch of 200 and a wire diameter of 50 μm was purchased from Tianjin Xingbo Guangwang Steel Co. Ltd. (China). 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FAS-17) was purchased from Sicongprotect Chemicals Co. (China). The molecular sieve was supplied by Liaoning Haitai Tech. Co. Ltd. (China). Other chemical reagents including NH4F, isopropanol, ethylene glycol, acetone, ethanol, NaSiO3, and HNO3 were of analytical-grade and purchased from Kelong Chemicals Co. (Chengdu, China). All the reagents were used as received without further purification.

spheronization granulation can be used to obtain completely spherical granules by introducing a superhydrophobic surface. Herein, the purpose of this study was to evaluate rollingspheronization granulation on a homemade superhydrophobic surface. In this study, a Cu mesh bowl with controllable superhydrophobicity was easily prepared by a combination of the electrochemical anodization of a Cu mesh and fluorosilanization with FAS-17; the Cu mesh bowl thus obtained was used as a tray for rolling-spheronization granulation. The feasibility and principle of rolling-spheronization granulation on the superhydrophobic surface was investigated, and the products were characterized. B


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Figure 4. Schematic diagram of adhesion test.

2.2. Preparation of Superhydrophobic Cu Mesh. The preparation of the superhydrophobic Cu mesh is shown schematically in Figure 2. Next, the Cu mesh was anodized, and the anodized Cu mesh was modified. The formability and flexibility of the soft Cu mesh can be attributed to the use of a Cu mesh, not Cu foil. First, the shape of the pure Cu mesh was manually prepared as a bowl with a diameter of 6 cm and a depth of 3 cm using a culture vessel with the same size as the template. Then, the shaped pure Cu mesh was washed sequentially with acetone, isopropanol, ethanol, and deionized water in a KQ5200DE ultrasonic cleaning machine to completely remove the grease on the surface of the Cu mesh. Next, the Cu mesh was chemically polished with a 15 wt % HNO3 solution for 10 s to remove the oxide layer and washed with deionized water immediately. After drying the Cu mesh in a vacuum oven under nitrogen, the clean Cu mesh bowl was anodized using the device shown in Figure 3. The electrolyte, 1 M NaOH aqueous solution, was deaerated with an anhydrous nitrogen stream for 1 h without stirring. The Cu mesh served as the anode; a homemade 304# stainless steel cylinder with a diameter of 2 cm and a height of 2 cm was used as the cathode. The distance between the two electrodes was 1 cm. The current intensity was maintained constant at 0.1 A, which was controlled by the chronopotentiometry module of a CHI660E electrochemical workstation. The anodizing time was set at 1800 s. Finally, the anodized Cu mesh was removed and cleaned with deionized water. Then, the sample was dried in an oven at 100 °C for 1 h to remove water and maintained at 150 °C for 3 h to obtain the final anodized product. A typical fluorinated silane, FAS-17, was used to perform the hydrophobic modification of the Cu mesh bowl. The FAS-17 solution was prepared by mixing 1 g of FAS-17 and 99 g of ethanol for 3 h under continuous stirring at a rate of 100 rpm. The superhydrophilic Cu mesh with a fine CuO nanostructure was immersed into the FAS-17 solution at ambient temperature for 12 h. Finally, the Cu mesh was washed with pure ethanol and dried in an oven at 100 °C for 1 h. 2.3. Stability and Adaptability of Superhydrophobic Cu Mesh. Stability of as-prepared superhydrophobic Cu mesh was tested by exposing it to air without any protection. Static water contact angles (CAs) of samples against exposure time

were measured using a CA meter (Powereach, JC2000C1) at room temperature, exhibiting its superhydrophobicity stability. Adaptability of superhydrophobic Cu mesh was determined by testing static contact angles of slurry with different solid content and water with different pH value. 2.4. Rolling-Spheronization Granulation. The rollingspheronization granulation was carried out using a homemade device. The superhydrophobic Cu mesh was designed and shaped manually as a bowl-like container with a diameter of 6 cm and a depth of 3 cm. The as-prepared bowl-like Cu tray was adhered onto a plastic plate to fix it on a stirring motor. A speed-controlling device was used to control the rotation of the plate and adhered to a Cu mesh bowl. The molecular sieve or other solid substances such as activated carbon, CaSO4, NaCl, Co(NO3)2, Ni(NO3)2, and Mn(CH3COO)2 were used as the powder raw material; water or a sodium silicate aqueous solution was used as the binder. Then, the molecular sieve was scattered on the bowl. Then, liquid droplets were injected using a 1 mL syringe with a needle diameter of 0.33 mm on the bottom of the rotating bowl, thus collecting the powder for the final formation of spherical granules. Finally, the formed granules were collected, dried in an oven at 85 °C for 12 h, and calcined in a muffle furnace at 550 °C for 2 h. 2.5. Adhesion of Superhydrophobic Cu Mesh. Figure 4 shows the basic steps of the adhesion experiment. Approximately, 15 g of molecular sieve and 15 g of water were blended to obtain a viscous slurry; the slurry was filled into bowls made up of a pure Cu mesh and the superhydrophobic Cu mesh. A stent preplaced in the bowl was fixed after filling the slurry. After a certain mass of the slurry was filled into the bowl, the bowl was left to stand for ∼3 h for aging and solidifying to form a cake. Then, a dynamometer was attached on the stent and lifted to remove the solidified cake vertically from the bowl. The reading of the dynamometer after subtracting the weight of the cake was used as the value of the adhesion between the slurry and Cu mesh. Five parallel experiments were conducted to determine the average adhesion value. 2.6. Characterization. Static contact angles (CAs) and rolling angles were measured using a CA meter (Powereach, JC2000C1) at room temperature. Water or slurry droplets of ca. 5 μL were gently deposited dropwise onto the surface of the sample. Three points of each sample were investigated; the average value of the three left and right CAs was calculated as C


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Figure 5. Characterization of the superhydrophobic Cu mesh. a: Voltage changing in anodization process; b: SEM graphics of pristine Cu mesh; c: SEM graphics of anodized Cu mesh; d: superhydrophilicity of anodized Cu mesh and fast penetration of water through it in 10 s.

Figure 6. Characterization of as-prepared superhydrophobic Cu mesh. a: SEM surface morphology; b: XRD pattern; c: FT-IR analysis; d: water static contact angel; e: water rolling angel; f: uniformity of the superhydrophobic Cu mesh.

D


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Figure 7. Stability and adaptability of superhydrophobic Cu mesh. a: Superhydrophobicity agaisnt time; b: superhydrophobicity against different solid content in slurry; c: superhydrophobicity against different pH value of water; d: SEM graphics of damaged Cu mesh after contacting acid solution.

growth of nanoneedle arrays on the surface by electrochemical anodization. These nanoneedle arrays provided the micro- and nanostructures for the surface superwettability. The length of the nanoneedles was ∼10 μm, and the tip diameter was ∼0.3 μm, as shown in the inserted SEM images (Figure 5c). The appearance and dimension of the nanowires grown on the surface of Cu wires were similar to those reported for the anodized nanowires on a Cu foil,43 confirming that the shape of the Cu substrate did not affect the anodization. The as-prepared anodized Cu mesh exhibited superhydrophilicity. As shown in Figure 5d, the water droplets on this surface spread quickly on the Cu wire mesh and penetrated the mesh in 10 s. 3.2. Characterization of Superhydrophobic Cu Mesh. The superhydrophilic Cu mesh was modified with silane FAS17, affording a superhydrophobic Cu mesh. Figure 6 shows the surface features of the as-prepared superhydrophobic Cu mesh for granulation application. The SEM images shown in Figure 6a confirmed that the modification did not significantly affect the morphology of the anodized nanowires. The XRD patterns of the as-prepared species indicated the presence of monoclinic CuO on the anodized Cu mesh (Figure 6b), belonging to the group C2/c (No. 15) with lattice constants a = 4.685 Å, b = 3.426 Å, and c = 5.130 Å. The characteristic peaks of Si−O−Si bonds at 810 and 1070 cm−1, CF3 groups at 1373 cm−1, and CF2 groups at 1200 cm−1 in the FT-IR spectrum of the modified Cu mesh44 (Figure 6c) indicated that the fluorinated silane with a low surface free energy was successfully immobilized on the Cu mesh surfaces. The determined water static CA of 156 ± 2° shown in Figure 6d and rolling angle of 4 ± 1° shown in Figure 6e confirmed the superhydrophobicity of the modified Cu mesh. The surface

the static CA. The surface morphology of the Cu mesh and formed molecular sieve granules was observed using a scanning electron microscope (SEM, JEOL, JSM-5900LV) and a polarizing microscope (Nikon, Eclipse Lv100pol). The crystal structure of the sample was obtained using an X-ray diffractometer (XRD, Karaltay, DX2700). The presence of FAS-17 was confirmed from the absorption spectra obtained using a Fourier transform infrared (FTIR) spectrometer (PerkinElmer, Spectrum Two, L1600300). The natural repose angle of the used molecular sieve powder was determined using a BT-1000 powder integrative characteristic tester (Bettersize Instruments Ltd., China). The compression strength of the asprepared spherical granules was determined using a ZQJ-II100N intelligent particle strength tester; the testing program was carried out following the Chinese Chemical Industry Standard HG/T 2782-2011.

3. RESULTS AND DISCUSSION 3.1. Anodization of Cu Mesh and Characterization. The polished Cu mesh bowl was anodized with a constant current in an alkali solution to obtain a subtle microstructure on the surface. Figure 5a shows the voltage change during the anodization of Cu mesh bowls with diameters of 6 and 12 cm. With the increase in the diameter of the Cu mesh bowl, the time required for reaching the constant voltage during the anodization proportionally increased. However, the voltage before and after the saltation point almost remained the same. The smooth surface of the Cu wires for mesh weaving shown in Figure 5b confirmed that no notable change occurred during the pretreating of the Cu mesh. The nanowires grown on the surface of the Cu wires shown in Figure 5c confirmed the E


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Figure 8. Device and product of rolling spheronization process on superhydrophobic Cu mesh. a: Adhesion tests of molecular sieve on superhydrophobic surface; b: Schematic diagram of the homemade device for rolling-spheronization granulation process and its working mechanism; c: the sphere product with different solid contents; d: SEM results of as-prepared sieve sphere granule and its surface.

damaging effect of the strong acidic solution to the microstructure of superhydrophobic Cu surface, as shown in Figure 7d. The shortening and destructive sticking and passivation of nanowires can be observed in comparison with the morphology of the original nanowires as shown in Figure 6a. The above-mentioned results confirm that the as-prepared superhydrophobic Cu mesh is an ideal alternative surface for the granulation of molecular sieve because of its high stability and wide adaptability. 3.4. Adhesion on Superhydrophobic Cu Mesh. The adhesion experiment was carried out by comparing the residual mass of the pristine Cu and superhydrophobic Cu mesh bowls in the exactly same dimension. The average mass increment of the superhydrophobic Cu mesh bowl was only 0.16 wt %, far less than 112 wt % mass increment of the pristine Cu mesh bowl. Hence, it was difficult to determine the adhesion force of the molecular sieve slurry on the superhydrophobic Cu mesh, as 0 N adhesion force indicates almost no adhesion affinity between the slurry and bowl. On the other hand, the average adhesion force between the slurry and pristine Cu mesh was as high as 0.81 N. These results are consistent with the optical images shown in Figure 8a: A negligible amount of the residual mass was observed in the black-colored superhydrophobic Cu mesh bowl, whereas a substantial amount of slurry was left inside the pristine Cu mesh bowl. The antiadhesion feature of the superhydrophobic surface is very important for its practical applications in wet granulation. 3.5. Rolling-Spheronization on Superhydrophobic Cu Mesh. A homemade device for rolling-spheronization granulation on a superhydrophobic surface was assembled as shown in Figure 8b. Inclination angle of the tray for granulation, θ, was selected arbitrarily as 38° to lower the vertical height for the

free energy of the superhydrophobic surfaces was estimated to be 0.2 mJ/m2 following the literature method.43 Clearly, the low surface free energy and the high surface roughness of the nanostructured surface endowed the desired superhydrophobicity for the Cu mesh. The uniformity of the superhydrophobicity of the modified Cu mesh was deduced from the random distribution of the water droplets at different locations on the surfaces of the asprepared species, as shown in the optical images (Figure 6f). 3.3. Stability and Adaptability of Superhydrophobicity. Because the as-prepared superhydrophobic Cu mesh was expected to be used in wet granulation, it is important to determine the stability and adaptability of such surfaces under the operating conditions. The good stability of the as-prepared superhydrophobic surface was ascertained by the result shown in Figure 7a, exhibiting only a slight decrease in the static water CA to

Preparation of Superhydrophobic Cu Mesh and Its Application in

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