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Sustainable and Biodegradable Superhydrophobic Coating from Epoxidized Soybean Oil and ZnO nanoparticles on Cellulosic Substrates for Efficient Oil/Water Separation Quan-Yong Cheng, Xu-Pei An, Yi-Dong Li, Cai-Li Huang, and Jian-Bing Zeng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02549 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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ACS Sustainable Chemistry & Engineering

Sustainable and Biodegradable Superhydrophobic Coating from Epoxidized Soybean Oil and ZnO nanoparticles on Cellulosic Substrates for Efficient Oil/Water Separation Quan-Yong Cheng,† Xu-Pei An,† Yi-Dong Li,† Cai-Li Huang,‡ Jian-Bing Zeng*, † †

School of Chemistry and Chemical Engineering, Southwest University, 2# Tiansheng

Road, Chongqing 400715, China ‡

Neutron Science Directorate, Oak Ridge National Laboratory, Bethel Valley Road

Tennessee 37830, USA *Corresponding author, Email: [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: Conventional superhydrophobic materials for oil/water separation were usually prepared from non-degradable and non-renewable resources, which would not only increase resource crisis but also cause environmental pollution after discarding. In this study, we report a renewable and biodegradable superhydrophobic material, which was fabricated via a two-step curing dip-coating method and was composed of cellulosic filter paper or fabric, cured epoxidized soybean oil (CESO), ZnO and stearic acid (STA) as the substrate, adhesive, rough structure and low energy modifier, respectively. The CESO combined ZnO nanoparticles with cellulosic substrate tightly to show excellent stability when suffering from immersing in water and oil for 7 days without losing superhydrophobicity. When the superhydrophobic coating was constructed onto the surface of cellulosic fabric, the material showed high separation efficiency with the values higher than 97 % during separation of various oil/water mixtures. The sustainable and biodegradable superhydrophobic material may find potential application in efficient oil/water separation without creating material waste after throwing away. KEYWORDS: superhydrophobicity; cured epoxidized soybean oil; biodegradable; oil/water separation

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INTRODUCTION Great efforts have been devoted to deal with oily wastewater pollutions, which are usually caused by never-ended oil spill accidents and ever-increasing consumption of fossil fuels and have been threatening the sustainable development of human beings seriously.1-5 Superhydrophobic materials, with water contact angle (WCA) above 150°,6-8 exhibiting excellent oil/water separation capability, have attracted increasingly attention in the treatment of oily wastewater.9-12 Superhydrophobic phenomenon was firstly observed on some plant leafs, such as lotus leaf.13-15 The superwettability mechanism of lotus leaf, also well-known as “lotus effect” attributes the unique superhydrophobicity to the combination of the rough structure with protrusions and the waxy layer with low surface energy on the surface of lotus leaf.16-18 Inspired by the lotus effect, a great deal of artificial superhydrophobic surfaces have been fabricated by constructing appropriate hierarchical surface geometry and surface chemical structure.19-22 The approach towards hydrophobic surfaces by mimicking lotus effect usually include two steps, i.e., creation of rough surface structure and subsequent low surface energy modification.23,24 One versatile strategy to fabricate hydrophobic surface is to coat nanoscale particles onto the surface of substrate to create the micro/nanoscale structure and subsequently low energy modification of the hierarchical structure. A variety of studies have created micro/nanoscale structures by coating nanoparticles onto various substrates with adhesives to improve stability of the superhydrophobic surface by improving the 3



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interfacial adhesion between the coating particles and the substrates.25-30 Both the substrates and the adhesives used in superhyrophobic materials for oil/water separation were usually from nonrenewable resources and were not biodegradable after throwing away.31-33 The widely used substrates included metal meshes, sponges, aerogels, and synthetic fiber fabrics,

3, 5, 31

and widely used

adhesives were diglycidyl ether of bisphenol-A (DGEBA) based epoxy thermosets, poly(methyl methacrylate) (PMMA), polystyrene (PS), and polydimethylsiloxane (PDMS).24-30, 34 The extensive use of these materials would not only result in fast consumption of non-renewable resource but also lead to secondary environmental pollution, since the non-degradable wastes of the used separation materials were usually burned or discarded directly after their life-time.31, 34, 35 Therefore, it is urgent to explore sustainable and biodegradable oil/water separation superhydrophobic materials from renewable resources to replace the nonrenewable and non-degradable analogues.25,

36-38

Although renewable and biodegradable cellulosic materials and

poly(lactic acid)-based nonwoven fabric have been used as the substrates in the recent studies. However, the adhesives and auxiliary components were still nonrenewable and non-degradable in these investigations. 31-33, 39, 40 In this study, we report a renewable and biodegradable superhydrophobic material consisting of sebacic acid cured epoxidized soybean oil (CESO) thermoset as the adhesive, cellulosic paper and fabric as the substrates, ZnO nanoparticles as the nano-/micro- structure component and stearic acid as the low energy modifier. The renewable and biodegradable diacids cured epoxidized soybean oil thermosets with 4


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plenty of hydroxyl, ester and epoxy groups showed excellent adhesive properties, thus can be used as the alternatives to the nonrenewable and non-degradable adhesives in preparation of eco-friendly superhydrophobic materials. In addition, all the other raw materials used in this study are derived from inexpensive renewable resources and are able to biodegrade after discarding.41-43 This work provides a novel cost-efficient pathway towards design and fabrication of the environmentally friendly oil/water separation material.

EXPERIMENTAL SECTION Materials. Epoxidized soybean oil (ESO) with epoxy value of ≥6.0% was purchased from Micxy Chemical Co., Ltd (Chengdu, China). The average number of epoxy group per an ESO molecule is 4.1 as calculated by NMR analysis. Sebacic acid (SA, 98.5%), stearic acid (STA, ≥99%), the catalyst 1, 8-Diazabicyclo [5.4.0] undec-7-ene (DBU, ≥98%) and cellulosic filter paper were obtained from Micxy Chemical Co., Ltd. Nano-ZnO (99.7%, 50±10 nm) and silicon oil were received from Aladdin Industrial Corporation and nano-ZnO was dried in an 80 °C oven for 1 hr prior to use. Decane was purchased from Shanghai Titanchem Co., Ltd. Chloroform, toluene, petroleum ether, absolute ethanol, NaCl (≥99.5%), KCl (≥99.5%), Na2HPO4·12H2O (≥99.0%) and KH2PO4 (≥99.5%) were obtained from Chuandong chemical plant (Chongqing, China). Cellulosic fabric was bought from local store. Curing of ESO with SA in the Presence of DBU. The curing process of ESO with SA as a curing agent and 1 wt% DBU as a catalyst was performed at 150 °C in a flask 5



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with a magnetic stirrer. The gel fraction with time was monitored by taking a part of products out of the flask at various time for solvent immersion measurement. The product (W1, ~1g) was immersed in 25 mL chloroform at room temperature for three days to dissolve the soluble part. The insoluble part was isolated from the solution and weighed (W2) after vacuum drying at 80 ℃ for 24 h. The gel fraction (GF) was calculated by: GF=

W2 ×100% W1

(1)

Construction of Superhydrophobic STA/ZnO/CESO Coating on Cellulosic Substrates. The construction of superhydrophobic coating on cellulosic substrates (filter paper and fabric) were performed via a two-step curing dip-coating method with STA, ZnO and CESO as the sustainable resources, as shown in Figure 1. The substrates in absolute ethanol were treated with ultrasound for 10 minutes to remove possible impurities and were then dried at an 80 °C oven for 1 hr prior to use. Ethanol solution containing 20 wt% ESO plus SA and 0.2 wt% DBU was prepared as the adhesive solution. The weight ratio of SA to ESO was 0.31:1 to get -COOH/epoxy molar ratio of 0.7:1.44, 45 The purified cellulosic substrate was then immersed in the adhesive solution for 30 min. Afterwards, the well soaked substrate was moved into an 80 °C fan-assisted oven for 10 min to remove the ethanol and then pre-cured at 150 °C for 30 min. After cooling to room temperature, the sample was immersed into a 0.04 g mL-1 ZnO water dispersion for 30 min. ZnO water dispersion was prepared by ultrasonication of the mixture of ZnO and water for an hour. After coated with ZnO, 6


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the samples were dried and then cured at 150 °C for 2.5 hr. The superhydrophobic material was then obtained by immering in 0.2 mol/L STA ethanol solution for 6 hr followed by drying at 120 °C for an hour. Chemical reaction between the surface hydroxyl group of ZnO and carboxyl group of STA would occur during the processing.46,47 For comparison, the control material with one-step curing dip-coating method was prepared. For the one-step curing dip-coating method, the sample was cured at 150 °C for 3 hr prior to immersing in ZnO dispersion. In addition, the control material without CESO was also prepared. The thickness of the pristine and coated substrates was measured with a thickness gauge (CH-1-ST, Liuling instruments, Shanghai, China).

Figure 1. Schematic illustration for construction of superhydrophobic coating on cellulosic substrate with STA, ZnO and CESO as renewable feedstocks. Fourier Transform Infrared (FT-IR). FT-IR spectra under reflective mode were recorded on RF-5301PC spectrophotometer in a range of wavenumbers from 4000 to 500 cm-1 with the resolution and scanning time of 4 cm-1 and 32 times, respectively. The sample was vacuum dried at 80 °C for 24 h before measuring. Water Contact Angle Measurement. The water contact angel (WCA) of surface was measured by JC2000C1 with 5 µL distilled water droplet at ambient temperature. 7



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Five measurements were performed for each sample and the averaged results were reported. X-ray Photoelectron Spectroscopy (XPS).

XPS spectra were recorded on a

Thermo Scientific X-ray photoelectron spectroscopy (Escalab 250Xi) at room temperature to calculate the surface elements composition. Scanning Electron Microscopy (SEM). The morphology for the surface of the sample was observed by a Hitachi (Japan) S-4800 scanning electron microscopy at an accelerating voltage of 20 kV. The sample was sprayed with a layer of foil prior to observation. Oil/Water Separation. 20 ml mixture of oil and water with volume ratio of 1:1 was used as mimic oily wastewater. The oil was colored by red dye. The as-prepared superhydrophobic material was used as a filter membrane, which was put above a small beaker in a big beaker. The oily wastewater was dropped onto the superhydrophbic material for separation. The oil fell into the small beaker while the water fell into the big beaker. Five mimic oily wastewaters containing different oils of decane, petroleum ether, toluene, chloroform and silicon oil were used to study the separation selectivity and efficiency of the prepared superhydrophobic materials. The weight of oil before (Wb) and after (Wa) the separation was determined to indicate the separation efficiency (SE) according to the following equation: SE=

Wa × 100% Wb

(2)

The flux was calculated by: 8


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F=

V St

(3)

where V represents the volume of the collected oil, S represents the surface area of separation material, and t was the time. Stability Measurement. To evaluate the water resistance of the constructed superhydrophobic material, the stability of the sample was investigated by immersion in water and oil for given times, and then were subjected to water contact angle measurement and SEM observation. Hydrolytic Degradation. Hydrolytic degradation in phosphate buffer solution (pH=7.4) was performed to characterize the biodegradation of the prepared superhydrophobic cellulosic materials. The phosphate buffer solution was composed of of 8.00 g NaCl, 0.20 g KCl, 2.89 g Na2HPO4 and 0.20 g KH2PO4 in 1 L distilled water. The samples with dimension of 10 mm×10 mm×0.4 mm and weight of W0 were used for hydrolytic degradation test. They were put into a glass container, which is filled with the buffer solution. The hydrolysis was performed at 37 ℃. The buffer solution was refreshed once a week. The samples were taken out of the solution after predetermined time. After washed with distilled water for three times, they were vacuum dried at 60 ◦C to a constant weight (W1). The weight loss of the sample was calculated from the following equation: Wloss =

W1 -W0 ×100% W0

(4)

Three specimens were used for each degradation time and the averaged results were reported. 9



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RESULTS AND DISCUSSION Curing of ESO with SA in the presence of DBU. In order to design the procedure for fabrication of superhydrophobic materials, we studied the curing of ESO with SA as a curing agent and DBU as a catalyst. Scheme 1 proposes the possible curing reaction between ESO and SA to form CESO thermoset. We monitored the gel fraction with time for the isothermal curing of ESO and SA at 150 °C, as shown in Figure 2a. No gel formed with time less than 45 min, corresponding to the formation of soluble precursors with branched structures.48,49 When the time increased to 60 min, the gel fraction drastically increased to 68.8 wt%, indicating the formation of network structured thermosets. With further increasing time, the curing reaction developed and the gel fraction increased gradually to 86.7 wt% at 2.5 h. The gel fraction only increased to 87.5 wt% at 3.0 h, indicating well developed network structure of the CESO thermoset.

Scheme 1. Curing reaction of ESO with SA in the presence of DBU as a catalyst.

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Figure 2a shows the FT-IR spectra of the cured products obtained by curing 30 min and 3.0 h. The characteristic absorptions at 1700 (C=O in carboxyl group) and 910 cm-1 (epoxy) of the spectrum of sample prepared by curing for 30 min indicate that plenty of carboxyl and epoxy groups remained after reaction for 30 min. These functional groups would be very helpful to combine ZnO nanoparticles for fabrication of superhydrophobic materials, as discussed in the following text. It is obvious that these two characteristic absorptions disappeared after curing for 3.0 h, indicating completion of curing. Based on these curing behaviors, we designed two procedures to fabricate the hydrophobic materials, i.e., one-step and two-step dip coating methods, to study the effect of curing behavior on the properties of the resultant products. For the one-step method, the substrate after coated with solution of ESO, SA and DBU was completely cured at 150 °C for 3 h before dipping in ZnO suspension. For the two-step method, the substrate after coated with solution of ESO, SA and DBU was first pre-cured at 150 °C for 0.5 h, then dipped in ZnO suspension, and finally cured at 150 °C for 2.5 h.

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Figure 2. Development of gel fraction with time for isothermal curing of ESA with SA in the presence of DBU as a catalyst at 150 °C (a) and the FT-IR spectra of the cured products obtained by curing for 30 min and 3 h (b). Surface Morphology and Wettability. The hydrophobic coating was first constructed onto the surface of cellulosic filter paper (CFP) via the method reported in section 2.3. For brevity, the sample without CESO was abbreviated to STA/ZnO/CFP, and the samples prepared via one-step or two-step curing procedures were abbreviated to STA/ZnO/1CESO/CFP and STA/ZnO/2CESO/CFP, respectively. The surface morphology was observed on SEM and the surface wettability was characterized by water contact angle (CA) measurement, as shown in Figure 3. The original CFP 12


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shows three-dimensional porous network structures consisting of smooth cellulosic fibers and the contact angle for the surface of the filter paper is not detectable due to the immediate soak of water after dropping, resulted from the hydrophilic nature of filter paper surface (Figure 3a1). The surface of the CFP became much rougher after modifying with ZnO, however, the water still penetrated through the surface due to the hydrophilic nature of ZnO, as shown in Figure 3a2. Significantly, the CA increased to 140º after modification with low energy substance STA, as shown in Figure 3a3. But, superhydrophobic surface was not achieved by simply treating CFP with ZnO and STA, since the final CA of the surface was still much lower than 150º. In addition, it was clearly seen that some coated particles peeled off the cellulosic fibers of the substrate, indicating a weak interfacial adhesion between ZnO and cellulosic microfibers, which would result in poor stability of the resultant coating. In order to bind ZnO with cellulosic fibers tightly, epoxy adhesive from curing of epoxidized soybean oil with sebacic acid, abbreviated to CESO, was used as the biobased and biodegradable adhesive between ZnO and the substrate. Two kind of dip-coating methods namely two-step and one-step curing procedures were applied to study the effect of curing ways of CESO on the morphology and surface wettability of CFP. In the two-step curing procedure, the filter paper coated with feedstocks of CESO was first pre-cured at 150 °C for 0.5 hr before dipping in ZnO dispersion and then cured at 150 °C for 2.5 hr before treating with STA. In the one-step curing procedure, the CFP with surface coated feedstocks of CESO was cured at 150 °C for 3.0 hr before dipping in ZnO dispersion. The surface morphology of CFP almost kept unchanged after 13



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coating with pre-cured (Figure 3b1) or cured (Figure 3c1) CESO. However, the CA increased to 100 and 105º, respectively, indicating that hydrophobic CESO was successfully coated onto the surface of CFP. After coated with ZnO, the surfaces became much rougher regardless of the curing steps. It was seen that some locations on the surface of ZnO/1CESO/CFP (Figure 3c2) were not covered by ZnO particles. In contrast, the surface of ZnO/2CESO/CFP (Figure 3b2) were all covered by ZnO particles, which accounted for the higher CA for the surface of ZnO/2CESO/CFP. The reason should be attributed to that the ZnO particles shows greater adhesion with pre-cured CESO than cured CESO, since there were plenty of epoxy groups remained for pre-cured CESO to provide strong adhesion with ZnO. By comparing Figure 3a2 and 3c2, it can be seen that ZnO particles on the surface of 1CESO/CFP were much denser than the surface of CFP, indicating that CFP coated with CESO has better adhesion with ZnO than the original CFP. The CA increased from 140º to 156º when the surface of ZnO/2CESO/CFP prepared by two-step curing procedure was treated with STA, indicating a superhydrophobic surface. In contrast, superhydrophobicity was not obtained for the one-step cured ZnO/1CESO/CFP by treating with STA, with the CA of 145º as shown in Figure 3c3.

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Figure 3. SEM images and contact angle for original CFP (a1), ZnO/CFP (a2), STA/ZnO/CFP (a3), 2CESO/CFP (b1), ZnO/2CESO/CFP (b2), STA/ZnO/2CESO/CFP (b3), 1CESO/CFP (c1), ZnO/1CESO/CFP (c2) and STA/ZnO/1CESO/CFP (c3). Surface Loading Analysis. In order to understand the insight into the difference in the final CA of the three surfaces, the loadings of CESO, ZnO and STA were analyzed according to the weight of CFP. The results are graphically shown in Figure 4. For STA/ZnO/CFP, the loadings of ZnO and STA were 8.9 and 17.4 wt%, respectively. For STA/ZnO/2CESO/CFP, the loading of CESO was 24.8 wt% and those of ZnO and STA were 35.8 and 21.8 wt%, respectively. In the case of STA/ZnO/1CESO/CFP, the loading of CESO was as high as 53.4 wt%, while those of ZnO and STA were 12.6 and 8.9 wt%. The loadings of ZnO were in accordance with particle compactness on the surface of the corresponding sample, as observed by SEM. The dissolution of 15



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unreacted feedstocks or uncured low molecular weight precursors, formed during pre-curing, during immersion in ZnO ethanol dispersion should result in the lower CESO loading of two-step cured sample in comparison with the one-step cured counterpart, of which the feedstocks cured completely before dipping in ZnO ethanol dispersion. What is more important is that the loadings of ZnO and STA of STA/ZnO/2CESO/CFP were much higher than those of STA/ZnO/1CESO/CFP, which was responsible for the difference in contact angle between the two samples, and also indicated that the pre-cured CESO precursors showed higher absorption capability toward ZnO than the completely cured CESO, due to the presence of unreacted epoxy group in pre-cured precursors which showed better affinity with ZnO.

Figure 4. Surface loading of CESO, ZnO and STA of the samples. XPS Analysis. The surface atomic compositions for the ZnO coated samples before and after treating with STA were measured by XPS. Figure 5 shows the corresponding XPS spectra of the samples and Table 1 summarizes their surface atomic compositions. The peaks of Zn element could be observed in all curves, indicating that ZnO was coated onto the surfaces of all samples. 16


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Figure 5. XPS scans for the surfaces of nano-ZnO coated filter paper before (a) and after treating with STA (b).

It was clear that the peak of C element strengthened while that of Zn element weakened after treating with STA by comparing the spectra of the samples before (Figure 5a) and after treating with STA (Figure 5b), which demonstrates that STA was successfully incorporated onto the surface of the samples. It was interesting to find that the surface Zn element content for STA/ZnO/CFP decreased considerably to 1.7% from

21.4%

of

ZnO/CFP,

while

those

of

STA/ZnO/1CESO/CFP

and

STA/ZnO/2CESO/CFP only reduced to 2.1% and 2.2% from 5.2% and 5.5% of 17



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ZnO/1CESO/CFP and ZnO/2CESO/CFP, respectively. The drastic reduction in Zn element content for ZnO/CFP by treating with STA was attributed to peeling out of ZnO from the original CFP surface, due to the poor interfacial adhesion, as observed by SEM in section 3.1. The contents of Zn and C elements on the surface of STA/ZnO/2CESO/CFP were higher than those of STA/ZnO/1CESO/CFP, which was in accordance with the results obtained by loading analysis. Table 1. Surface element composition of the samples Sample

Atomic percent (mol %) C

O

Zn

ZnO/2CESO/CFP

70.7

23.8

5.5

ZnO/1CESO/CFP

73.8

21.0

5.2

ZnO/CFP

38.2

40.3

21.5

STA/ZnO/2CESO/CFP

88.4

9.4

2.2

STA/ZnO/1CESO/CFP

87.9

10.0

2.1

STA/ZnO/CFP

84.4

13.9

1.7

Stability. The hydrophobic materials were designed for oil/water separation. They would contact with water and oil during practical applications. Therefore, it is necessary to study the water resistance of the surfaces to evaluate their stability. For stability analysis, the samples were dried after immersing in water for given times and were then subjected to water contact angle measurement, SEM experiment and weight loss testing. Figure 6a shows the variation of CA versus water immersion time of the samples. STA/ZnO/CFP and STA/ZnO/1CESO/CFP exhibited poor stability with contact angle decreased obviously with increase in immersion time. The contact angle of STA/ZnO/CFP and STA/ZnO/1CESO/CFP decreased from 145º and 147º to 125º 18


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and 137º, respectively, after immersion in water for 7 days. In contrast, STA/ZnO/2CESO/CFP showed excellent stability with contact angle almost keeping unchanged with immersion time, and the value was 152° after immersion in water for 7 days. The different stabilities of the samples were attributed to the different interfacial bindings between ZnO and the substrates. The weakest interfacial adhesion occurred for STA/ZnO/CFP, due to the absence of adhesive between ZnO and CFP, and the strongest interfacial adhesion existed for STA/ZnO/2CESO/CFP, since the complete cure of adhesive took place after absorption of ZnO. The interfacial adhesion of STA/ZnO/1CESO/CFP should be smaller than STA/ZnO/2CESO/CFP, because some epoxy groups remained for STA/ZnO/2CESO/CFP but almost no epoxy group remained in the case of STA/ZnO/1CESO/CFP during dip-coating with ZnO. The epoxy groups provided strong interfacial adhesion between ZnO and substrate for STA/ZnO/2CESO/CFP. The interfacial adhesion between ZnO and the different substrates could be qualitatively compared by the weight loss of the sample during water immersion measurement, because the surface coated STA/ZnO may fall off the substrate during water immersion and the stronger the interfacial adhesion, the smaller the weight loss. After immersion in water for 7 days, STA/ZnO/2CESO/CFP, STA/ZnO/1CESO/CFP and STA/ZnO/CFP showed the weight loss of 1.7, 10.2 and 12.5 wt%, respectively. The decrement in weight for the samples showed similar behavior with the decrement in contact angle during water immersion experiment. Furthermore, the oil resistance of as-prepared surfaces was also investigated by immersing in decane for different time. Figure 6b shows the variation of water contact 19



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angle with immersion time for the samples. All the samples showed good oil resistance as evidenced by the little fluctuated water contact angle with immersion time. Although STA/ZnO/1CESO/CFP and STA/ZnO/CFP showed good oil resistance, they

were

not

stable

when

suffering

from

water

immersion.

The

STA/ZnO/2CESO/CFP sample showed excellent both water and oil immersion stability could hopefully find durable application in oil/water separation.

Figure 6. The variation of water contact angle with water (a) and oil (b) immersion time for the samples. The weight loss arisen from STA/ZnO peeling off the surface of the samples would change their surface morphology. Figure 7 shows the surface morphology of the 20


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as-prepared samples and the counterparts after immersion in water for 3 and 7 days. Obvious STA/ZnO peeling off occurred for STA/ZnO/CFP after immersion in water for 3 days (Figure 7a2) and the peeling off aggravated with increase time to 7 days (Figure 7a3). For STA/ZnO/2CESO/CFP, the surface morphology almost kept unchanged regardless of water immersion time (Figure 7b1~b3). In the case of STA/ZnO/1CESO/CFP, compared to the as-prepared sample (Figure 7c1), the roughness decreased after immersion in water for 3 days (Figure 7c2) in spite of the absence of obvious STA/ZnO peeling off, and obvious peeling off occurred when the immersion time increased to 7 days (Figure 7c3). The results were in accordance with the change in the CA and weight loss of the samples when suffering from water immersion, and once again indicated that the sample prepared via two-step curing procedure possessed strongest interfacial binding between ZnO and the substrate to exhibit excellent stability without obvious change in surface morphology.

21



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Figure 7. Surface morphology of STA/ZnO/CFP (a), STA/ZnO/2CESO/CFP (b) and STA/ZnO/1CESO/CFP (c) after immersion in water for different time: 0 day (a1, b1 and c1), 3 days (a2, b2 and c2) and 7 days (a3, b3 and c3). Oil/Water Separation. The oil/water separation performance of superhydrophobic STA/ZnO/2CESO/CFP was evaluated by using it as filter paper to separate the mixtures of water and decane colored by red dye, as shown in Figure 8. The dyed oily decane fell into the inner small beaker after penetrating through the superhydrophobic filter paper, while the water could not penetrate through the paper and thus was excluded to the outer big beaker. The mixtures of water and decane were separated completely, as indicated in Figure 8d. However, it was found that the separation rate was very low. Two hours were required to separate 20 mL of decane/water with volume ratio of 1:1. The slow separation rate should be resulted from the blocking off most holes on the surface of filter paper after constructing of superhydrophobic coating, which could be observed from the surface morphology of the sample (Figure 3b3).

22


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Figure 8. Schematic illustration for oil/water separation of superhydrophobic STA/ZnO/2CESO/CFP filter paper.

Constructing Superhydrophobic Coating onto the Surface of Cellulosic Fabric for Fast Oil/Water Separation. In order to obtain sustainable and biodegradable superhydrophobic materials with fast oil/water separation rate, we constructed superhydrophobic STA/ZnO/CESO coating onto the surface of cellulosic fabric (CFB) via the same two-step curing procedure. Large pores of CFB would showed fast oil penetrating rate if superhydrophobicity was achieved. Figure 9 shows the surface morphology and contact angle of original CFB, CESO/CFB, ZnO/CESO/CFB and STA/ZnO/CESO/CFB. The original CFB showed porous network weave structure with smooth cellulosic fibers, and the water droplet infiltrated the surface immediately, as shown in Figure 8a1 and a2. After coated with CESO, the fibers were still smooth and the individual fiber is hard to discriminate (Figure 9b2) since it was adhered with other fibers, due to the presence of CESO, which increased the contact angle to 110°. It was seen that the weave structure of CFB remained unchanged and the pores were not blocked, as shown in Figure 9b1. The weave structure of the cellulosic fabric was well covered after being dip-coated with ZnO and the pores remained (Figure 9c1). The high-resolution SEM image in Figure 9c2 clearly shows that ZnO fully covers the individual cellulosic fibers, of which the surface changed to rough and the contact angle enhanced significantly to 140°. After modification with low energy STA, the porous network structure retained and superhydrophobic surface with contact angle of 158º was successfully achieved, as shown in Figure 9d1 and d2. 23



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The superhydrophobic CFB with porous network structure would exhibit faster oil/water separation rate than the superhydrophobic CFP, of which the pores were blocked off significantly. The application of superhydrophobic CFB in oil/water separation was also investigated with the similar method to that of superhydrophobic CFP. The time required to completely separate 20 mL mixtures of decane and water reduced to 5 min, compared to 2 hr for superhydrophobic CFP. With fast separation efficiency, the sustainable and biodegradable superhydrophobic CFB could find potential application in oil/water separation.

Figure 9. The SEM image and contact angle for original CFB (a1, a2), CESO/CFB (b1, b2), ZnO/CESO/CFB (c1, c2) and STA/ZnO/CESO/CFB (d1, d2).

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In order to show the separation selectivity and efficiency of the materials, five different oil/water mixtures consisting of water with decane, petroleum ether, toluene, chloroform or silicon oil were separated using the prepared superhydrophobic cellulosic fabric. Five time separations were performed for each oil/water mixture and the separation efficiencies are shown in Figure 10. It is noted that the separation efficiency for the first time was not high enough, around 90%, regardless of the composition of the mixture. The relatively low separation efficiency was due to the inevitable oil absorption of the cellulosic fabric. It is interesting that the separation efficiency increased to 97~99% for the latter four times separation of all the oil/water mixtures. The separation efficiency is comparable to other non-degradable superhydrophobic materials coated fabrics, such as PA−Mn@PDMS-coated materials,9 polyaniline and fluorinated alkyl silane coated cotton fabrics, ormosil@fabrics

51

50

PDMS–

and polystyrene grafted cotton fabrics.52 The results indicate that

the prepared superhydrophobic cellulosic fabric is an efficient and versatile separation material, which is able to separate various oil/water mixtures containing different oily compounds effectively. The fluxes of different oils were measured, as shown in Figure 10b. The silicon oil with highest viscosity showed the lowest flux with the value of ~480 L m-2 h-1. Other liquids with low viscosities showed high flux with the values in the range of 23.5 k-33.8 k L m-2 h-1, depending on the type of oils mainly due to their different viscosities. The values were much higher than the superhydrophobic and superoleophilic polybenzoxazine/TiO2 modified polyester non-woven fabric53 and superhydrophobic PLA nonwoven fabric with the treatment of SiO2 nanoparticles and 25



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PS microspheres on the polydopamine (PDA)-modified PLA fabric.31

Figure 10. The separation efficiency of different oil/water mixtures for five cycles of each sample. Thickness of Coatings on the Different Substrates Surfaces. The thickness of coated and uncoated surfaces was measured and the results are summarized in Figure 11. The thickness of pristine CFP, CESO/CFP and STA/ZnO/CESO/CFP was 167, 172 and 210 µm, respectively. The thickness of CESO layer was 5 µm. After coated with ZnO and STA, the thickness got a 38 µm increase. Therefore, the superhydrophobic coating thickness on CFP was 43 µm. The thickness of pristine CFB, CESO/CFB and STA/ZnO/CESO/CFB were 191 um, 235 um and 279 um, respectively. The 26


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superhydrophobic coating thickness on CFB substrate was 88 µm. It seems there is no direct relationship between the coating thickness and the separation behavior, since superhydrophobic CFB with thicker coatings showed better separation property. The separation behavior is more dependent on textile structure of the substrates. Although the coating thickness on CFB is higher, the holes on the fabric are not covered after coating (Figure 9d1), which makes the oils to penetrate the substrate easily to exhibit better separation property.

Figure 11. Thickness of the pristine and coated CFP and CFB. Hydrolytic Degradation. As mentioned in the introduction section, most of the previously

reported

superhydrophobic

oil/water

separation

materials

are

non-degradable, waste pollutions would accumulate after use of those separation materials. All the raw materials used in this study to fabricate superhydrophobic material are biodegradable. The biodegradation of the adhesive, i.e., dicarboxylic cured ESO thermoset, has been reported in literature.41 In order to confirmed the biodegradability of the prepared superhydrophobic materials, the hydrolytic degradation of the samples in 37 °C phosphate buffer solution (pH=7.4) were 27



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evaluated. Figure 12 shows the weight loss of CFB, CESO/CFB and STA/ZnO/CESO/CFB during hydrolytic degradation. It is obvious that the weight loss increased gradually with increasing time for all the samples, indicating that they are all biodegradable. For a given time, the degradation rate followed the order: CFB>CESO/CFB>STA/ZnO/CESO/CFB, as the value of weight loss decreased in this sequence. After degradation for 70 days, the weight losses of CFB, CESO/CFB and STA/ZnO/CESO/CFB were 18.1, 15.9 and 5.9 wt%, respectively. The results are reasonable because the degradation rate is highly related to the hydrophilicity of the materials when subjecting to hydrolytic degradation.54 CFB is highly hydrophilic thus is easily attacked by water molecules to undergo relatively faster degradation. After coated with CESO, the hydrophilicity of CFB reduced, thus, CESO/CFB showed relatively smaller degradation rate. Accordingly, STA/ZnO/CESO/CFB showed the smallest degradation rate, because the difficulty for the water molecules to attack the bulk of the material increased significantly due to the superhydrophobic surface.

Figure 12. Hydrolytic degradation of CFB, CESO/CFB and STA/ZnO/CESO/CFB.

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CONCLUSION In summary, sustainable and biodegradable superhydrophobic coating was successfully constructed onto the surface of cellulosic substrates via a two-step curing dip-coating method. The surface coated CESO of cellulosic substrate formed via a two-step curing procedure worked as a very efficient adhesive to combined ZnO with the substrate very tightly to withstand water and oil immersion for 7 days without losing superhydrophobicity. In contrast, the hydrophobic surface without CESO or with one-step cured CESO showed poor stability when suffering from water immersion, due to the poor interfacial adhesion between ZnO particles and the substrate, which caused particles peeling off the substrate during water immersion. It was found that when the superhydrophobic coating was constructed onto the surface of filter paper, the oil/water separation rate was very slow, as most of the pores were blocked off to slow down the penetrating rate of oil through the filter paper. When the hydrophobic coating was built onto the surface of cellulosic fabric, the superhydrophobic cellulosic fabric showed fast oil/water separation rate and high separation efficiency and was versatile in separation of water mixtures with various oil compounds. In addition, the material was biodegradable as demonstrated by degradation measurement in phosphate buffer solution. With high separation efficiency, excellent durability and biodegradability, the sustainable superhydrophobic cellulosic fabric can be used as an ideal ecofriendly oil/water separation material for oily wastewater treatment.

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AUTHOR INFORMATION Corresponding Authors Email: [email protected] (J.B. Zeng) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Science Foundation of China (51673158) and Fundamental Research Funds for the Central Universities (XDJK2017A016 and XDJK2017C022).

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284x207mm (96 x 96 DPI)


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Sustainable and Biodegradable Superhydrophobic Coating from

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