- Email: [email protected]
water separation
Applied Surface Science 481 (2019) 883–891
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Surface modification of polyamide meshes and nonwoven fabrics by plasma etching and a PDA/cellulose coating for oil/water separation
T
⁎
Pei Zhao, Ning Qin, Carolyn L. Ren, John Z. Wen
Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: A hybrid coating of PDA/cellulose Plasma etching Oil/water separation Meshes and nonwoven fabrics Underwater oleophobicity Durability
This work investigated a two-step surface modification of polyamide meshes and nonwoven fabrics for oil/water separation and looked into the durability of such modified polyamide. The two-step modification included 1) pre-etching the polyamide surface using plasma treatment and 2) coating the pre-etched surface by eco-friendly polydopamine (PDA)/cellulose. The pre-etching increased the surface roughness, which further improved the underwater superoleophobicity of the coating. Therefore, the modified polyamide was able to separate various oil/water mixtures and showed a higher intrusion pressure than the original sample and the samples which were only etched or only coated. The grooves on the surface that resulted from the pre-etching prevented the coating from peeling off. In durability tests, after 6 repeated uses, the modified nonwoven sample lost its underwater oleophobicity due to severe oil fouling, coming to a complete failure in oil/water separation. After 19 cycles, the modified mesh was still able to separate a certain amount of oil/water but showed reduced intrusion pressure because of slight oil contamination. Filters with different structures, like meshes with one layer of pores and nonwoven fabrics with complex three dimensional pores, had different oil fouling levels that affected oil/water separation. The recoverability of filters from oil contamination should be considered for practical applications.
1. Introduction Expanding oily wastewater from industry and increasing oil leakage have driven extensive studies of oil/water separation. Novel filters with opposite wettability to water and oil, such as hydrophobic/oleophilic, underwater oleophobic or underoil hydrophobic, selectively allow one of oil/water to penetrate and resist the other [1–6]. The wetting properties of the filters depend on the surface materials and topography [7,8]. Filters (metal or polymer) (meshes, membranes or sponges) possessing underwater oleophobicity have been widely used for gravitydriven oil/water separation since water is heavier than most types of oils. And the underwater oleophobicity is considered to make the solidwater-oil interface antifouling [9,10]. Some polymer filters are etched by using plasma with a high power (100 W–600 W, a treatment time of 2–6 min) to achieve superoleophobicity (> 150°) in water [11–14]. Plasma treatment could create an increased roughness on the polymer surface, which enhances the oleophobicity in water according to Wenzel or Cassie models [15–17]. Plasma with a low power (30–100 W, a treatment time of 5–60 s) doesn't work for etching polymer but can activate the surface with some functional groups of –OH and –COOH, which is used for temporarily changing the wettability of the surface or for subsequent bonding with other materials [18]. Many filters are ⁎
modified by special coatings with underwater oleophobic properties, such as hydrogel, graphene oxide, cellulose, polydopamine (PDA) and tetrahydrofuran oxidized PDA [19–28]. The underwater oleophobic filters have been developed either by the surface construction or by special coatings. In order to combine the required properties of the coating and the large roughness, we propose a two-step strategy that would lead to advanced filters for efficient oil/water separation: firstly, the filter surface is etched by using plasma; secondly, the pre-etched surface is coated by a special coating. Regarding the special coating that would be applied onto the preetched surface of filters, we propose a hybrid coating of PDA/cellulose. The PDA/cellulose is eco-friendly, which would be beneficial for environmental protection. PDA is a harmless bio material that adheres to various surfaces [29–31]. Additionally, PDA has been used as a coating material for oil/water separation, because it has the underwater oleophobic property [26,27]. The underwater oleophobicity of materials increases with the increase of the hydrophilicity [32,33]. However, PDA shows limited hydrophilicity, e.g. the water contact angle (WCA) on PDA coated glass or silicon wafer is around 55° [34], the WCA on PDA coated PVDF membrane is ranged from 110° to 55° depending on the coating time [26], and the WCA on PDA coated steel mesh is around 67° [35]. In order to reach superhydrophilicity and underwater
Corresponding author. E-mail address: [email protected] (J.Z. Wen).
https://doi.org/10.1016/j.apsusc.2019.03.152 Received 8 November 2018; Received in revised form 6 March 2019; Accepted 15 March 2019 Available online 18 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
superoleophobicity, PDA is sometimes used as “bio-glue” to fabricate hybrid coatings through a mussel-inspired one-step method where superhydrophilic species mixed with dopamine (DA) are immobilized onto the substrate during the PDA formation, such as PDA/TEOS [25], PDA/LPAA [35], PDA/PEI [36] and PDA/KH560 [37]. For developing eco-friendly coatings that have less environmental concern for large scale fabrication and applications, we propose the hybrid coating of PDA/cellulose. Cellulose with plenty of hydroxyl groups is superhydrophilic and has attracted much attention for its good bio-compatibility [23,24]. Therefore, we propose a hybrid coating of PDA/cellulose, which is eco-friendly and would be underwater superoleophobic. Here, we developed the environmentally friendly two-step method of surface modification for polyamide filters, which could be applied to other polymers, to achieve highly efficient oil/water separation. Polyamide meshes and nonwoven fabrics were pre-etched by using plasma and then coated by the hybrid PDA/cellulose. Such modified polyamide samples possessed superoleophobicity in water and showed increased intrusion pressure than the original, the only coated or the only plasma treated polyamide. The rugged surface resulted from the pre-etching reinforced the coating adherence and enhanced the coating stability. During repeated uses, polyamide meshes with one layer of pores and nonwoven fabrics with layers of complex pores had different oil fouling levels, which affected their oil/water separation. The mesh was more durable than the nonwoven fabric due to less oil contamination, which should be considered for practical applications.
etched polyamide (7 cm × 7 cm) was immersed into the above solution for 6 h with 600 rpm at room temperature. After that, the sample was washed by ethanol and water and dried. Then we had the original, only coated, only plasma treated and two-step modified meshes and nonwoven fabrics. 2.3. Oil/water separation A piece of polyamide (7 cm × 7 cm) mesh or nonwoven fabric was fixed under a tube with a clamp to form a filter (Fig. S1). Before the separation, water was fed into the tube to prewet the polyamide. Then the oil/water mixture (1:2, 20 ml) was poured into the tube, where water flowed through the polyamide and oil was held on top of the polyamide. It was gravity driven separation. The filtrated water was collected to evaluate the separation efficiency η, defined by (1 − C1) × 100%, where C1 is the oil concentration in the filtrated water and measured by a UV–Vis spectrometer [14]. The intrusion pressure (Pintru) of oil was obtained by keeping adding the oil into the tube till the maximum height (hmax), when the oil started to penetrate the polyamide, according to Pintru = ρoil g hmax. 2.4. Durability tests To evaluate the durability of the coated and the two-step modified polyamide, each sample was used for separation for a number of cycles. In each cycle, the sample was used to separate 20 ml heavy mineral oil/ water (1:2) and then intruded by the oil by adding more oil in order to obtain the intrusion pressure. That meant the sample was contaminated by the oil in each cycle. After each cycle, the sample was washed by ethanol and water and dried.
2. Experimental 2.1. Materials Polyamide (nylon 6.6) meshes with a pore size of 64 μm were purchased from Component Supply, Tennessee. Polyamide nonwoven fabrics (thermal bonded, PBN-II®, Type 30) with a density of 2 oz per square yard (osy) were provided by Cerex Advanced Fabrics, Florida. Heavy mineral oil, silicone oil, dopamine hydrochloride (DA) and ammonia solution with a concentration of 28–30% were bought from Sigma-Aldrich, Canada.
2.5. Characterization The microstructures and morphologies of the polyamide samples were observed by using an Atomic Force Microscopy (AFM, MultimodeTM SPM, Digital Instruments) in a tap mode and a Scanning Electron Microscope (SEM, Zeiss ULTRA Plus). The oil contact angle in water on the polyamide samples were measured using a lab-made contact angle meter, set up by a syringe needle, a side-view microscope, and a camera. Fourier transform infrared (FTIR) spectra on the sample surface were obtained with a Bruker Tensor 27 FTIR spectrometer by using the attenuated total reflectance (ATR) mode.
2.2. Surface modification The two-step surface modification for polyamide meshes and nonwoven fabrics included plasma treatment and the following coating procedure, as shown in Fig. 1. The Phantom Reactive Ion Etching system (RIE, Trion Technology) was employed as an oxygen plasma source to etch the polyamide surface (14 cm × 14 cm) for a period of 5 min with a power of 200 W. The pressure was kept as 100mTorr and oxygen was fed into the chamber at 50 sccm. In the coating procedure, 0.4% ammonia solution, 0.2% DA and 0.4% cellulose were put into a mixture of 100 ml water and ethanol (7:3). A piece of original or plasma
3. Results and discussion Microstructures of the two-step modified polyamide mesh and nonwoven are shown in Figs. 2 and 3, respectively (written as the modified mesh/nonwoven sometimes in the following). Microstructures of the original, only coated and only plasma treated polyamide mesh
Fig. 1. The schematic of the two-step modification method. 884
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Fig. 2. SEM images of the top (a, b) and side (c, d) views of the two-step modified polyamide mesh. The top view was facing plasma during the pre-etching step. Typical AFM images of the two-step modified polyamide mesh (e).
was more and more rugged when approaching the plasma. The side view of the only plasma treated polyamide also had the gradient structure (Figs. S2f and S3d–f). The plasma greatly etched the top surface of polyamide. When the plasma dived into the pores of polyamide, its power was weakened. The surface roughness of the meshes was determined by AFM analysis as shown in Table 1. Typical AFM images of the two-step modified and coated meshes are shown in Figs. 2e and S4, respectively. The AFM images of the plasma treated and original meshes were shown in another paper [14]. The two-step modified mesh achieved a very large roughness, 1.5 times of the plasma
and nonwoven are presented in Figs. S2 and S3 for comparison. The original polyamide surface was smooth. After the first step of plasma treatment, the surface became rough with many etched grooves (Figs. S2e, g, h, i and S3e). The different plasma doses didn't affect the microstructure significantly (Fig. S2e, g, h, i). After the second step of coating, PDA/cellulose was successfully coated on the pre-etched surface. The finished top surface, facing the plasma during the pre-etching, was very rough due to the etched grooves and the coated particles (Figs. 2a, b and 3a–c). The side view of the modified mesh and nonwoven presented a gradually varied structure (Figs. 2c, d and 3d–f). It
Fig. 3. SEM images of the top (a, b, c) and side (d, e, f) views of the two-step modified nonwoven polyamide. The top view was facing plasma during the pre-etching step. 885
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Table 1 The roughness of the two-step modified, plasma treated, coated and original polyamide meshes determined by AFM.
Ra (nm) r (surface/projected area)
Two-step modified
Plasma treated [14]
Coated
Original [14]
300.1 ± 95.3 1.59 ± 0.23
206.9 ± 84.6 1.44 ± 0.07
47.1 ± 16.9 1.21 ± 0.08
13.9 ± 1.5 1.07 ± 0.02
and vibrations were observed at 2923 and 2856 cm−1 [13,38]. And the peaks arising at 1633, 1535 and 1371 cm−1 were ascribed to the amide carbonyl C]O stretching vibration of the secondary amide band, the NeH bending motion, and wagging of NH amide groups, respectively [38,39]. Compared to the original mesh (or nonwoven), the modified mesh (or nonwoven) showed a broadened band at around 3300 cm−1, and the relative intensity of this band to the rest peaks was much higher. And this peak shifted a little towards the shorter wavelength from 3300 cm−1 (the original mesh) to 3293 cm−1 (the modified mesh) and from 3297 cm−1 (the original nonwoven) to 3265 cm−1 (the modified nonwoven). Those differences were caused by the coating. One of the coating materials, cellulose, had plenty of –OH, the characteristic peak of which is located at the wavelength of 3200–3600 cm−1, stronger and broader than the signal wavelength of the NH bond (3300–3500 cm−1) enriched in the original polyamide [40]. The molecule interactions between the coating materials and polyamide could have caused the little shift of this peak. When the modified samples were prewetted by water, they were able to separate water and various oils, allowing water to flow through their pores and resisting the oil (supplementary video 1 and Fig. S1). Three types of oils with different physical properties as shown in Table 2, heavy mineral oil, silicone oil, and peanut oil, were used as model oils here. Fig. 5 shows the separation efficiencies of all the polyamide samples. The error bars here and in the following figures were standard deviations. The separation efficiencies of the original polyamide were in the range of 90%–99%. The two-step modified polyamide showed the increased efficiencies of 92%–99%. The reason why the samples could separate oil/water was the underwater oleophobicity that can be proved by the oil contact angle in water (Fig. 6a). Compared to the original, plasma treated and coated polyamide mesh (or nonwoven), the two-step modified mesh (or nonwoven) had the largest oil contact angle in water due to the combination of its largest roughness and the properties of the coating. The contact angle (CA) would be affected by the surface roughness according to Cassie (cosθ = f(cosθY + 1) − 1) or Wenzel (cosθ = r cos θY) models, where θ is measured or apparent CA on a real rough surface, θY is Young CA on an ideal flat surface, f is the fraction of solid surface in contact with the droplet and r is the ratio of the actual over the projected surface area [15,16,32]. Therefore, the hydrophilicity and hydrophobicity of a surface would be improved with the increase of the
Fig. 4. FTIR spectra of the original and the two-step modified polyamide meshes and nonwoven fabrics.
treated one and 5 times larger than the coated one. Therefore, the twostep strategy brought in the required coating as well as the large roughness. Fig. 4 shows the FTIR spectra of the original and the two-step modified polyamide meshes and nonwoven fabrics. The main peaks located within the range of 1300–3800 cm−1, although a range of 500–4000 cm−1 was scanned. The peaks of the different samples were almost overlapping. Since ATR mode was used to obtain the FTIR spectra here, the detecting depth was several microns. Even for the modified samples, a considerable part of the signals were from the polyamide substrate. For the spectra of all the samples, one band was present at around 3300 cm−1, which can be assigned to the stretching of NeH bonds [13,38]. The CH2 asymmetric and symmetric stretching
Fig. 5. The separation efficiencies of the original, plasma treated, coated and two-step modified polyamide meshes (left) and nonwoven fabrics (right) prewetted by water. 886
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
sample was contaminated by the oil in each cycle. Fig. 7 shows the intrusion pressure and the oil concentration in the separated water during the duty cycles of the two-step modified mesh. After 19 cycles, the intrusion pressure began to decrease (Fig. 7a). However, the oil concentration in the filtrated water did not change much during 21 cycles (Fig. 7b). It was in the range of 8 ± 3 ppm and accordingly, the separation efficiency was 98.9%–99.5% during the 21 cycles. It was because in the tube with an inner diameter of 2.2 cm, which was used for the separation, the to be separated 20 ml oil/water mixture corresponded to a 0.15 kPa hydraulic pressure of oil, still smaller than the reduced intrusion pressure of around 0.75 kPa at the 20th and 21st cycles. That meant after 19 cycles the modified mesh started to show the decreased intrusion pressure, but it was still capable of separating a certain amount of oil/water with a high efficiency. The reason for the reduced intrusion pressure of the used two-step modified mesh was investigated. As shown in Fig. 8a, the superhydrophilicity of the new modified mesh (0 cycle) was transformed to hydrophobicity after 21 cycles and the oil contact angle in water decreased, which directly caused the reduced intrusion pressure according 2γ cos θ to Pintru = − ow d O / W . FTIR spectrum of the modified mesh after 21 cycles is shown in Fig. 8c. Compared to the new sample, the relative intensities of the peaks at 2923 cm−1 and 2856 cm−1 that were ascribed to the CH2 stretching were increased. The heavy mineral oil shows intense peaks at those locations [46]. It implied that after being intruded by the oil for cycles the modified mesh was contaminated by the oil probably inside the pores, which could not be easily cleaned by ethanol. Fig. 9a shows the microstructures of the modified mesh that had been used for 21 cycles, which were basically the same as the new one shown in Fig. 2. The coating did not fall out of the substrate. Therefore, the most possible reason for the change of the wettability and the reduced intrusion pressure could be the oil contamination.
Table 2 Oil properties at room temperature of 22–25 °C [13,41–45]. Oil
Heavy mineral oil
Silicone oil
Peanut oil
Density (g/cm3) Surface tension (mN/m) Oil/water interfacial tension (mN/ m)
0.86 28.5 ~52
0.93 21.4 38.75
0.92 34.5 ~23
roughness. Among all the polyamide meshes (or nonwoven) the two-step modified one presented the largest intrusion pressure of oil (Fig. 6b). The oil intrusion happens when the oil/water interfacial tension is not strong enough to hold or resist the oil according to 2γ cos θ Pintru = ρoil g hmax = − ow d O / W , where γow is oil/water interfacial tension, θO/W is oil contact angle in water and d is the pore size [19]. For the same polyamide sample, the intrusion pressure of heavy mineral oil was larger than that of the other two oils, as the heavy mineral oil/ water interfacial tension is the largest as shown in Table 2. A larger intrusion pressure of oil means a higher robustness of the sample for oil/water separation and a larger capacity of oil/water that can be separated each time, which was one target of the proposed two-step strategy that was achieved. Overall, the two-step modified polyamide showed improved performances of oil/water separation than the original and one-step modified (plasma treated or coated) polyamide. The durability of the two-step modified polyamide mesh was investigated by using the sample for cycles and compared with that of the coated mesh. In each cycle, the sample was used to separate 20 ml heavy mineral oil/water (1:2) and then intruded by the oil by adding more oil in order to obtain the intrusion pressure. That meant the
Fig. 6. Oil contact angle in water (a) and the intrusion pressure of oil (b) for the original, plasma treated, coated and two-step modified polyamide meshes (left) and nonwoven fabrics (right). The inset images are the underwater heavy mineral oil droplets on the polyamide samples. 887
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Fig. 7. The intrusion pressure of oil (a) and the oil concentration in the filtrated water (b) during the duty cycles of the two-step modified and coated polyamide meshes.
Fig. 8. Water drops dyed by methyl orange in air (left) and water drops in oil (right) on the two-step modified mesh (a) and on the coated mesh (b). FTIR spectra of the coated and two-step modified polyamide meshes (c).
the oil contamination after 13 cycles, indicated by the increased intensities of the peaks at 2923 cm−1 and 2856 cm−1 in the FTIR spectrum (Fig. 8c). Different from the two-step modified mesh, however, the coating partially peeled out of the substrate (Fig. 9b). Both the oil contamination and the peeling of the coating could be the reasons that caused the change of the wettability and the decreased intrusion pressure for the used coated mesh.
Similar with the two-step modified mesh, the coated mesh showed the decreased intrusion pressure after 11 cycles (Fig. 7a), but during the 13 cycles the oil concentration in the separated water stayed in a certain range of 9 ± 3 ppm (Fig. 7b) and the separation efficiency was 98.8%–99.4%. After 13 cycles, as shown in Fig. 8b, the coated mesh became hydrophobic and the underwater oil contact angle decreased, conforming to the reduced intrusion pressure. The coated mesh also had
Fig. 9. SEM images of the two-step modified polyamide mesh after 21 cycles of uses (a) and the coated mesh after 13 cycles of uses (b). 888
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Fig. 10. The intrusion pressure of oil (a) and the oil concentration in the filtrated water (b) during the duty cycles of the two-step modified and coated nonwoven fabrics.
Fig. 11. Water drops dyed by methyl orange in air (left) and water drops in oil (right) on the two-step modified (a) and the coated (b) nonwoven fabrics. FTIR spectra of the coated and two-step modified nonwoven fabrics (c).
8 ± 4 ppm and 10 ± 4 ppm, respectively. The separation efficiency was around 98.6%–99.6%. But after those cycles, the oil slowly dripped through the used nonwoven sample after water being filtrated (supplementary video 2 and Fig. S5). The oil could not be resisted anymore, because the used nonwoven was not underwater oleophobic anymore, with its oil contact angle in water being zero as shown in Fig. 11a, b. FTIR spectra of the used nonwoven samples are shown in Fig. 11c. Compared to the new sample (0 cycle), the relative intensities of the peaks at 2923 cm−1, 2856 cm−1, 1461 cm−1, and 1376 cm−1 were greatly increased. The increase was much more significant than that of the used mesh (Fig. 8c). And a new peak appeared at 2952 cm−1. All of those peaks were characteristic peaks of the heavy mineral oil [46]. It indicated that the nonwoven samples were heavily contaminated by the oil after only several cycles of uses. The contamination was much more severe than that of the used mesh. Since the used nonwoven did not show any change of microstructures or any peeling of the coating (Fig. S6), the heavy oil contamination could be the reason of its failure in the oil/water separation. The mesh was more easily recovered from oil contamination by the ethanol/water wash, more durable in other words, compared to the nonwoven fabric. There was likely some oil residual inside the micro pores of polyamide after each cycle of use, which could not be completely washed away. With being used for more and more cycles, the samples could accumulate enough oil to change their wettability and affect their performances of oil/water separation. The nonwoven fabric with complex three dimensional porous structures (Fig. 3) was more difficult to be cleaned than the mesh with simple one layer of pores (Fig. 2). Therefore, the two-step modified mesh showed the reduced intrusion pressure after 19 cycles, while the modified nonwoven came to the complete failure in oil/water separation (zero intrusion pressure) after only 6 cycles. For most durability investigations in literature, in each cycle of oil/ water separation, pores of the target sample were not intruded by the oil (the intrusion pressure wasn't measured), and the separation
Fig. 12. The oil concentration in the filtrated water during the duty cycles of the two-step modified mesh and nonwoven fabric. In each cycle, the sample was used to separate 20 ml oil/water, the intrusion pressure was not tested and the sample was not intruded by the oil.
Compared to the coated mesh (11 cycles), the two-step modified mesh (19 cycles) was more durable and the coating on it was more stable. The rugged surface resulted from the pre-etching reinforced the coating adherence and enhanced the coating stability during repeated uses. The durability investigation was also carried out on the two-step modified and coated nonwoven fabrics. The intrusion pressure decreased to zero after 6 and 5 cycles for the modified and coated samples, respectively (Fig. 10a). During the 6 and 5 cycles, the oil concentration in the filtrated water, presented in Fig. 10b, was in a small range of 889
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
efficiency or/and wettability were tested. That meant the oil did not touch or contaminate the pores of the sample. Under these circumstances, during 25 duty cycles (Fig. 12), the oil concentration in the filtrated water did not change much and the separation efficiency was 98.9%–99.6% and 98.6%–99.5% for the two-step modified mesh and nonwoven fabric, respectively. Without being intruded by the oil in each cycle, the modified mesh and nonwoven fabric were equally durable. However, in our typical durability tests here, pores of the sample were intruded (contaminated) by the oil in each cycle. Then it was found that after repeated uses filters with different porous structures (mesh and nonwoven) had different oil fouling levels that affected their oil/water separation. The mesh was more durable than the nonwoven fabric. A method to efficiently clean the viscous oil in the micro pores could help to recover the intrusion pressure or the oil/water separation ability. The recoverability of the filter from oil contamination should be considered for practical applications. It gives a new insight into filter design and durability investigations.
[8] S. Abbott, J. Ralston, G. Reynolds, R. Hayes, Reversible wettability of photoresponsive pyrimidine-coated surfaces, Langmuir 15 (1999) 8923–8928. [9] Z. Wang, S. Ji, F. He, M. Cao, S. Peng, Y. Li, One-step transformation of highly hydrophobic membranes into superhydrophilic and underwater superoleophobic ones for high-efficiency separation of oil-in-water emulsions, J. Mater. Chem. A 6 (2018) 3391–3396. [10] F. Zhang, W.B. Zhang, Z. Shi, D. Wang, J. Jin, L. Jiang, Nanowire-haired inorganic membranes with superhydrophilicity and underwater ultralow adhesive superoleophobicity for high-efficiency oil/water separation, Adv. Mater. 25 (2013) 4192–4198. [11] I. Sadeghi, A. Aroujalian, A. Raisi, B. Dabir, M. Fathizadeh, Surface modification of polyethersulfone ultrafiltration membranes by corona air plasma for separation of oil/water emulsions, J. Membr. Sci. 430 (2013) 24–36. [12] G. Li, Y. Lu, P. Wu, Z. Zhang, J. Li, W. Zhu, Y. Hu, D. Wu, J. Chu, Fish scale inspired design of underwater superoleophobic microcone arrays by sucrose solution assisted femtosecond laser irradiation for multifunctional liquid manipulation, J. Mater. Chem. A 3 (2015) 18675–18683. [13] F. Chen, J. Song, Z. Liu, J. Liu, H. Zheng, S. Huang, J. Sun, W. Xu, X. Liu, Atmospheric pressure plasma functionalized polymer mesh: an environmentally friendly and efficient tool for oil/water separation, ACS Sustain. Chem. Eng. 4 (2016) 6828–6837. [14] P. Zhao, N. Qin, C.L. Ren, J.Z. Wen, Polyamide 6.6 separates oil/water due to its dual underwater oleophobicity/underoil hydrophobicity: role of 2D and 3D porous structures, Appl. Surf. Sci. 466 (2019) 282–288. [15] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (8) (1936) 988–994. [16] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [17] Y.C. Jung, B. Bhushan, Wetting behavior of water and oil droplets in three-phase interfaces for hydrophobicity/philicity and oleophobicity/philicity, Langmuir 25 (2009) 14165–14173. [18] B. Cheng, Y. Inouea, K. Ishihara, Surface functionalization of polytetrafluoroethylene substrate with hybrid processes comprising plasma treatment and chemical reactions, Colloids Surf. B: Biointerfaces 173 (2019) 77–84. [19] Z. Xue, S. Wang, L. Lin, L. Chen, M. Liu, L. Feng, L. Jiang, A novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh for oil/water separation, Adv. Mater. 23 (2011) 4270–4273. [20] H. Zhu, P. Guo, Z. Shang, X. Yu, Y. Zhang, Fabrication of underwater superoleophobic metallic fiber felts for oil-water separation, Appl. Surf. Sci. 447 (2018) 72–77. [21] Y. Dong, J. Li, L. Shi, X. Wang, Z. Guo, W. Liu, Underwater superoleophobic graphene oxide coated meshes for the separation of oil and water, Chem. Commun. 50 (2014) 5586–5589. [22] K. Hou, Y. Zeng, C. Zhou, J. Chen, X. Wen, S. Xu, J. Cheng, Y. Lin, P. Pi, Durable underwater superoleophobic PDDA/halloysite nanotubes decorated stainless steel mesh for efficient oil–water separation, Appl. Surf. Sci. 416 (2017) 344–352. [23] X. Gao, L.P. Xu, Z. Xue, L. Feng, J. Peng, Y. Wen, S. Wang, X. Zhang, Dual-scaled porous nitrocellulose membranes with underwater superoleophobicity for highly efficient oil/water separation, Adv. Mater. 26 (2014) 1771–1775. [24] F. Lu, Y. Chen, N. Liu, Y. Cao, L. Xu, Y. Wei, L. Feng, A fast and convenient cellulose hydrogel-coated colander for high-efficiency oil–water separation, RSC Adv. 4 (2014) 32544–32548. [25] Z. Wang, X. Jiang, X. Cheng, C.H. Lau, L. Shao, Mussel-inspired hybrid coatings that transform membrane hydrophobicity into high hydrophilicity and underwater superoleophobicity for oil-in-water emulsion separation, ACS Appl. Mater. Interfaces 7 (2015) 9534–9545. [26] Y. Xiang, F. Liu, L. Xue, Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation, J. Membr. Sci. 476 (2015) 321–329. [27] Q. Zhang, Y. Cao, N. Liu, W. Zhang, Y. Chen, L. Feng, A facile approach for fabricating dual-function membrane: simultaneously removing oil from water and adsorbing water-soluble proteins, Adv. Mater. Interfaces 3 (2016) 1600291. [28] X. Li, H. Shan, M. Cao, B. Li, Mussel-inspired modification of PTFE membranes in a miscible THF-Tris buffer mixture for oil-in-water emulsions separation, J. Membr. Sci. 555 (2018) 237–249. [29] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426–430. [30] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057–5115. [31] B. Shang, Y. Wang, B. Peng, Z. Deng, Bioinspired polydopamine particles-assisted construction of superhydrophobic surfaces for oil/water separation, J. Colloid Interface Sci. 482 (2016) 240–251. [32] X. Tian, V. Jokinen, J. Li, J. Sainio, R.H.A. Ras, Unusual dual superlyophobic surfaces in oil–water systems: the design principles, Adv. Mater. 28 (2016) 10652–10658. [33] J.W. Grate, K.J. Dehoff, M.G. Warner, J.W. Pittman, T.W. Wietsma, C. Zhang, M. Oostrom, Correlation of oil–water and air–water contact angles of diverse silanized surfaces and relationship to fluid interfacial tensions, Langmuir 28 (2012) 7182–7188. [34] Y. Liu, C.P. Chang, T. Sun, Dopamine-assisted deposition of dextran for nonfouling applications, Langmuir 30 (2014) 3118–3126. [35] L. Xu, N. Liu, Y. Cao, F. Lu, Y. Chen, X. Zhang, L. Feng, Y. Wei, Mercury ion responsive wettability and oil/water separation, ACS Appl. Mater. Interfaces 6 (2014) 13324–13329. [36] H.C. Yang, K.J. Liao, H. Huang, Q.Y. Wu, L.S. Wan, Z.K. Xu, Mussel-inspired
4. Conclusions We demonstrated an environmentally friendly two-step method of surface modification for polyamide to achieve underwater superoleophobicity and efficient oil/water separation. Polyamide meshes and nonwoven fabrics were etched by using plasma first and then coated by the hybrid PDA/cellulose. Compared to the original, only plasma treated or only coated polyamide, the two-step modified polyamide showed the increased oil contact angle in water and improved intrusion pressure. The rugged surface resulted from the pre-etching reinforced the coating adherence and enhanced the coating stability during repeated uses. The nonwoven fabric with a complex three dimensional porous structure was less durable than the mesh with one layer of pores due to much heavier oil contamination of the nonwoven fabric. A method to efficiently clean the viscous oil residual in the micro pores could benefit the durability. The recoverability of the filter from oil contamination should be considered for practical applications. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.03.152. Acknowledgements This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would like to thank MW Canada Ltd. and especially Lindsay Barber for technical discussions and support. Conflict of interest The authors declare no conflict of interest. References [1] Z. Wang, S. Ji, J. Zhang, F. He, Z. Xu, S. Peng, Y. Li, Dual functional membrane with multiple hierarchical structures (MHS) for simultaneous and high-efficiency removal of dye and nano-sized oil droplets in water under high flux, J. Membr. Sci. 564 (2018) 317–327. [2] M.V. del Blanco, E.J. Fischer, E. Cabane, Underwater superoleophobic wood cross sections for efficient oil/water separation, Adv. Mater. Interfaces 4 (2017) 1700584. [3] Z. Wang, X. Yang, Z. Cheng, Y. Liu, L. Shao, L. Jiang, Simply realizing “water diode” Janus membranes for multifunctional smart applications, Mater. Horiz. 4 (2017) 701–708. [4] X. Yang, Z. Wang, L. Shao, Construction of oil-unidirectional membrane for integrated oil collection with lossless transportation and oil-in-water emulsion purification, J. Membr. Sci. 549 (2018) 67–74. [5] G. Ren, Y. Song, X. Li, Y. Zhou, Z. Zhang, X. Zhu, A superhydrophobic copper mesh as an advanced platform for oil-water separation, Appl. Surf. Sci. 428 (2018) 520–525. [6] H. Cao, W. Gu, J. Fu, Y. Liu, S. Chen, Preparation of superhydrophobic/oleophilic copper mesh for oil-water separation, Appl. Surf. Sci. 412 (2017) 599–605. [7] K.R. Shull, T.E. Karis, Dewetting dynamics for large equilibrium contact angels, Langmuir 10 (1) (1994) 334–339.
890
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
[37]
[38]
[39] [40] [41]
modification of a polymer membrane for ultra-high water permeability and oil-inwater emulsion separation, J. Mater. Chem. A 2 (2014) 10225–10230. Z.X. Wang, C.H. Lau, N.Q. Zhang, Y.P. Bai, L. Shao, Mussel-inspired tailoring of membrane wettability for harsh water treatment, J. Mater. Chem. A 3 (2015) 2650–2657. J. Song, S. Huang, Y. Lu, X. Bu, J.E. Mates, A. Ghosh, R. Ganguly, C.J. Carmalt, I.P. Parkin, W. Xu, C.M. Megarids, Self-driven one-step oil removal from oil spill on water via selective-wettability steel mesh, ACS Appl. Mater. Interfaces 6 (22) (2014) 19858–19865. J.D. Martin, S.D. Hudson, Mass transfer and interfacial properties in two-phase microchannel flows, New J. Phys. 11 (2009) 115005. L.R. Fisher, E.E. Mitchell, N.S. Parker, Interfacial tensions of commercial vegetable oils with water, J. Food Sci. 50 (1985) 1201–1202. T. Glawdel, C.L. Ren, Droplet formation in microfluidic T-junction generators
[42]
[43]
[44]
[45] [46]
891
operating in the transitional regime. III. Dynamic surfactant effects, Phys. Rev. E 86 (2012) 026308. T. Svitova, O. Theodoly, R.M. Hill, C.J. Radke, Wetting behavior of silicone oils on solid substrates immersed in aqueous electrolyte solutions, Langmuir 18 (2002) 6821–6829. L.A. Díaz-Alejo, E.C. Menchaca-Campos, J.U. Chavarín, R. Sosa-Fonseca, M.A. García-Sánchez, Effects of the addition of ortho- and para-NH2 substituted tetraphenylporphyrins on the structure of nylon 66, Int. J. Polym. Sci. 2013 (2013) 323854. J. Charles, G.R. Ramkumaar, S. Azhagiri, S. Gunasekaran, FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66, E-J. Chem. 6 (1) (2009) 23–33. http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html. https://www.medicinescomplete.com/mc/excipients/2012/218025.htm.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Surface modification of polyamide meshes and nonwoven fabrics by plasma etching and a PDA/cellulose coating for oil/water separation
T
⁎
Pei Zhao, Ning Qin, Carolyn L. Ren, John Z. Wen
Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: A hybrid coating of PDA/cellulose Plasma etching Oil/water separation Meshes and nonwoven fabrics Underwater oleophobicity Durability
This work investigated a two-step surface modification of polyamide meshes and nonwoven fabrics for oil/water separation and looked into the durability of such modified polyamide. The two-step modification included 1) pre-etching the polyamide surface using plasma treatment and 2) coating the pre-etched surface by eco-friendly polydopamine (PDA)/cellulose. The pre-etching increased the surface roughness, which further improved the underwater superoleophobicity of the coating. Therefore, the modified polyamide was able to separate various oil/water mixtures and showed a higher intrusion pressure than the original sample and the samples which were only etched or only coated. The grooves on the surface that resulted from the pre-etching prevented the coating from peeling off. In durability tests, after 6 repeated uses, the modified nonwoven sample lost its underwater oleophobicity due to severe oil fouling, coming to a complete failure in oil/water separation. After 19 cycles, the modified mesh was still able to separate a certain amount of oil/water but showed reduced intrusion pressure because of slight oil contamination. Filters with different structures, like meshes with one layer of pores and nonwoven fabrics with complex three dimensional pores, had different oil fouling levels that affected oil/water separation. The recoverability of filters from oil contamination should be considered for practical applications.
1. Introduction Expanding oily wastewater from industry and increasing oil leakage have driven extensive studies of oil/water separation. Novel filters with opposite wettability to water and oil, such as hydrophobic/oleophilic, underwater oleophobic or underoil hydrophobic, selectively allow one of oil/water to penetrate and resist the other [1–6]. The wetting properties of the filters depend on the surface materials and topography [7,8]. Filters (metal or polymer) (meshes, membranes or sponges) possessing underwater oleophobicity have been widely used for gravitydriven oil/water separation since water is heavier than most types of oils. And the underwater oleophobicity is considered to make the solidwater-oil interface antifouling [9,10]. Some polymer filters are etched by using plasma with a high power (100 W–600 W, a treatment time of 2–6 min) to achieve superoleophobicity (> 150°) in water [11–14]. Plasma treatment could create an increased roughness on the polymer surface, which enhances the oleophobicity in water according to Wenzel or Cassie models [15–17]. Plasma with a low power (30–100 W, a treatment time of 5–60 s) doesn't work for etching polymer but can activate the surface with some functional groups of –OH and –COOH, which is used for temporarily changing the wettability of the surface or for subsequent bonding with other materials [18]. Many filters are ⁎
modified by special coatings with underwater oleophobic properties, such as hydrogel, graphene oxide, cellulose, polydopamine (PDA) and tetrahydrofuran oxidized PDA [19–28]. The underwater oleophobic filters have been developed either by the surface construction or by special coatings. In order to combine the required properties of the coating and the large roughness, we propose a two-step strategy that would lead to advanced filters for efficient oil/water separation: firstly, the filter surface is etched by using plasma; secondly, the pre-etched surface is coated by a special coating. Regarding the special coating that would be applied onto the preetched surface of filters, we propose a hybrid coating of PDA/cellulose. The PDA/cellulose is eco-friendly, which would be beneficial for environmental protection. PDA is a harmless bio material that adheres to various surfaces [29–31]. Additionally, PDA has been used as a coating material for oil/water separation, because it has the underwater oleophobic property [26,27]. The underwater oleophobicity of materials increases with the increase of the hydrophilicity [32,33]. However, PDA shows limited hydrophilicity, e.g. the water contact angle (WCA) on PDA coated glass or silicon wafer is around 55° [34], the WCA on PDA coated PVDF membrane is ranged from 110° to 55° depending on the coating time [26], and the WCA on PDA coated steel mesh is around 67° [35]. In order to reach superhydrophilicity and underwater
Corresponding author. E-mail address: [email protected] (J.Z. Wen).
https://doi.org/10.1016/j.apsusc.2019.03.152 Received 8 November 2018; Received in revised form 6 March 2019; Accepted 15 March 2019 Available online 18 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
superoleophobicity, PDA is sometimes used as “bio-glue” to fabricate hybrid coatings through a mussel-inspired one-step method where superhydrophilic species mixed with dopamine (DA) are immobilized onto the substrate during the PDA formation, such as PDA/TEOS [25], PDA/LPAA [35], PDA/PEI [36] and PDA/KH560 [37]. For developing eco-friendly coatings that have less environmental concern for large scale fabrication and applications, we propose the hybrid coating of PDA/cellulose. Cellulose with plenty of hydroxyl groups is superhydrophilic and has attracted much attention for its good bio-compatibility [23,24]. Therefore, we propose a hybrid coating of PDA/cellulose, which is eco-friendly and would be underwater superoleophobic. Here, we developed the environmentally friendly two-step method of surface modification for polyamide filters, which could be applied to other polymers, to achieve highly efficient oil/water separation. Polyamide meshes and nonwoven fabrics were pre-etched by using plasma and then coated by the hybrid PDA/cellulose. Such modified polyamide samples possessed superoleophobicity in water and showed increased intrusion pressure than the original, the only coated or the only plasma treated polyamide. The rugged surface resulted from the pre-etching reinforced the coating adherence and enhanced the coating stability. During repeated uses, polyamide meshes with one layer of pores and nonwoven fabrics with layers of complex pores had different oil fouling levels, which affected their oil/water separation. The mesh was more durable than the nonwoven fabric due to less oil contamination, which should be considered for practical applications.
etched polyamide (7 cm × 7 cm) was immersed into the above solution for 6 h with 600 rpm at room temperature. After that, the sample was washed by ethanol and water and dried. Then we had the original, only coated, only plasma treated and two-step modified meshes and nonwoven fabrics. 2.3. Oil/water separation A piece of polyamide (7 cm × 7 cm) mesh or nonwoven fabric was fixed under a tube with a clamp to form a filter (Fig. S1). Before the separation, water was fed into the tube to prewet the polyamide. Then the oil/water mixture (1:2, 20 ml) was poured into the tube, where water flowed through the polyamide and oil was held on top of the polyamide. It was gravity driven separation. The filtrated water was collected to evaluate the separation efficiency η, defined by (1 − C1) × 100%, where C1 is the oil concentration in the filtrated water and measured by a UV–Vis spectrometer [14]. The intrusion pressure (Pintru) of oil was obtained by keeping adding the oil into the tube till the maximum height (hmax), when the oil started to penetrate the polyamide, according to Pintru = ρoil g hmax. 2.4. Durability tests To evaluate the durability of the coated and the two-step modified polyamide, each sample was used for separation for a number of cycles. In each cycle, the sample was used to separate 20 ml heavy mineral oil/ water (1:2) and then intruded by the oil by adding more oil in order to obtain the intrusion pressure. That meant the sample was contaminated by the oil in each cycle. After each cycle, the sample was washed by ethanol and water and dried.
2. Experimental 2.1. Materials Polyamide (nylon 6.6) meshes with a pore size of 64 μm were purchased from Component Supply, Tennessee. Polyamide nonwoven fabrics (thermal bonded, PBN-II®, Type 30) with a density of 2 oz per square yard (osy) were provided by Cerex Advanced Fabrics, Florida. Heavy mineral oil, silicone oil, dopamine hydrochloride (DA) and ammonia solution with a concentration of 28–30% were bought from Sigma-Aldrich, Canada.
2.5. Characterization The microstructures and morphologies of the polyamide samples were observed by using an Atomic Force Microscopy (AFM, MultimodeTM SPM, Digital Instruments) in a tap mode and a Scanning Electron Microscope (SEM, Zeiss ULTRA Plus). The oil contact angle in water on the polyamide samples were measured using a lab-made contact angle meter, set up by a syringe needle, a side-view microscope, and a camera. Fourier transform infrared (FTIR) spectra on the sample surface were obtained with a Bruker Tensor 27 FTIR spectrometer by using the attenuated total reflectance (ATR) mode.
2.2. Surface modification The two-step surface modification for polyamide meshes and nonwoven fabrics included plasma treatment and the following coating procedure, as shown in Fig. 1. The Phantom Reactive Ion Etching system (RIE, Trion Technology) was employed as an oxygen plasma source to etch the polyamide surface (14 cm × 14 cm) for a period of 5 min with a power of 200 W. The pressure was kept as 100mTorr and oxygen was fed into the chamber at 50 sccm. In the coating procedure, 0.4% ammonia solution, 0.2% DA and 0.4% cellulose were put into a mixture of 100 ml water and ethanol (7:3). A piece of original or plasma
3. Results and discussion Microstructures of the two-step modified polyamide mesh and nonwoven are shown in Figs. 2 and 3, respectively (written as the modified mesh/nonwoven sometimes in the following). Microstructures of the original, only coated and only plasma treated polyamide mesh
Fig. 1. The schematic of the two-step modification method. 884
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Fig. 2. SEM images of the top (a, b) and side (c, d) views of the two-step modified polyamide mesh. The top view was facing plasma during the pre-etching step. Typical AFM images of the two-step modified polyamide mesh (e).
was more and more rugged when approaching the plasma. The side view of the only plasma treated polyamide also had the gradient structure (Figs. S2f and S3d–f). The plasma greatly etched the top surface of polyamide. When the plasma dived into the pores of polyamide, its power was weakened. The surface roughness of the meshes was determined by AFM analysis as shown in Table 1. Typical AFM images of the two-step modified and coated meshes are shown in Figs. 2e and S4, respectively. The AFM images of the plasma treated and original meshes were shown in another paper [14]. The two-step modified mesh achieved a very large roughness, 1.5 times of the plasma
and nonwoven are presented in Figs. S2 and S3 for comparison. The original polyamide surface was smooth. After the first step of plasma treatment, the surface became rough with many etched grooves (Figs. S2e, g, h, i and S3e). The different plasma doses didn't affect the microstructure significantly (Fig. S2e, g, h, i). After the second step of coating, PDA/cellulose was successfully coated on the pre-etched surface. The finished top surface, facing the plasma during the pre-etching, was very rough due to the etched grooves and the coated particles (Figs. 2a, b and 3a–c). The side view of the modified mesh and nonwoven presented a gradually varied structure (Figs. 2c, d and 3d–f). It
Fig. 3. SEM images of the top (a, b, c) and side (d, e, f) views of the two-step modified nonwoven polyamide. The top view was facing plasma during the pre-etching step. 885
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Table 1 The roughness of the two-step modified, plasma treated, coated and original polyamide meshes determined by AFM.
Ra (nm) r (surface/projected area)
Two-step modified
Plasma treated [14]
Coated
Original [14]
300.1 ± 95.3 1.59 ± 0.23
206.9 ± 84.6 1.44 ± 0.07
47.1 ± 16.9 1.21 ± 0.08
13.9 ± 1.5 1.07 ± 0.02
and vibrations were observed at 2923 and 2856 cm−1 [13,38]. And the peaks arising at 1633, 1535 and 1371 cm−1 were ascribed to the amide carbonyl C]O stretching vibration of the secondary amide band, the NeH bending motion, and wagging of NH amide groups, respectively [38,39]. Compared to the original mesh (or nonwoven), the modified mesh (or nonwoven) showed a broadened band at around 3300 cm−1, and the relative intensity of this band to the rest peaks was much higher. And this peak shifted a little towards the shorter wavelength from 3300 cm−1 (the original mesh) to 3293 cm−1 (the modified mesh) and from 3297 cm−1 (the original nonwoven) to 3265 cm−1 (the modified nonwoven). Those differences were caused by the coating. One of the coating materials, cellulose, had plenty of –OH, the characteristic peak of which is located at the wavelength of 3200–3600 cm−1, stronger and broader than the signal wavelength of the NH bond (3300–3500 cm−1) enriched in the original polyamide [40]. The molecule interactions between the coating materials and polyamide could have caused the little shift of this peak. When the modified samples were prewetted by water, they were able to separate water and various oils, allowing water to flow through their pores and resisting the oil (supplementary video 1 and Fig. S1). Three types of oils with different physical properties as shown in Table 2, heavy mineral oil, silicone oil, and peanut oil, were used as model oils here. Fig. 5 shows the separation efficiencies of all the polyamide samples. The error bars here and in the following figures were standard deviations. The separation efficiencies of the original polyamide were in the range of 90%–99%. The two-step modified polyamide showed the increased efficiencies of 92%–99%. The reason why the samples could separate oil/water was the underwater oleophobicity that can be proved by the oil contact angle in water (Fig. 6a). Compared to the original, plasma treated and coated polyamide mesh (or nonwoven), the two-step modified mesh (or nonwoven) had the largest oil contact angle in water due to the combination of its largest roughness and the properties of the coating. The contact angle (CA) would be affected by the surface roughness according to Cassie (cosθ = f(cosθY + 1) − 1) or Wenzel (cosθ = r cos θY) models, where θ is measured or apparent CA on a real rough surface, θY is Young CA on an ideal flat surface, f is the fraction of solid surface in contact with the droplet and r is the ratio of the actual over the projected surface area [15,16,32]. Therefore, the hydrophilicity and hydrophobicity of a surface would be improved with the increase of the
Fig. 4. FTIR spectra of the original and the two-step modified polyamide meshes and nonwoven fabrics.
treated one and 5 times larger than the coated one. Therefore, the twostep strategy brought in the required coating as well as the large roughness. Fig. 4 shows the FTIR spectra of the original and the two-step modified polyamide meshes and nonwoven fabrics. The main peaks located within the range of 1300–3800 cm−1, although a range of 500–4000 cm−1 was scanned. The peaks of the different samples were almost overlapping. Since ATR mode was used to obtain the FTIR spectra here, the detecting depth was several microns. Even for the modified samples, a considerable part of the signals were from the polyamide substrate. For the spectra of all the samples, one band was present at around 3300 cm−1, which can be assigned to the stretching of NeH bonds [13,38]. The CH2 asymmetric and symmetric stretching
Fig. 5. The separation efficiencies of the original, plasma treated, coated and two-step modified polyamide meshes (left) and nonwoven fabrics (right) prewetted by water. 886
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
sample was contaminated by the oil in each cycle. Fig. 7 shows the intrusion pressure and the oil concentration in the separated water during the duty cycles of the two-step modified mesh. After 19 cycles, the intrusion pressure began to decrease (Fig. 7a). However, the oil concentration in the filtrated water did not change much during 21 cycles (Fig. 7b). It was in the range of 8 ± 3 ppm and accordingly, the separation efficiency was 98.9%–99.5% during the 21 cycles. It was because in the tube with an inner diameter of 2.2 cm, which was used for the separation, the to be separated 20 ml oil/water mixture corresponded to a 0.15 kPa hydraulic pressure of oil, still smaller than the reduced intrusion pressure of around 0.75 kPa at the 20th and 21st cycles. That meant after 19 cycles the modified mesh started to show the decreased intrusion pressure, but it was still capable of separating a certain amount of oil/water with a high efficiency. The reason for the reduced intrusion pressure of the used two-step modified mesh was investigated. As shown in Fig. 8a, the superhydrophilicity of the new modified mesh (0 cycle) was transformed to hydrophobicity after 21 cycles and the oil contact angle in water decreased, which directly caused the reduced intrusion pressure according 2γ cos θ to Pintru = − ow d O / W . FTIR spectrum of the modified mesh after 21 cycles is shown in Fig. 8c. Compared to the new sample, the relative intensities of the peaks at 2923 cm−1 and 2856 cm−1 that were ascribed to the CH2 stretching were increased. The heavy mineral oil shows intense peaks at those locations [46]. It implied that after being intruded by the oil for cycles the modified mesh was contaminated by the oil probably inside the pores, which could not be easily cleaned by ethanol. Fig. 9a shows the microstructures of the modified mesh that had been used for 21 cycles, which were basically the same as the new one shown in Fig. 2. The coating did not fall out of the substrate. Therefore, the most possible reason for the change of the wettability and the reduced intrusion pressure could be the oil contamination.
Table 2 Oil properties at room temperature of 22–25 °C [13,41–45]. Oil
Heavy mineral oil
Silicone oil
Peanut oil
Density (g/cm3) Surface tension (mN/m) Oil/water interfacial tension (mN/ m)
0.86 28.5 ~52
0.93 21.4 38.75
0.92 34.5 ~23
roughness. Among all the polyamide meshes (or nonwoven) the two-step modified one presented the largest intrusion pressure of oil (Fig. 6b). The oil intrusion happens when the oil/water interfacial tension is not strong enough to hold or resist the oil according to 2γ cos θ Pintru = ρoil g hmax = − ow d O / W , where γow is oil/water interfacial tension, θO/W is oil contact angle in water and d is the pore size [19]. For the same polyamide sample, the intrusion pressure of heavy mineral oil was larger than that of the other two oils, as the heavy mineral oil/ water interfacial tension is the largest as shown in Table 2. A larger intrusion pressure of oil means a higher robustness of the sample for oil/water separation and a larger capacity of oil/water that can be separated each time, which was one target of the proposed two-step strategy that was achieved. Overall, the two-step modified polyamide showed improved performances of oil/water separation than the original and one-step modified (plasma treated or coated) polyamide. The durability of the two-step modified polyamide mesh was investigated by using the sample for cycles and compared with that of the coated mesh. In each cycle, the sample was used to separate 20 ml heavy mineral oil/water (1:2) and then intruded by the oil by adding more oil in order to obtain the intrusion pressure. That meant the
Fig. 6. Oil contact angle in water (a) and the intrusion pressure of oil (b) for the original, plasma treated, coated and two-step modified polyamide meshes (left) and nonwoven fabrics (right). The inset images are the underwater heavy mineral oil droplets on the polyamide samples. 887
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Fig. 7. The intrusion pressure of oil (a) and the oil concentration in the filtrated water (b) during the duty cycles of the two-step modified and coated polyamide meshes.
Fig. 8. Water drops dyed by methyl orange in air (left) and water drops in oil (right) on the two-step modified mesh (a) and on the coated mesh (b). FTIR spectra of the coated and two-step modified polyamide meshes (c).
the oil contamination after 13 cycles, indicated by the increased intensities of the peaks at 2923 cm−1 and 2856 cm−1 in the FTIR spectrum (Fig. 8c). Different from the two-step modified mesh, however, the coating partially peeled out of the substrate (Fig. 9b). Both the oil contamination and the peeling of the coating could be the reasons that caused the change of the wettability and the decreased intrusion pressure for the used coated mesh.
Similar with the two-step modified mesh, the coated mesh showed the decreased intrusion pressure after 11 cycles (Fig. 7a), but during the 13 cycles the oil concentration in the separated water stayed in a certain range of 9 ± 3 ppm (Fig. 7b) and the separation efficiency was 98.8%–99.4%. After 13 cycles, as shown in Fig. 8b, the coated mesh became hydrophobic and the underwater oil contact angle decreased, conforming to the reduced intrusion pressure. The coated mesh also had
Fig. 9. SEM images of the two-step modified polyamide mesh after 21 cycles of uses (a) and the coated mesh after 13 cycles of uses (b). 888
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
Fig. 10. The intrusion pressure of oil (a) and the oil concentration in the filtrated water (b) during the duty cycles of the two-step modified and coated nonwoven fabrics.
Fig. 11. Water drops dyed by methyl orange in air (left) and water drops in oil (right) on the two-step modified (a) and the coated (b) nonwoven fabrics. FTIR spectra of the coated and two-step modified nonwoven fabrics (c).
8 ± 4 ppm and 10 ± 4 ppm, respectively. The separation efficiency was around 98.6%–99.6%. But after those cycles, the oil slowly dripped through the used nonwoven sample after water being filtrated (supplementary video 2 and Fig. S5). The oil could not be resisted anymore, because the used nonwoven was not underwater oleophobic anymore, with its oil contact angle in water being zero as shown in Fig. 11a, b. FTIR spectra of the used nonwoven samples are shown in Fig. 11c. Compared to the new sample (0 cycle), the relative intensities of the peaks at 2923 cm−1, 2856 cm−1, 1461 cm−1, and 1376 cm−1 were greatly increased. The increase was much more significant than that of the used mesh (Fig. 8c). And a new peak appeared at 2952 cm−1. All of those peaks were characteristic peaks of the heavy mineral oil [46]. It indicated that the nonwoven samples were heavily contaminated by the oil after only several cycles of uses. The contamination was much more severe than that of the used mesh. Since the used nonwoven did not show any change of microstructures or any peeling of the coating (Fig. S6), the heavy oil contamination could be the reason of its failure in the oil/water separation. The mesh was more easily recovered from oil contamination by the ethanol/water wash, more durable in other words, compared to the nonwoven fabric. There was likely some oil residual inside the micro pores of polyamide after each cycle of use, which could not be completely washed away. With being used for more and more cycles, the samples could accumulate enough oil to change their wettability and affect their performances of oil/water separation. The nonwoven fabric with complex three dimensional porous structures (Fig. 3) was more difficult to be cleaned than the mesh with simple one layer of pores (Fig. 2). Therefore, the two-step modified mesh showed the reduced intrusion pressure after 19 cycles, while the modified nonwoven came to the complete failure in oil/water separation (zero intrusion pressure) after only 6 cycles. For most durability investigations in literature, in each cycle of oil/ water separation, pores of the target sample were not intruded by the oil (the intrusion pressure wasn't measured), and the separation
Fig. 12. The oil concentration in the filtrated water during the duty cycles of the two-step modified mesh and nonwoven fabric. In each cycle, the sample was used to separate 20 ml oil/water, the intrusion pressure was not tested and the sample was not intruded by the oil.
Compared to the coated mesh (11 cycles), the two-step modified mesh (19 cycles) was more durable and the coating on it was more stable. The rugged surface resulted from the pre-etching reinforced the coating adherence and enhanced the coating stability during repeated uses. The durability investigation was also carried out on the two-step modified and coated nonwoven fabrics. The intrusion pressure decreased to zero after 6 and 5 cycles for the modified and coated samples, respectively (Fig. 10a). During the 6 and 5 cycles, the oil concentration in the filtrated water, presented in Fig. 10b, was in a small range of 889
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
efficiency or/and wettability were tested. That meant the oil did not touch or contaminate the pores of the sample. Under these circumstances, during 25 duty cycles (Fig. 12), the oil concentration in the filtrated water did not change much and the separation efficiency was 98.9%–99.6% and 98.6%–99.5% for the two-step modified mesh and nonwoven fabric, respectively. Without being intruded by the oil in each cycle, the modified mesh and nonwoven fabric were equally durable. However, in our typical durability tests here, pores of the sample were intruded (contaminated) by the oil in each cycle. Then it was found that after repeated uses filters with different porous structures (mesh and nonwoven) had different oil fouling levels that affected their oil/water separation. The mesh was more durable than the nonwoven fabric. A method to efficiently clean the viscous oil in the micro pores could help to recover the intrusion pressure or the oil/water separation ability. The recoverability of the filter from oil contamination should be considered for practical applications. It gives a new insight into filter design and durability investigations.
[8] S. Abbott, J. Ralston, G. Reynolds, R. Hayes, Reversible wettability of photoresponsive pyrimidine-coated surfaces, Langmuir 15 (1999) 8923–8928. [9] Z. Wang, S. Ji, F. He, M. Cao, S. Peng, Y. Li, One-step transformation of highly hydrophobic membranes into superhydrophilic and underwater superoleophobic ones for high-efficiency separation of oil-in-water emulsions, J. Mater. Chem. A 6 (2018) 3391–3396. [10] F. Zhang, W.B. Zhang, Z. Shi, D. Wang, J. Jin, L. Jiang, Nanowire-haired inorganic membranes with superhydrophilicity and underwater ultralow adhesive superoleophobicity for high-efficiency oil/water separation, Adv. Mater. 25 (2013) 4192–4198. [11] I. Sadeghi, A. Aroujalian, A. Raisi, B. Dabir, M. Fathizadeh, Surface modification of polyethersulfone ultrafiltration membranes by corona air plasma for separation of oil/water emulsions, J. Membr. Sci. 430 (2013) 24–36. [12] G. Li, Y. Lu, P. Wu, Z. Zhang, J. Li, W. Zhu, Y. Hu, D. Wu, J. Chu, Fish scale inspired design of underwater superoleophobic microcone arrays by sucrose solution assisted femtosecond laser irradiation for multifunctional liquid manipulation, J. Mater. Chem. A 3 (2015) 18675–18683. [13] F. Chen, J. Song, Z. Liu, J. Liu, H. Zheng, S. Huang, J. Sun, W. Xu, X. Liu, Atmospheric pressure plasma functionalized polymer mesh: an environmentally friendly and efficient tool for oil/water separation, ACS Sustain. Chem. Eng. 4 (2016) 6828–6837. [14] P. Zhao, N. Qin, C.L. Ren, J.Z. Wen, Polyamide 6.6 separates oil/water due to its dual underwater oleophobicity/underoil hydrophobicity: role of 2D and 3D porous structures, Appl. Surf. Sci. 466 (2019) 282–288. [15] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (8) (1936) 988–994. [16] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551. [17] Y.C. Jung, B. Bhushan, Wetting behavior of water and oil droplets in three-phase interfaces for hydrophobicity/philicity and oleophobicity/philicity, Langmuir 25 (2009) 14165–14173. [18] B. Cheng, Y. Inouea, K. Ishihara, Surface functionalization of polytetrafluoroethylene substrate with hybrid processes comprising plasma treatment and chemical reactions, Colloids Surf. B: Biointerfaces 173 (2019) 77–84. [19] Z. Xue, S. Wang, L. Lin, L. Chen, M. Liu, L. Feng, L. Jiang, A novel superhydrophilic and underwater superoleophobic hydrogel-coated mesh for oil/water separation, Adv. Mater. 23 (2011) 4270–4273. [20] H. Zhu, P. Guo, Z. Shang, X. Yu, Y. Zhang, Fabrication of underwater superoleophobic metallic fiber felts for oil-water separation, Appl. Surf. Sci. 447 (2018) 72–77. [21] Y. Dong, J. Li, L. Shi, X. Wang, Z. Guo, W. Liu, Underwater superoleophobic graphene oxide coated meshes for the separation of oil and water, Chem. Commun. 50 (2014) 5586–5589. [22] K. Hou, Y. Zeng, C. Zhou, J. Chen, X. Wen, S. Xu, J. Cheng, Y. Lin, P. Pi, Durable underwater superoleophobic PDDA/halloysite nanotubes decorated stainless steel mesh for efficient oil–water separation, Appl. Surf. Sci. 416 (2017) 344–352. [23] X. Gao, L.P. Xu, Z. Xue, L. Feng, J. Peng, Y. Wen, S. Wang, X. Zhang, Dual-scaled porous nitrocellulose membranes with underwater superoleophobicity for highly efficient oil/water separation, Adv. Mater. 26 (2014) 1771–1775. [24] F. Lu, Y. Chen, N. Liu, Y. Cao, L. Xu, Y. Wei, L. Feng, A fast and convenient cellulose hydrogel-coated colander for high-efficiency oil–water separation, RSC Adv. 4 (2014) 32544–32548. [25] Z. Wang, X. Jiang, X. Cheng, C.H. Lau, L. Shao, Mussel-inspired hybrid coatings that transform membrane hydrophobicity into high hydrophilicity and underwater superoleophobicity for oil-in-water emulsion separation, ACS Appl. Mater. Interfaces 7 (2015) 9534–9545. [26] Y. Xiang, F. Liu, L. Xue, Under seawater superoleophobic PVDF membrane inspired by polydopamine for efficient oil/seawater separation, J. Membr. Sci. 476 (2015) 321–329. [27] Q. Zhang, Y. Cao, N. Liu, W. Zhang, Y. Chen, L. Feng, A facile approach for fabricating dual-function membrane: simultaneously removing oil from water and adsorbing water-soluble proteins, Adv. Mater. Interfaces 3 (2016) 1600291. [28] X. Li, H. Shan, M. Cao, B. Li, Mussel-inspired modification of PTFE membranes in a miscible THF-Tris buffer mixture for oil-in-water emulsions separation, J. Membr. Sci. 555 (2018) 237–249. [29] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426–430. [30] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057–5115. [31] B. Shang, Y. Wang, B. Peng, Z. Deng, Bioinspired polydopamine particles-assisted construction of superhydrophobic surfaces for oil/water separation, J. Colloid Interface Sci. 482 (2016) 240–251. [32] X. Tian, V. Jokinen, J. Li, J. Sainio, R.H.A. Ras, Unusual dual superlyophobic surfaces in oil–water systems: the design principles, Adv. Mater. 28 (2016) 10652–10658. [33] J.W. Grate, K.J. Dehoff, M.G. Warner, J.W. Pittman, T.W. Wietsma, C. Zhang, M. Oostrom, Correlation of oil–water and air–water contact angles of diverse silanized surfaces and relationship to fluid interfacial tensions, Langmuir 28 (2012) 7182–7188. [34] Y. Liu, C.P. Chang, T. Sun, Dopamine-assisted deposition of dextran for nonfouling applications, Langmuir 30 (2014) 3118–3126. [35] L. Xu, N. Liu, Y. Cao, F. Lu, Y. Chen, X. Zhang, L. Feng, Y. Wei, Mercury ion responsive wettability and oil/water separation, ACS Appl. Mater. Interfaces 6 (2014) 13324–13329. [36] H.C. Yang, K.J. Liao, H. Huang, Q.Y. Wu, L.S. Wan, Z.K. Xu, Mussel-inspired
4. Conclusions We demonstrated an environmentally friendly two-step method of surface modification for polyamide to achieve underwater superoleophobicity and efficient oil/water separation. Polyamide meshes and nonwoven fabrics were etched by using plasma first and then coated by the hybrid PDA/cellulose. Compared to the original, only plasma treated or only coated polyamide, the two-step modified polyamide showed the increased oil contact angle in water and improved intrusion pressure. The rugged surface resulted from the pre-etching reinforced the coating adherence and enhanced the coating stability during repeated uses. The nonwoven fabric with a complex three dimensional porous structure was less durable than the mesh with one layer of pores due to much heavier oil contamination of the nonwoven fabric. A method to efficiently clean the viscous oil residual in the micro pores could benefit the durability. The recoverability of the filter from oil contamination should be considered for practical applications. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.03.152. Acknowledgements This work was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would like to thank MW Canada Ltd. and especially Lindsay Barber for technical discussions and support. Conflict of interest The authors declare no conflict of interest. References [1] Z. Wang, S. Ji, J. Zhang, F. He, Z. Xu, S. Peng, Y. Li, Dual functional membrane with multiple hierarchical structures (MHS) for simultaneous and high-efficiency removal of dye and nano-sized oil droplets in water under high flux, J. Membr. Sci. 564 (2018) 317–327. [2] M.V. del Blanco, E.J. Fischer, E. Cabane, Underwater superoleophobic wood cross sections for efficient oil/water separation, Adv. Mater. Interfaces 4 (2017) 1700584. [3] Z. Wang, X. Yang, Z. Cheng, Y. Liu, L. Shao, L. Jiang, Simply realizing “water diode” Janus membranes for multifunctional smart applications, Mater. Horiz. 4 (2017) 701–708. [4] X. Yang, Z. Wang, L. Shao, Construction of oil-unidirectional membrane for integrated oil collection with lossless transportation and oil-in-water emulsion purification, J. Membr. Sci. 549 (2018) 67–74. [5] G. Ren, Y. Song, X. Li, Y. Zhou, Z. Zhang, X. Zhu, A superhydrophobic copper mesh as an advanced platform for oil-water separation, Appl. Surf. Sci. 428 (2018) 520–525. [6] H. Cao, W. Gu, J. Fu, Y. Liu, S. Chen, Preparation of superhydrophobic/oleophilic copper mesh for oil-water separation, Appl. Surf. Sci. 412 (2017) 599–605. [7] K.R. Shull, T.E. Karis, Dewetting dynamics for large equilibrium contact angels, Langmuir 10 (1) (1994) 334–339.
890
Applied Surface Science 481 (2019) 883–891
P. Zhao, et al.
[37]
[38]
[39] [40] [41]
modification of a polymer membrane for ultra-high water permeability and oil-inwater emulsion separation, J. Mater. Chem. A 2 (2014) 10225–10230. Z.X. Wang, C.H. Lau, N.Q. Zhang, Y.P. Bai, L. Shao, Mussel-inspired tailoring of membrane wettability for harsh water treatment, J. Mater. Chem. A 3 (2015) 2650–2657. J. Song, S. Huang, Y. Lu, X. Bu, J.E. Mates, A. Ghosh, R. Ganguly, C.J. Carmalt, I.P. Parkin, W. Xu, C.M. Megarids, Self-driven one-step oil removal from oil spill on water via selective-wettability steel mesh, ACS Appl. Mater. Interfaces 6 (22) (2014) 19858–19865. J.D. Martin, S.D. Hudson, Mass transfer and interfacial properties in two-phase microchannel flows, New J. Phys. 11 (2009) 115005. L.R. Fisher, E.E. Mitchell, N.S. Parker, Interfacial tensions of commercial vegetable oils with water, J. Food Sci. 50 (1985) 1201–1202. T. Glawdel, C.L. Ren, Droplet formation in microfluidic T-junction generators
[42]
[43]
[44]
[45] [46]
891
operating in the transitional regime. III. Dynamic surfactant effects, Phys. Rev. E 86 (2012) 026308. T. Svitova, O. Theodoly, R.M. Hill, C.J. Radke, Wetting behavior of silicone oils on solid substrates immersed in aqueous electrolyte solutions, Langmuir 18 (2002) 6821–6829. L.A. Díaz-Alejo, E.C. Menchaca-Campos, J.U. Chavarín, R. Sosa-Fonseca, M.A. García-Sánchez, Effects of the addition of ortho- and para-NH2 substituted tetraphenylporphyrins on the structure of nylon 66, Int. J. Polym. Sci. 2013 (2013) 323854. J. Charles, G.R. Ramkumaar, S. Azhagiri, S. Gunasekaran, FTIR and thermal studies on nylon-66 and 30% glass fibre reinforced nylon-66, E-J. Chem. 6 (1) (2009) 23–33. http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html. https://www.medicinescomplete.com/mc/excipients/2012/218025.htm.