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Colloids and Surfaces A

journal homepage: www.elsevier.com/locate/colsurfa

Mechanically robust superhydrophobic coating from sawdust particles andcarbon soot for oil/water separation

Usama Zulfiqara,b,⁎,1, Syed Zajif Hussaina,1, Tayyab Subhanib, Irshad Hussaina,Habib-ur-Rehmana,⁎

a Department of Chemistry, SBA-School of Science and Engineering, Lahore University of Management Sciences, DHA, Lahore Cantt 54792, Pakistanb Composite Research Center, Department of Materials Science and Engineering, Institute of Space Technology, Islamabad, Pakistan

G R A P H I C A L A B S T R A C T

A R T I C L E I N F O

Keywords:Saw dustSuperhydrophobicOil/water separationSelf-cleaningMesh

A B S T R A C T

We propose a facile, cost-effective and commercially viable method to prepare durable superhydrophobic sur-faces using a bio-waste material. The naturally available nanostructured sawdust particles were combined withpolychlroprene adhesive, carbon soot and silicon polymer to formulate superhydrophobic coatings. Saw dustparticles were bonded on the substrate using polychloroprene adhesive, which was followed by depositing si-licone by dip-coating to produce a superhydrophobic surface. To further improve the mechanical and super-hdyrophobic properties, a thin layer of carbon soot was deposited and stabilized by second layer of silicone.Electron microscopy, spectroscopy and goniometry were employed for microstructural analysis, chemical natureand the contact angle measurements of the coatings. After quantifying the excellent response against mechanicalabrasion, the superhydrophobic coating was employed on a mesh to demonstrate its application for oil/waterseparation. It was observed that the coated mesh successfully separated both light and heavy oils from oil/watermixtures with high separation efficiency and therefore has the potential for large-scale separation of oil/watermixtures.

https://doi.org/10.1016/j.colsurfa.2017.12.047Received 19 October 2017; Received in revised form 15 December 2017; Accepted 18 December 2017

⁎ Corresponding authors at: Department of Chemistry, SBA-School of Science and Engineering, Lahore University of Management Sciences, DHA, Lahore Cantt 54792, Pakistan.

1 These authors contributed equally to this work.E-mail addresses: usamazulfiqar@live.com (U. Zulfiqar), habib.rehman@lums.edu.pk (Habib-ur-Rehman).

Colloids and Surfaces A 539 (2018) 391–398

Available online 19 December 20170927-7757/ © 2017 Elsevier B.V. All rights reserved.

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1. Introduction

The oil/water mixtures resulting from mishaps during transporta-tion or industrial accidents are detrimental to our ecosystem. Toxichabitats arising from these mixtures may not only imperil marine lifebut also cost human resources. To solve this problem, several strategieshave been devised which include skimmers, in-situ burning, dis-persants, and absorbents. However, the physical separation of oil/watermixture through absorbents is one of the most promising techniques asit requires less energy while producing minimum environmental pol-lution. Despite an easy approach, unmodified absorbents attract theunwanted liquid phase during separation process which restricts theirapplication for effective separation of oil/water mixtures. A novel ap-proach is to design selectively wettable materials which are only at-tracted towards desirable liquid phase to realize effective oil-waterseparation. In this regard, superhydrophobic materials have the po-tential to serve the purpose of oil/water separation as these completelyrepel water while having high affinity towards oil. These materials arealready finding applications in various engineering fields such as bio-materials, building materials, anti-corrosion coatings and materials foroil/water separations to either replace or to improve the performanceof existing materials [1–16]. Therefore, multiple approaches have beenespoused to construct superhydrophobic materials for oil/water se-paration. A few of these include superhydrophobic and superoleophobicmetallic meshes [17–24], fabrics [25,26,[1–16], foams [27–33], mem-branes [34–36] and aerogels [37], which have been employed to ef-fectively separate different kinds of oil/water mixtures. For instance, afibrous sorbent consisting of polystyrene fibers and magnetic nano-particles was fabricated by electrospinning method for oil/water se-paration [38]. In a recent report, copper nanoparticles were pre-cipitated on a fabric substrate followed by chemical modification toprepare superhydrophobic medium for the separation of oily mixtures[39].

To realize superhydrophobic and selective absorption character-istics on a surface, it must have a combination of hierarchical structureroughness and low surface energy. The roughness and surface energyare the two main parameters, which regulate the wetting properties of asurface. The roughness is generally created using nanomaterials of si-lica, titania, zinc oxide, tin oxide and carbon [40,30], while the surfaceenergy is controlled by depositing a thin surface layer of hydrophobicpolymers or by functionalizing with non-polar moieties. So far, manyprocesses including sol-gel, chemical vapor deposition, lithography,etching and polymer templating have been used to generate nanoscalefeatures; however, the surfaces formed using these methods lack me-chanical strength along with high cost and involve manufacturingprocesses that are complex and difficult to control [41,42]. Moreover,the introduction of synthetic nanomaterials in any design significantlyincreases the cost, which is generally associated with the requirementof specialized equipment and the complexity of processes involved intheir synthesis. Although superhydrophobic materials are finding in-creasing acceptability in many fields, their widespread commercialviability is tied to their cost-effectiveness and long-term chemical andmechanical stabilities. The development of a versatile super-hydrophobic coating meeting the requirements of durability, low costand large-scale productions is, therefore, the ultimate solution.

Herein, we report a simple and cost-effective method to fabricateversatile and mechanically stable superhydrophobic coatings by usingcommercially available low-cost materials. Instead of using conven-tional nanomaterials, we utilized sawdust (SD) which is a naturallyavailable bio-waste. SD is a by-product of wood machining operationcontaining lignin and cellulose as its main ingredients. Only a limitedamount of SD is used in building materials while the major chunk isburnt for energy purposes [43].This over-looked bio-waste holds avariety of interesting opportunities due to its attractive nanoscale fea-tures. The porous structure of wood, when undergoes traditional ma-chining operations, creates small size particles/rods and a myriad of

roughness on porous features. We have proposed to employ this natu-rally available rough scaffold to create stable and durable super-hydrophobic coatings. For this purpose, uniform micrometer-sizedporous particles were bonded to the substrate materials using com-mercially available polychloroprene, which is an easy-to-use adhesiveand work as a strong bonding medium between SD and substrate. Tothe best of our knowledge, polychloroprene has been used for the firsttime as an adhesive to form a strong and durable bond between SD andglass slide providing a novel framework of roughness. The roughnessproduced by this method results in durable and robust multifunctionalsuperhydrophobic surface once modified with low energy materials. Forfurther amplification of superhydrophobic characteristic and mechan-ical durability, a film of carbon soot (CS)was deposited and stabilizedby second layer silicon polymer. The multilayer coating produced bythis method holds high mechanical strength and strong potential to beused in the separation of both light and heavy oil/water mixtures.Moreover, the simplicity of manufacturing route without any need ofspecialized equipment and low-cost raw materials make this processattractive for large-scale fabrication.

2. Materials and methods

2.1. Materials

SD was purchased from a local timber market in Islamabad,Pakistan. Polychloroprene glue was purchased from Samad RubberWorks(Pvt.) Ltd., and RTV-1 silicone adhesive from GMSA IndustrialLtd. Paraffin wax candle was purchased from local market to be used asa source for carbon soot. Glass slide and commercially available me-tallic mesh was used as substrates for superhydrophobic coating. All thesolvents used in the present study were of commercial grade and dis-tilled twice before use.

2.2. Methods

Locally available SD was ground and sieved through meshes of gritsizes 200 and 400 to obtain micrometer size particles in the size rangeof 37–74 μm. The achieved micrometer-sized and nanometer-structuredSD particles were bonded to different substrates using polychloropreneadhesive. Initially the commercial grade polychloroprene adhesive wasdeposited on glass slide using a metal strip followed by application ofSD particles with a brush. The glass slide was dried at 50 °C for 15minand rinsed in methanol to remove loosely adhered SD particles.Subsequently the sample was dip-coated in 4 wt.% solution of com-mercial grade RTV-1 silicone in n-hexane. After dip-coating, the sub-strate was held in air to remove the excess solution and dried at 50 °Cfor 1 h. A thin layer of carbon soot was then deposited by holding theglass slide above the flame of a candle. The glass slide was swept overthe flame to uniformly deposit the carbon soot. The schematic of carbonsoot deposition is shown in Fig. S1. Later the sample was rinsed withmethanol to remove the excess carbon particles and dried, which wasfollowed by dipping in the same RTV-1 silicone solution again to de-posit another layer of silicone. The same scheme was followed to coatmesh samples to prepare superhydrophobic surfaces as prepared onglass slide. The schematic in Fig. 1 shows the fabrication steps, design ofsuperhydrophobic surface and its application in oil/water separation.

2.3. Characterizations

The chemical functionalities of the coating were studied usingFourier transform infrared (FTIR) spectroscopy. The microscopic ana-lysis of sawdust and coatings was performed by scanning electron mi-croscopy (SEM) while the goniometer was employed to measure thewater contact angles. At least 10 values of each sample surface wererecorded from different locations and the average value was presented.For the durability test, a glass slide loaded with 200 g weight was

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placed on an emery paper of grit size 320, which was subsequentlydisplaced through a distance of 200mm. The same sample was used forthe following abrasion cycles under similar conditions. Similarly, 100 gsand was dropped from the height of 500mm on a sample placed at anangle of 45°. After each cycle of durability test, at least three values ofcontact angle were recorded and the average value was presented. Theseparation experiments were performed by preparing mixtures of waterwith four different organic solvents including n-hexane, toluene,chloroform and dichloromethane in 1:1 ratio; 50ml of each liquid wasused during the preparation of oil/water mixtures. For convenience,these solvents are referred as oil in rest of the manuscript. The se-paration efficiency of superhydrophobic mesh was calculated by usingthe following expression:

E= (V−Vº/V)× 100 (1)

V=The volume of oil in oil/water mixtures.Vº=The volume of the oil lost during separation of oil/water

mixture using superhdyrophobic mesh.

3. Results and discussion

The SEM images of pristine SD show a variety of features includingmicrometer-sized morphologies (Figs. 2 and S2) along with narrowed-down nanometer-sized fibers, spheres and layered structures. The sur-face roughness of SD particles is associated with their productiontechnique and can lead to host functional properties. The emergence ofnanometer-scale features on micrometer-sized SD particles generatehierarchical micro/nano framework with the capability to edifice de-sired air cushion for superhydrophobic properties after modificationwith suitable low energy materials. For this purpose, we have usedcommercially available RTV-1 silicone, which readily cures and forms astrong film while retaining a rough texture of SD.RTV-1 silicone is onecomponent system consisting of polydimethylsiloxane (PDMS), curingagent and filler. PDMS is an elastic, chemically stable, bio-compatibleand hydrophobic polymer [44]; once cross-linked, thin conformalcovering of PDMS on SD particles can effectively lower the surfaceenergy of SD coatings while maintaining the desired surface roughness.

The superhydrophobic surface formed by SD particles is shown inFig. 3a. It is clear from the SEM images (Fig. 3a) that the silicone hasfully covered the rough features of SD while preserving the desired

roughness. The stable superhydrophobic surface manufactured by thismethod demonstrates a contact angle of 150 ± 1.5°. It is important tomention here that it is the average value of several contact anglemeasurements and some of the measurements were less than 150°. Tofurther enhance superhydrophobicity and generate a robust super-hydrophobic surface, CS was deposited on the intricate porous frame-work of SD particles followed by the second modification with RTV-1silicone (Figs. 3b and S3). The introduction of CS layer to the porousarchitecture of SD particles further enhanced the roughness at nan-ometer-scale ensuring improved superhydrophobic properties underharsh conditions. It is obvious from Fig. 3b that CS has uniformlycovered the entire network of SD particles and thus the coatingroughness improved further. It is important to note that the roughfeatures of SD particles are still present in the final coating and finestructure of CS particles provides additional roughness. Siliconepolymer was blended well with CS particles thus making the film me-chanically and chemically robust. The intensification of roughnesscoupled with low surface energy of the resulting film increased itscontact angle from 150 ± 2° to 155 ± 4°. The proposed and im-plemented design served two purposes: (a) it enhanced super-hydrophobicity and (b) improved mechanical integrity of the coating byenhancing its resistance to mechanical wear or abrasion. The secondlayer of silicone stabilized the CS film, which otherwise is prone todeterioration upon interaction with water or any surface in contact. Ithas been demonstrated in Fig. 4 a and b that CS film cannot sustainimpact from sand particles while its endurance increases tremendouslyonce stabilized by a silicone film. The CS film without silicone was sofragile that it underwent immediate abrasion with sand particles whilethe silicon stabilized coating was able to resist the sand impact withoutany visible damage. Previously paraffin has been used to stabilize thecarbon soot against water impact [45] and we instead have utilizedsilicone to avoid deterioration of coating.

FTIR spectra of polychloroprene, sawdust, silicone and super-hydrophobic coating are shown in Fig. 5. The peak around 3500 cm−1

in the sawdust spectrum confirms the presence of hydroxyl groups,which originates from cellulose, lignin and hemicelluloses moieties ofsawdust particles; the hydrophilic character was altered by wrappingthem with a layer of low energy silicone polymer. The appearance ofpeaks at 1258 and 790 cm−1 in the silicon andsuperhydrophobiccoating is associated with Si-C bond. The peak at 1006–1082 cm−1inthe spectra of silicon and superhdyrophobic coating correspond to

Fig. 1. Schematic showing the steps for the fabrication of coating (a),superhydrophobic surface and its application in oil/water separation (b).

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SieOeSi. These bonds are not present in the spectrum of pristinesawdust. It verifies the surface modification of sawdust by silicone.These values are in close agreement with literature [46,47].

To evaluate the mechanical stability of the prepared coatings, thecoating with and without CS layers were tested against mechanical

abrasion tests. The coatings were found to resist the severe mechanicalabrasion and retained their superhydrophobic properties after extendedexposure to mechanical abrasion. Fig. 6a shows water contact angles ofthe coatings after abrasion against emery paper. The coating without CSlayer experienced severe mechanical damage and the corresponding

Fig. 2. SEM images of sawdust micrometer sizedparticles containing micrometer scale roughness (aand b), nanosized fibers (c) and layered structure (d).

Fig. 3. Digital and SEM images of superhydrophobicsurfaces prepared from sawdust (a) and after de-position of carbon soot and stabilization with secondlayer of silicon (b).

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contact angle experienced a substantial drop from its initial value of 150° to140° after the abrasion distance of 600mm. The contact angle was droppedto 143° for an abrasion distance of 200mm only. However, the coatingwith CS layer demonstrated better resistance to mechanical abrasion as isevident from water contact angle, which only decreased from 155° to 151°after 600mm abrasion distance. From the analysis of SEM images that itcan be stated that the mechanical damage, in the case of coating with CS,was resisted by the upper CS surface while the underneath surface did notlose its integrity and superhydrophobic character. SEM analysis of theabraded surface confirmed that the upper layer of CS sustained the damageduring abrasion, as a result the underneath architecture of SD and the finecarbon particles deposited on SD layer retained its integrity thus sustainingits superhydrophobic properties (Fig. S4).

The performance of the coatings was further evaluated by sandimpact test combined with ultrasonication. The coating was bombardedwith 100 g of sand particles followed by sonication for 10min in me-thanol. The cycle was repeated three times while the contact angle wasrecorded after each cycle. The combined effect of sand test and soni-cation resulted in lowering the contact angles of both coatings (Fig. 6b);however, the drop was not significant to completely deteriorate thehydrophobic character of the surface. Again, the carbon containing

Fig. 4. Digital images showing the abrasion ofcarbon soot with sand impact (a) without silicone (b)after silicone modification.

Fig. 5. FTIR spectra of polychloroprene, sawdust, silicone and superhydrophobic coating.

Fig. 6. Variation in water contact angle of superhydrophobic surface with increasing abrasion distance (a), variation in water contact angle of superhydrophobic surface with increasingsand weight (b).

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coating demonstrated superior properties compared to the coatingwithout CS layer. It clearly demonstrates that the coatings can with-stand both high-speedsand impact and long sonication in the solvent(Fig. S5).

For practical oil/water separation applications, a superhydrophobicsurface may interact with wet slurries during separation with waste-waters. That’s why simulated slurry was forcibly interacted with thesuperhydrophobic surface to observe its response. For this purpose,

Fig. 7. EDX analysis of superhydrophobic surface after slurry test using map scan mode.

Fig. 8. Digital image of superhydrophobic surface demonstratingself-cleaning phenomenon (a), superhydrophobic mesh for oil/water separation (b).

Fig. 9. A superhydrophobic mesh demonstrating the separation of oil/water mixture (a) and collection of oil from oil/water mixture (b).

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surface coated with SD and CS was subjected to grinding against alu-mina slurry. A fibrous grinding cloth was mounted on a spin coater and5ml of 10 wt% alumina suspension was dropped onto it. The coatingwas ground against the wet cloth for 1min at 300 rpm followed byultrasonication in methanol for 20min. The contact angle measure-ments did not indicate any substantial damage to the coating. Though afew alumina particles were found to be incorporated in the frameworkof the coating as verified by EDX analysis using map scan mode (Fig. 7)and SEM images (Fig. S6), this, however, did not adversely impact thecoating performance. Although alumina nanoparticles hold hydrophiliccharacter and are, therefore, expected to adversely affect the waterrepellent properties of the coating, however, the coating retained con-tact angle of 149 ± 1° demonstrating their excellent durability.

As confirmed by the above-mentioned tests, the prepared coating isfairly robust and mechanically stable and can be used for various appli-cations especially in oil/water separation. The excellent self-cleaningcharacteristic (Fig. 8) of the coating may also open new avenues in theemerging field of protecting materials against moisture and water decays.The potential of these coatings for oil/water separation is demonstrated inFig. 9 and Fig. S7. Fig. S7 typically shows a mesh coated with super-hydrophobic material holding water without any leakage. The super-hydrophobic mesh was used for the separation of oil/water mixtures asshown in Fig. 9. The mesh readily separated the oil while blocking the flowof water through its pores. Water was not found in the separated oil, whichconfirms the excellent superhydrophobic and superoleophilic properties.The steel mesh with excellent mechanical stability modified with durablesuperhydrophobic coating can adopt various shapes to be used as amedium for oil/water separation. As shown in Fig. S7, the mesh was foldedto make a boat which was then used to separate four different oil/watermixtures. The mesh readily allowed the oil to penetrate through its poreswhile completely blocking the water. After separation, the collected fluidswere used for the calculation of separation efficiencies. It can be seen inFig. 10 that the mesh demonstrated the very high efficiency of>90% in allcases (n-hexane, toluene, chloroform and dichloromethane). There was noindication of water in the separated oil. A small volume of oil may havebeen lost during handling or absorbed on mesh surface, which justifies theminor decrease in oil volume.

It can be inferred from the results that the produced super-hydrophobic mesh is suitable for the separation of both light oils (n-hexane and toluene) and heavy oils (chloroform and dichloromethane).To examine the stability of the prepared mesh, two oils were selectedand five cycles of separation were performed. It can be seen in Fig.10bthat the mesh maintained its performance and the separation efficiencyof> 90% was recorded in both cases. It is worth-noting that thenaturally occurring nanostructured waste material is used to producesuperhydrophobic surfaces without any complex processing steps.These durable superhydrophobic materials prepared from commerciallyavailable adhesive and bio-waste have great potential to address few ofthe long-standing issues related to oil/water separation and many otherproblems demanding superhydrophobic functionalities.

4. Conclusions

A cheap, simple and scalable route was adopted to develop me-chanically robust and superhydrophobic coatings from the bio-wastematerial. The combination of SD micrometer-scale particles, poly-chloroprene adhesive, and silicone polymer formed a durable super-hydrophobic coating. The incorporation of CS and silicone to constitutethe second coating, which further enhanced the water-repellent char-acteristic. SEM revealed a variety of micrometer-sized morphologiesalong with nanometer-sized fibers, spheres and layers thus producingnovel surface roughness. After verifying the durability of super-hydrophobic coating, its application in oil/water separation was in-vestigated. Several cycles of separation confirmed that the super-hydrophobic porous coating has the potential to offer a viable solutionto separate oil/water mixtures. It is believed that the present approachhas a great potential to meet the contemporary needs of several fieldsespecially purifying the oil/water mixtures.

Acknowledgements

This work was partially supported by Directorate of Science andtechnology (DoST) Khyber Pakhtunkhwa, Pakistan. We also thank SBASchool of Science & Engineering (SSE), LUMS to provide support foraccomplishing this project. The authors wish to express their gratitudeto Mr. Huzaifa Zulfiqar for providing unconditional support during thisproject.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.colsurfa.2017.12.047

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