PVDF-HFP Composite 3D

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One Step Dipping Fabrication of FeO/PVDF-HFP Composite 3D Porous Sponge for Magnetically Controllable Oil-Water Separation Jiatu Li, Mizuki Tenjimbayashi, Nicole S. Zacharia, and Seimei Shiratori ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02035 • Publication Date (Web): 01 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Sustainable Chemistry & Engineering

One Step Dipping Fabrication of Fe3O4/PVDF-HFP Composite 3D Porous Sponge for Magnetically Controllable Oil-Water Separation Jiatu Li†, Mizuki Tenjimbayashi†, Nicole S. Zacharia‡, and Seimei Shiratori†* †

Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1

Hiyoshi, Yokohama, 223-8522, Japan. ‡

Department of Polymer Engineering, University of Akron, 302 Buchtel Common, Akron, OH 44325

U.S.A. *

[email protected]

ABSTRACT Industrial oil spills in various bodies of water is a worldwide environmental problem that requires effective oil absorbents with remote controllability, which would be a clear improvement upon currently used technologies. One approach for adding remote controllability is embedding magnetic particles into the oil absorbent materials, however, there are currently few reports of magnetic oil absorbents and most of them are prepared through multi-step processes or using hazardous materials, which inhibits their practical use. In this study, we introduce a single step dipping method to simultaneously provide both magnetic and hydrophobic/oleophilic functions to melamine foam by combining the hydrophobic flexible co-polymer

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poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), and magnetic Fe3O4 nanoparticles synthesized by a method suitable for mass production. The as-fabricated coating performs effectively for oil collection on oil spilled on water, and the movement of the foam on the water’s surface can be effectively controlled under magnetic field without touching it directly. Also, the coating is capable of regenerating its oil absorbing property by being wrung out after the initial absorption owing to its flexibility and separation of oil in a water-in-oil emulsion. Such a simple method for the creation of multifunction material could potentially be helpful for the development of commercial remediation materials. KEYWORDS; superoleophilic, magnetic manipulation, one step dipping method, water purification, surface chemistry

INTRODUCTION Oil spills can be a severe environmental problem for many species in the world including human beings.1 For instance, the disastrous BP Deepwater Horizon oil spill in the Gulf of Mexico in 2010 severely damaged not only the marine environment such as the coastline and seawater but also a range of living organisms.2 It takes an enormous amount of time and cost to collect oil, which continuously spreads on the air/water interface owing to interfacial energy considerations and to recover from such a disaster. Thus, the development of highly efficient oil absorbent materials as well as oil/water separation techniques is required for solving such severe environmental problems. Recently, many researchers have reported functional materials with superwettability for many applications such as self-cleaning,3-5 anti-icing,6,7 anti-fogging,8,9 liquid transportation10,11 and oil-water separation.12-14 There are several kinds of material with superwettability for oil-water separation such as porous materials, filters15-17 and chromatography columns.18 Porous materials with selective wettability


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are useful for selectively absorbing a target liquid from a mixture and maintaining that target liquid inside of itself. However, 2D substrates such filters or meshes are always too thin to possess high volume capacities. In contrast, 3D porous materials including polyurethane sponges or biomass aerogels have a large surface area and high adsorption capacity so that they provide extensive reaction sites for oil adsorption.19-25 Compared with existing methods such as carbon fibers or mineral materials, this technique presented here is energy saving, safe, and cost-effective for facing problems such as largescale oil spills.26,27 Well known methods to fabricate 3D porous materials with selective wettability include tethering alkyl or fluorine groups on the outermost surface to achieve superhydrophobicity and superoleophilicity,28,29 or adding hydroxy groups to achieve superhydrophilicity and superoleophobicity in raw materials.30 In addition to these previously mentioned chemical functionalizations, control over the micro- /or nanoscale surface roughness of a material’s surface is crucial to controlling the wettability on that surface. Using the methods mentioned above, very high separation efficiencies have already been achieved (> 99%,31 even including successful emulsion separation).32,33 Although there are many publications on superwettability for oil absorbance, most of these works are still unsafe and impractical for clean oil collection. Recent interest has been shown in adding other functions suitable for practical application such as reusability, durability, transparency and remote controllability.24,34-36 The ability to have remote control over the material specifically is essential for water remediation in toxic environments in order that people can avoid entering these hazardous areas and health risks while collecting the spilled oil.37,38 This type of functionality also helps to maintain the cleanliness of the device for controlling the absorbent as it will work from a distance and will allow for the safe, remote collection of spilled materials. To impart remote controllability to an oil absorbent, the introduction of magnetic force is a promising approach.39 The addition of magnetic particles inside 3D porous materials may allow the materials to be carried by a magnetic drone, a crane truck with a magnet, or an external magnetic field, which allows oil collection to be performed safely without touching the contaminated water or the ACS Paragon Plus Environment

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absorbent material. There are a few existing reports of magnetic oil absorbent materials. Du et al. reported the fabrication of particles/sheets hybrid dimensional magnetic microstructures by dipping, a two-step heating process, a two-step freezing treatment and finally a vapor deposition.40 Wu et al. reported the fabrication of a polyurethane@Fe3O4@SiO2@fluoropolymer sponge in 3 steps.25 These different methods are expensive and complex and usually require multiple steps (at least two processes) and a high temperature; the fabrication procedures includes both (i) addition of magnetic particles and (ii) adding hydrophobic components.41 This reduces the potential for mass production and practical application of these materials. Herein, we propose a single step dipping fabrication method of a magnetically controllable oil absorbent with a hydrophobic resin/magnetic nanoparticle composite. These functionalities were imparted to a 3D porous material simply by dipping that material into a solution containing magnetic nanoparticles and a hydrophobic co-polymer (i.e. a one-step dipping method). Fe3O4 nanoparticles were synthesized by a spray method which requires shorter times than conventional coprecipitation methods, is suitable for mass-production and is used to provide magnetic properties and well as to increase the surface roughness of the foam to control the wettability.36,42,43 Although some researchers have reported achieving water repellency by treating magnetic particles, there are few reports regarding the synthesis of magnetic particles themselves. The quick synthesis of Fe3O4 nanoparticles and simple process of inducing both hydrophobicity and magnetism can simplify the practical fabrication of oil absorbents. A fluorinated co-polymer was used in this work, poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), which has a low surface energy of 25 mN/m achieving hydrophobicity and also an ability to adhere the magnetic particles to the 3D porous material.44 A melamine sponge was chosen as the 3D porous material having many properties that are suitable for use as an absorbent such as thermal resistance, elasticity and chemical stability.45-47 The Fe3O4/resin composite coated sponge exhibited a high hydrophobicity and remote


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controllability by a magnetic force. It could float on water and be controlled without being touched. In addition to these properties, the absorbed oil could be easily extracted so that the sponge could be reused by merely squeezing owing to its flexibility which originates from the elastic properties of both the melamine sponge and PVDF-HFP. The resultant material could be remotely manipulated, a functionality that is not wellrepresented in the literature. The remote controllability of the oil absorbent could potentially lower the cost of the oil collection process in polluted areas. This method will resolve many difficulties that hamper the practical application of oil absorbents, such as the use of toxic materials, limited preparation structure and weak magnetism.

EXPERIMENTAL SECTION Materials. Commercially available melamine sponge was purchased from Seria (Mizu-pika cube, Yokohama, Japan). FeCl3・6H2O was purchased from Yoneyama Yakuhin Kogyo Co., Ltd. (Osaka, Japan), FeSO4・7H2O, 28 wt.% NH4OH aq., oil red O (C26H24N4O), and oleic acid (C18H34O2) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). Poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-HFP, (-CH2CH2-)m[-CF2CF(CF3)-]n, Mw ~400,000, Mn ~130,000, m:n = 10:1 (molar ratio)) was purchased from Sigma-Aldrich Co., LLC (St. Louis, America). Ethyl-2cyanoacrylate was kindly provided by Toagosei Co., Ltd. (Tokyo, Japan). THF (C4H8O), toluene (C7H8), hexane (C6H14), cyclohexane (C6H12), and petroleum ether were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Gasoline (grade: 90-91 octane) was purchased from a gas station. Deionized water with a high resistivity (> 18.2 MΩ) was obtained from a three-stage Millipore Mill-Q Plus 185 system (Academic) and was used in all experiments.


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Synthesis of Fe3O4 Particles. The Fe3O4 particles were fabricated by a conventional drop coprecipitation method or a spray coprecipitation method suitable for mass-production in a short time. A solution of 1.39 g of FeSO4・7H2O and 2.70 g of FeCl3・6H2O dissolved in 50 g of deionized water was mixed with NH4OH aq. In the conventional drop method, 20 mL of NH4OH aq. was added in 20 portions at 10 min intervals with stirring. For the spray method, the precursor aqueous solution was directly sprayed into ammonia at a pressure of 0.7 MPa and the spray distance was 18 cm. The mixture was separated into supernatant and precipitation layers by a magnet. After removing the supernatant layer, deionized water was used to rinse the deposition three times. The deposition was dried under vacuum for 12 hours.

Preparation of Magnetically Controllable Oil Absorbent. First, 20 g of THF containing different weights (0.2g, 0.5 g, and 0.7 g) of PVDF-HFP was stirred at 60℃ for 1 h. The mass concentration of PVDF-HFP was 1.0 wt%, 2.4 wt%, 3.4 wt%, respectively. After PVDF-HFP was dissolved, 0.1 g of Fe3O4 particles (fabricated by the spray method) were added to the solution and sonicated by ultrasonication (ASU-3M; AS ONE Co., Osaka, Japan, oscillatory frequency of 42 kHz, output power of 80 W) for 3 min. Finally, melamine sponge (2×2×2 cm) was immersed in the solution containing the Fe3O4 dispersion for 4 h (sonicated for 3 min every hour) and dried in air for 12 h to evaporate the absorbed THF.

Characterization. The particle size distribution of Fe3O4 was measured by a particle counter (LA-960, HORIBA, Japan). The images of the Fe3O4 particles were taken using a scanning electron microscope (SEM, INSPECT F50, FEI, America) and transmission electron microscope (TEM, Tecnai G2 F20, FEI, America). The images of the samples’ surface were taken using an optical microscope (VH-Z500R, KEYENCE, Japan) and a scanning electron microscope (SEM, INSPECT S50, FEI, America). The chemical bonds within the sponges were examined by Fourier transform infrared spectroscopy (FT-IR, ACS Paragon Plus Environment

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ALPHA-T, Bruker, Billerica, MA, USA). Energy dispersive X-ray spectrometry (EDX) was conducted using an attachment to the SEM. The magnetic curve was taken using a magnetic property measurement system (MPMS, Quantum Design, America).

Measurement of Water and Oil Contact Angles. Water and oil contact angles were measured as part of an assessment of wettability. Water and oil (toluene) drops were released onto the upper surface of the sample at 5 different points. The volume of each drop was 10 µL. The droplets were captured by a CCD camera and analyzed with the Image J software (U. S. National Institutes of Health, Bethesda, Maryland, USA).

Oil Absorbency and Reusability. First, the weight of the dry sample was measured before it absorbed oil. Then the sample was immersed in toluene at room temperature for 1 min and the weight was measured after it was taken out of the solution using a microbalance (accuracy of 0.1mg). After it was squeezed, the sample was weighed again to check its performance for the removal of the absorbed oil, which was related to its reusability. The sample was sequentially immersed in toluene and squeezed 10 times. The mechanical strength of the uncoated sponge and fabricated samples was evaluated by 50 cycles of compression and release. The separation efficiency of the oil-water mixture was calculated by the following equation: separation efficiency [%] =

×100

(1)

in which wcollected and woil are the weight of the oil collected by the absorbent and oil, respectively. The absorbency of various oils was also examined. At least three measurements were conducted for each sample and the average value was presented.


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RESULTS AND DISCUSSION Synthesis of Fe3O4 nanoparticles TEM images of the Fe3O4 nanoparticles fabricated by the two methods are shown in Figure 1a and b. Although the coprecipitation method is a popular method for fabricating Fe3O4 particles, a spray method was also employed. This method requires shorter times and is better suited for mass-production of particles. The average size of the Fe3O4 particles fabricated by the conventional coprecipitation method was approximately several hundred nanometers, whereas the particle size distribution was broad with particles as large as approximately 100 µm owing to aggregation. The size distribution of the Fe3O4 particles fabricated by the two different methods was measured by a particle counter (Figure S1). The average particle size is 9.4±1.9 nm for the drop method, and 9.3±1.5 nm for the spray method (Figure S2). The main aim of coprecipitation is to simultaneously precipitate several kinds of insoluble salts, which then results in particulate matter with high uniformity. The two graphs in Figure S2 indicated almost the same size distribution, but the average particle size of the spray method was a little bit smaller than that of the conventional method. This was because the sprayed Fe3O4 precursor droplet was apparently smaller than the ammonia droplet used in the conventional method during the coprecipitation.48 The reaction of the yellow precursor solution and clear ammonia solution happened immediately, and the solution changed into a nearly black, opaque liquid. Almost the same distribution of Fe3O4 particle size was achieved, which is useful for mass-production on an industrial scale.

Morphology & Surface Wettability The magnetic oil absorbent was fabricated according to the scheme in Figure 2a. The diameter of the melamine fibers was approximately 5 µm and the pore size was approximately 100 µm. Because the


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melamine sponge has large macropores, the dissolved PVDF-HFP and dispersed Fe3O4 particles penetrated the sponge very easily. It was also observed that the pores are mainly unblocked after deposition, allowing oil to be taken in by the sponge. After taking the sponge out from the solvent, the PVDF-HFP containing Fe3O4 particle coating hardened around the fibers as the THF evaporated. The surface morphology of the plain, uncoated melamine sponge and the coated sample are shown in Figure 2b-e. PVDF-HFP was observed as a white film that adhered to the melamine fibers. Fe3O4 was observed as black particles adhered on the surface and the inner structure. A large particle was observed in Figure 2e, which was even larger than the aggregated Fe3O4 particles seen in Figure 1, and attributed to the aggregation of Fe3O4/PVDF-HFP. The Fe3O4 particles covered with a PVDF-HFP film or aggregated with PVDF-HFP retained inside the absorbent even after it was used for oil-water separation. Because large particles sink in the bottom of the solvent, they were not included in the 3D porous material. The surface chemistry of the resultant material was analyzed with results shown in Figure 3. The surface was composed of melamine fibers (hydrogen, carbon, nitrogen and oxygen), PVDF-HFP (hydrogen, carbon and fluorine) and Fe3O4 particles (oxygen and iron). The analysis of the components was also conducted, as shown in Figure S3. The FT-IR data showed that a new peak at 875 cm-1 (corresponding to the C-H bending vibration) was present in the spectra of the melamine sponge containing 1.0 wt% PVDF-HFP solution (Figure 3a). The presence of fluorine and iron (Figure 3b) indicated that both the PVDF-HFP and Fe3O4 particles are present on the sample’s surface. As shown in Figure 3c, the elements mentioned above were all included in the material. Most of the melamine fibers were firmly covered with PVDF-HFP and Fe3O4 particles and the surface roughness was increased owing to the presence of Fe3O4 particles. Considering the thermodynamic stability and volume ratio, PVDF-HFP which has lower surface tension than the Fe3O4 can exist on the surface of the coating. There were also Fe3O4 particles entirely coated with PVDF-HFP which increased the overall magnetic performance. The contact angle is an important characterization technique for discussing wettability. Static water contact angles (WCA) and oil (toluene) contact angles (OCA) on the melamine sponge surface ACS Paragon Plus Environment

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were measured (Figure 4). This included the samples of the uncoated melamine sponge, the melamine sponge coated with PVDF-HFP only, and the melamine sponges coated with both PVDF-HFP and Fe3O4 nanoparticles. The OCA values of the three samples were 0°, namely, they were superoleophilic and oil completely spread on these surfaces. However, the plain melamine sponge absorbed water as expected, the PVDF-HFP-coated samples repelled water and showed high hydrophobicity (> 120°). This change in wettability resulted from the combination of fluorine-containing polymer (PVDF-HFP) and surface roughness of the particles (synergistic effect of nanoparticles and melamine sponge). There was little change in the WCA in the presence of Fe3O4 nanoparticles. The hydrophobicity and the surface roughness were enhanced by the Fe3O4 nanoparticles, which were smaller than the melamine fibers. The WCA of the samples containing different PVDF-HFP concentrations are shown in Figure 4. The sample prepared with 3.4 wt% PVDF-HFP immersion liquid incorporated the most hydrophobic material of the three; however, the sample prepared with 2.4 wt% PVDF-HFP showed the highest contact angle. The addition of PVDF-HFP clearly increased the hydrophobicity; however, too much PVDF-HFP filled the pores of the melamine sponge and decreased its surface roughness and wettability. By comparing Figure 2c with Figure S4, it can be seen that the pours were filled as the concentration of PVDF-HFP increased. At higher deposition concentrations the PVDF-HFP formed sheet-like structures as shown in Figure 2e and did not contribute to increase surface roughness.

Magnetic Force Analysis Magnetic curves of the fabricated samples with different PVDF-HFP concentrations at 300 K are shown in Figure 5a. All three samples showed ferrimagnetism with no hysteresis after removal of the external magnetic field, which is the same property as standard Fe3O4 particles, meaning that there is no loss of magnetic property in the composite material. From among the three samples, the sample prepared with ACS Paragon Plus Environment

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2.4 wt% PVDF-HFP showed the strongest magnetism of 4.91 emu/g. The weight of the samples prepared with PVDF-HFP concentration of 1.0 wt%, 2.4 wt% and 3.4 wt% in the dry state was 0.14 g, 0.30 g and 0.33 g, respectively, a piece of plain sponge weighed 0.08 g, so the weight of the coatings (Fe3O4 particles + PVDF-HFP) was 0.06 g, 0.22 g and 0.25 g, respectively. The weight did increase as the concentration increased but the sample prepared with 1.0 wt% PVDF-HFP had an extremely low weight. This was because a limited amount of Fe3O4 particles were adhered. It is desirable that as many magnetic particles are attached to the sample as possible to enhance the magnetism, as some critical ratio of PVDF-HFP to Fe3O4 particles is required. However, the addition of too much PVDF-HFP not only increased the sample’s weight, but it also diminished its magnetism because the propotion of magnetic particles was more influential than the total content. A sample containing many magnetic particles does not necessarily mean it has large magnetism if there are also other impurities that have no magnetic character. Because the fabricated samples contained magnetic particles, they could be quickly lifted using a magnet, as shown in Figure 5b. The upper surface of the pictures shows the bottom when dipped and dried so that magnetic particles which were deposited in the fabrication process could be utilized. The absorbent could be controlled without being touched and applied for remotecontrolling the collection of oil. Because of the presence of Fe3O4, the material can be magnetically controlled over the surface of the water surface by a magnet, as shown in Figure 6a. A red dyed oil phase, toluene, was floated on uncolored water and the absorbent was magnetically operated to collect the oil without directly touching it. In addition to the magnetic property of the absorbent, the material also exhibited hydrophobicity and floated on water. These unique properties are helpful to enable remotely controllable oil absorption and a brand-new oil-water separation method. The reusability of the coated samples is shown in Figure 6b. All the samples maintained their


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absorbtive capacity after 10 oil absorption/desorption cycles. As the concentration of the precursor solution decreased, the oil absorbency increased because the mass of the absorbent decreased whereas the volume of absorbed oil was almost the same as the cube size when changing the concentration. Assuming that the sample volume was the same after completion of oil absorption, it was the case that the sample fabricated with 1.0 wt% PVDF-HFP had the largest weight change before and after absorbing oil. The addition of 1.0 wt% PVDF-HFP was still enough to achieve hydrophobicity which was shown in Figure 4. The material also had good compression capability equivalent to the uncoated sponge (Figure 6c). The oil-water separation efficiency of the oil/water mixtures containing different oils was >95% (Figure 6d). The samples repeatedly absorbed aproximately 20 times their own weight and the capacity was the same level that has been reported in previous studies.25 There is certainly an advantage in being able to achieve the same capacity by using a more straightforward fabrication method. The oil absorbency of the fabricated sample for various oils was measured, showing a weight gain of 29.23-43.73 times their weight as shown in Figure 6e, which is higher than similar previous reports. The oil absorbency of the fabricated sample for toluene, hexane and petroleum ether was 43.73 g/g, 30.28 g/g and 29.23 g/g, respectively, compared with 20.19 g/g, 20.42 g/g and 19.93 g/g, respectively, reported in similar studies.

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A comparison of the performance of these materials with

other materials reported in the literature is presented in Table 1. Compared with other magnetic oil absorbents, the materials prepared in this study not only had good oil absorption performance but were also cost-effective. The fabrication of other materials reported in the literature required a high temperature (the highest one required a temperature of 400℃) and multiple processes processes for fabrication, whereas our dipping method can be conducted in one-step and at a low temperature (60℃). The water absorbency of the uncoated melamine sponge was also examined to estimate the maximum adsorption capacity of the composite, which was 92.32 [g/g]. This was much higher than the


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oil absorbency of the fabricated samples because water has a higher density compared to this series of oils. The physical parameters of the various oils used is compared in Table S1 and the correlation between density, surface tension of the oil and its absorbency is shown in Figure S5. In this study, the degree of absorbance was mainly dependent on the density and surface tension of the oil. This absorbent can also be used for extracting pure oil from a water-in-oil emulsion (Figure 7). This reusability and selective oil absorption performance can be helpful for practical application.

CONCLUSIONS The fabrication of a magnetic and hydrophobic/oleophilic melamine sponge was achieved by a single dipping step, adhering Fe3O4 nano and microparticles on the melamine sponge by using PVDF-HFP. Fe3O4 particles were synthesized by a method suitable for mass-production. The PVDF-HFP film formed across the pores defined by the melamine fibers and Fe3O4 particles appeared within and at the surface of the film, acting to increase the surface roughness and changing the film wettability. The fabricated sample exhibited hydrophobicity, superoleophilicity, magnetic controllability and reusability. The foam could be manipulated remotely by a magnet and used as an oil absorbent without touching the oil. Such a simple method of designing a functional unique wetting system can be quickly increased for practical use and helpful for the development of water remediation materials.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng. ####### Distribution of Fe3O4 particle size (containing aggregate), distribution of Fe3O4 particle size, EDX spectra, and optical images of magnetic oil absorbent AUTHOR INFORMATION Corresponding Author *[email protected]

Present Addresses †Center for Material Design Science, School of Integrated Design Engineering, Keio University, 3-14-1 Hiyoshi, Yokohama, 223-8522, Japan.

Author Contributions J.L. designed the experiment, wrote a paper, conducted the experiment, and analyzed the data. M.T. and N.Z. discussed the data and provided scientific advice. S. S. supervised the project, provided scientific advice and commented on the manuscript. Funding Sources This work was partially supported by JSPS KAKENHI (grant number JP 16J06070, JP 17K04992) and JST SENTAN (grant number YYN6031). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We are grateful to our laboratory member whose meticulous comments were of enormous help.


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FIGURES

Figure 1. (a) SEM images and (b) TEM images of Fe3O4 particles synthesized by a conventional method, and (c) SEM images and (d) TEM images of Fe3O4 particles synthesized by a spray method.


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Figure 2. (a) Schematic illustration of the one-step dipping fabrication of Fe3O4/PVDF-HFP composite 3D material, (b) optical images of plain melamine sponge (4×4×3.2 cm), (c) optical images of the magnetic oil absorbent (fabricated with 3.4 wt% PVDF-HFP solution, 2×2×2 cm), (d) SEM images of


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plain melamine sponge, and (e) SEM images of the magnetic oil absorbent. Melamine fibers were bent by the adhesion of the PVDF-HFP film.


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Figure 3. (a) FT-IR spectra of plain melamine sponge (MS) and MS with 1.0 wt% PVDF-HFP solution, (b) EDX spectra of the surface of the magnetic oil absorbent (3.4 wt% PVDF-HFP solution), (c) elemental distribution of carbon, nitrogen, oxygen, fluorine and iron.


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Figure 4. Water contact angles of uncoated melamine sponge (MS), MS with PVDF-HFP (2.0 wt%) and MS with PVDF-HFP (1.0 wt%, 2.4 wt% and 3.4 wt%) and Fe3O4 magnetic particles (MP). All water droplets used were 10 µL in volume. The uncoated melamine sponge showed hydrophilicity and samples containing PVDF-HFP showed hydrophobicity.


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Figure 5. (a) Magnetic curves of the fabricated samples with three different PVDF-HFP solutions (1.0 wt%, 2.4 wt% and 3.4 wt%) at 300 K. (b) Fabricated sample (1.0 wt% PVDF-HFP solution) lifted by a neodymium magnet without directly touching it. White arrows and black arrows indicate the gravitational force (Fg) applied to the sample and the magnetic force (FM), respectively. The green arrows indicate the direction that the magnet is going to move. As the magnet gets closer to the sample, the magnetic force applied to the sample become larger. The sample is lifted as soon as the magnetic force becomes dominant. The sample was put upside down for utilizing deposited Fe3O4 particles in the bottom while dipping.


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Figure 6. (a) Fabricated oil absorbent (1.0 wt% PVDF-HFP solution) floated on uncolored water and magnetically controlled by a magnet to collect red-dyed oil phase (toluene). (b) Weight changes by absorbing (green line) and squeezing (blue line) of each sample prepared with different PVDF-HFP solution (1.0 wt%, 2.4 wt% and 3.4 wt%). (c) Cyclical pressing measurements for uncoated MS and fabricated oil absorbent. (d) Oil-water separation efficiency and (e) oil absorption capacity of the fabricated oil absorbent (1.0 wt% PVDF-HFP solution) towards different oils.


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Table 1. Comparison of magnetic oil absorbents

sorbent hydrophobic/magnetic materials

oil

hexane polyurethane sponge Fe2O3/methyltrichlorosilane crude oil

required highest sorption number temperature in capacity [g/g] of steps for fabrication fabrication [℃] 62 89

crude oil

4

diesel

4

polyurethane sponge Fe3O4/fluoropolymer

petrol

13

polyurethane sponge Fe3O4/straw soot

lubricating oil 30 waste

polyester sponge sheet Fe3O4/TEOS, HDTES

melamine sponge Fe3O4/PVDF-HFP

petroleum ether 20

toluene

44

petroleum ether 29

ref

3

400

39

2

70

42

3

110

25

3

70

41

1

60

present work


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Figure 7. Photographs of a water/toluene emulsion (1:10 w/w) (a) before and (b) after purification. Optical images of a water/toluene emulsion (1:10 w/w) (c) before and (d) after purification. (e) Color histogram for a water/toluene emulsion (1:10 w/w) before and after purification. The water-in-oil emulsion was prepared by mixing water and toluene in 1:10 w: w and sonicated by ultrasonication for 3 min.


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Table of Contents

One-step dipping fabrication of remote-controllable oil absorbents for mass production and practical application.


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PVDF-HFP Composite 3D

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