聚酯织物表面耐用超疏水涂层的制备及在油水分离中的应用

李　杨　汪家道*　樊丽宁　陈大融(清华大学摩擦学国家重点实验室，北京100084)

聚酯织物表面耐用超疏水涂层的制备及在油水分离中的应用

李杨汪家道*樊丽宁陈大融
(清华大学摩擦学国家重点实验室，北京100084)

使用在含有甲基MQ(M：单官能团Si―O单元R3SiO1/2，Q：四官能团Si―O单元SiO2)硅树脂与疏水SiO2的二甲苯溶液中浸渍的方法，在聚酯织物表面制备了耐用超疏水涂层。经过处理后，微米级聚酯纤维表面被紧密的疏水纳米颗粒包裹，通过这种方法降低了纤维的表面能。聚酯织物展现出良好的超疏水特性，与水滴的静态接触角为156°，滚动角为5°。得到的超疏水聚酯织物在机械磨损、酸碱环境及紫外线照射条件下，表现出了良好的稳定性。此外，用超疏水聚酯织物作为过滤材料得到的油水分离效率达99%以上。该方法为大面积工业制备超疏水织物提供了新的思路。

超疏水；聚酯织物；纳米颗粒；耐用性；油水分离

[Article]

www.whxb.pku.edu.cn

It is reported that the wettability of surface is decided by the chemical composition and the morphological structure24.A quantity of researchers have studied the wettabilities of different surfaces by experiments25,calculations26,and simulations27.They conclude that superhydrophobic surfaces need creating hierarchical structures on low surface energy materials or modifying low surface energy materials on rough structures.Recently,Zhu et al.28prepared a superhydrophobic polyester fabric with mechanical stability and easy-repairability by coating with Ag nanoparticles and surface fluorination.Similarly,Zhou et al.29demonstrated a durable coating on polyester fabrics using polydimethylsiloxane (PDMS),fluorinated alkyl silane,and silica nanoparticles,which could sustain washing,boiling,and acid-alkali attack.However, the existing preparation methods to get the durable superhydrophobic fabric are some complex and time-consuming.Most strategies rely on external equipment to complete the fabrication, which sets up obstacles to easy preparation technique in largescale fabrication.In application of oil-water separation,the modified fabrics show limited efficiency and recyclability.To address the problem,an efficient and facile fabrication is very desirable.

In this work,a superhydrophobic polyester fabric was fabricated with the assistant of methyl MQ silicon resin to bond the hydrophobic silica nanoparticles and the surfaces.The whole fabrication process was facile and efficient without special devices.Moreover,the as-prepared fabrics showed excellent water repellency with water contact angle higher than 150°and maintained the repellence of water after mechanical abrasion by sandpaper,acid-alkali attack,and UV irradiation.In addition,oilwater separation experiments have been demonstrated with the asprepared fabric which show high separation efficiency above 99% with good stability and recyclability.Moreover,the extreme environmental conditions such as mechanical abrasion,acidic and alkaline attack,and UV irradiation have little effect on the high separation efficiency.This rapid and simple method to fabricate superhydrophobic fabric provides more possibility for large-scale application in the industry.

Fig.1　Schematic of the fabrication of superhydrophobic polyester fabrics through a simple solution-immersion method

2　Experimental

2.1Materials

Polyester fabrics were purchased from Wujiang Ouyang Weaving Co.,Ltd.,China.Hydrophobic fumed silica(modified with dimethyldichlorosilane,AEROSIL R972)with surface area of 130 m2∙g－1and the average particle size of 16 nm was obtained from Degussa,Germany.Methyl MQ silicone resin, whichcontainsmono-functionalsilicon-oxygenunit(CH3)3SiO1/2(M)and tetra-functional silicon-oxygen unit SiO4/2(Q)(M/Q= 0.8,Mw=8000 g∙mol－1),was purchased from Shenzhen Haili Chemical Co.,Ltd.,China.Dimethylbenzene(99%),ethanol (99.7%),hydrochloric acid(36%－38%),and sodium hydroxide (96%)were obtained from Beijing Chemical Works,Co.,Ltd., China.All chemicals were used as received.

2.2Sample preparation

The fabrication of superhydrophobic fabric was facile and efficient.As shown in Fig.1,firstly,the hydrophobic silica nanoparticles and the methyl MQ silicone resin were dissolved in dimethylbenzene,the mass fraction of 1%(w)separately,and dispersed by magnetic stirring for 15 min.After that,the polyester fabric(10 cm×10 cm)cleaned with ethanol and deionized water was steeped into the as-prepared solution for approximately 15 min,removed,and dried at ambient temperature.The fabric was not damaged by the dimethylbenzene in fabrication process.There was no need of external devices for further treatment.The fabric was cut into pieces for different tests.As a contrast,another hydrophobic fabric was prepared only by the methyl MQ silicone resin dissolved in dimethylbenzene(1%(w)).

2.3Characterization

The micrographs of the polyester fabric were obtained by a field-emission scanning electron microscope(FEI Quanta 200 FEG,Netherlands).Before the observation,all the samples were coated with gold.Static contact angle measurements with deionized water was accomplished using a Dataphysics DCAT 21 (Dataphysics,Germany)apparatus at room temperature.The volume of an individual droplet was 5 μL and each result was obtained from an average of five measurements.The sliding angle of water droplet was captured by a high speed imaging system (AcutEye,RockeTech Technology Corp.,Ltd.,China).The chemical composition of surface was analyzed by an X-ray photoelectron spectroscopy(XPS,EscaLab 250XI,ThermoScientific,USA).

3　Results and discussion

The preparation of superhydrophobic polyester fabrics described in the experimental section was illustrated in Fig.1.The cleaned fabric was steeped in the hydrophobic SiO2/methyl MQ silicon resin and dried in room temperature.This obtained fabric was looked the same as the pristine in Fig.2(a),and the pristine hydrophilic fabric was transitioned to superhydrophobic surface after the immerse(Fig.2(b)).The water droplet spread on and permeated through the pristine fabric in a few seconds while the water droplet sat on the coated fabric as a marked contrast.It indicated that the coated fabric exhibited excellent superhydrophobic property.When the coated fabric was immersed into water by an external force,it shows a mirror-like appearance which could be attributed to the existence of trapped air between water and superhydrophobic fabric corresponding to the Cassie-Baxter model(Fig.2(c))30.Moreover,the water contact angle of pristine fabric was 0°(Fig.2(d)).After coating,the water contact angle and sliding angle of fabric were 156°and 5°(Fig.2(e)and 2(f)).

The surface wettability depends on both the chemical composition and the surface morphological structure.As shown in Fig.3(a),SEM images of pristine polyester fabrics presented regular three-dimensional microfibers.In addition,the surface of fiber was smooth with an average diameter of 12 μm(Fig.3(b)), which created a primary structure of superhydrophobic surface. It is difficult to realize the repellence to water of the fabric leaning on this single microstructure.Besides that,nanostructure and low surface energy material are needed to form superhydrophobic surfaces.As a contrast,another hydrophobic fabric was fabricated only using methyl MQ silicon resin with water contact angle of 135°(Fig.3(c)).It was shown in Fig.3(c)that silicon resin wrapped the fiber and there were few nanosized protuberances.Such a structure cannot fully meet the demand of micro-nano hierarchical roughness.After the immersion step in methyl MQ silicon resin/ silica nanoparticle solution,smooth surface of fiber was covered by a homogeneous nanoscale rough structure(Fig.3(d)and 3(e)). The higher magnification of surface showed the morphology of compact nanoscale granules caused by particle aggregation of hydrophobic SiO2under the influence of silicon resin.More than that,the nanostructure was made up of hydrophobic SiO2and methyl MQ silicon resin which contained methyl group(―CH3) with low surface energy.It is the other essential condition for superhydrophobic surfaces which will be discussed in the next part.The combination of micro-nano hierarchical structure and low surface energy material met the requirement for the morphology and the chemistry of the superhydrophobicity.

Fig.2　Different wettabilities before and after immersion

Researchers have found that simplex material with low surface energy is difficult to be bonded with the substrate,and can be easily destroyed by an external force,like finger wipe and current scour.To overcome the problem,Lu et al.31found an innovative approach to address the problem using a“paint+double-sided tape/spray adhesive+substrates”method,and the results exhibited excellent adhesive ability.It is reported that methyl MQ silicone resin has been widely used in adhesives because of its low cost and excellent properties,like radiation resistance and antifriction32.Especially,the methyl MQ silicone resin has hydrophobicity that is favorable to prepare superhydrophobic surface33,34.Moreover,methyl MQ silicone resin is able to crosslink and cure at room temperature,which is efficient in industry.In consideration of simplifying the fabrication process,methyl MQ silicon resin,which was used as adhesive to strengthen the adhesion to the substrate,was mixed with silica nanoparticles in solvent.In this study,XPS was used to analyze the chemical composition on surfaces of polyester fabrics.Only peaks corresponding to C and O are observed in Fig.4(a)for pristine fabrics,and no other impurities can be detected.After the immersion in mixed solution, a new Si peak appears in Fig.4(b).Specifically,an insert image in Fig.4(b)shows the high resolution Si 2p spectrum of the coated fabrics.There are two peaks at a binding energy of 103.9 eV for Si―O bonds and 102.0 eV for Si―CH3groups,which are corresponding to the chemical structure of methyl MQ silicon resin and hydrophobic SiO2.The methyl group reduces the surface energy and gives the hydrophobicity to the fabric.The results directly prove that the existence of methyl MQ silicon resin and SiO2on the polyester fabrics.Furthermore,there were no new chemical bonds existing between the silicon resin and fabric by FTIR analysis,so the bindings between the coating and fabric was assigned to adhesion of the silicon resin as adhesive.Therefore, the superhydrophobicity of fabric is attributed to the micro-nano roughness structure and the coverage of methyl MQ silicon resin/ silica nanoparticles.Besides the good water repellence,methyl MQ silicon resin gives the coating excellent adhesion,which is important in practical application.

Fig.3　SEM images of the pristine fabric(a,b),the methyl MQ silicon resin coated fabric(c),and the methyl MQ silicon resin/silica nanoparticles coated fabric(d,e,f)

Fig.4　XPS survey spectra of the pristine(a)and the coated fabrics(b)

Oil-water separation is an important pursuit because of increasing worldwide oil pollution35.As a functional material,the superhydrophobic fabric shows ability in separating oil from water efficiently in experiments.Fig.5(a,b)shows the simple oil-absorbing process.Hexadecane dyed with oil red floated over water in a petri dish,and when the modified fabric was put into the dish, most of the hexadecane was absorbed quickly after a few seconds. Moreover,the as-prepared fabric floated on the water,and in Fig.5 (c),the water droplets sat on the hexadecane contaminated modified fabric.It indicated the fabric was superoleophilic andsuperhydrophobic at the same time.Due to the poor uptake capacity of polyester fabric,the former method to remove oil was inefficient and might be restricted.Therefore,a simple oil-water separator was designed as shown in Fig.6(a).The modified fabric was folded and placed onto funnel as a filter.When a mixture of gasoline and water was poured into the filter,only gasoline could get through the fabric and drop into the beaker due to the capillary effect36.Water was prevented to pass through the fabric because of the superhydrophobicity,and after the separating process,water was poured into another beaker(Fig.6(c)).The separation efficiency was got by calculating the ratio between the weight of water before and after the separation37.The separation efficiency of the as-prepared fabric was above 99.5%.Therefore,it is believed that the superhydrophobic fabric in our study has great potential in separation of oil-water mixtures.

Fig.5　Images of oil-water separation using the modified fabric(a)hexadecane dyed with oil red over water;(b)absorption of hexadecane by the modified fabric; (c)water droplets sitting on the hexadecane contaminated modified fabric

Fig.6　Separation process of gasoline-water mixture

Table 1　Comparison of mechanical abrasion experiments in recent studies with the coating in this article

In order to adapt to the extreme environment,the stability of superhydrophobic coating is required.Without stability,this functional surface has limited prospect in daily life or industry,so a wear-resistingtest andanenvironmental durabilitytest havebeen performed.Table 1 provides a representative summary of mechanical abrasionexperiments inpublishedworkandinthis article. It is obvious that most methods need annealing to improve the bonding of coating and remove the solvent,but not in this study. Moreover,aroughersandpaperof600meshandthehigherpressure indicate better performance of the coated fabric in this study.As shown in Fig.7(a),the insert picture illustrates the methodology of abrasion test.The as-prepared fabric was placed under different weights andwas facedtothesandpaper.Thenthefabricwas moved back and forth(20 cm for one cycle).It was found that the water contact angle of the surface was still higher than 150°after 100 abrasion cycles under a 200 g mass.With the increase of load,the water contact angle slightly deceased,but the surface still kept the water repellence.The water contact angle was 135.2°after 100 severeabrasioncycles under a 1000gmass(equal to25kPa).After 100 abrasion cycles under different masses,oil-water separation experiments were repeated.Fig.7(b)showed the high separation efficiency above 99%in all experiments,and that indicated the durability under extreme environment.For further study,the sur-face morphology of fabric rubbed at 25 kPa was observed.After such a severe abrasion,surfaces of fibers were destroyed in different degrees.Fig.8(a)showed the outermost fibers appeared cracked and broken severely,and the worse was most coating on fibers were peeled off.On the other hand,the deeper regions were protected by the 3D microstructure of fabric38,as a result,the coating was broken and planished partly as shown in Fig.8(b).The left micro and nano structure kept the water repellence property, although the water contact angle decreased slightly.The fabricated fabricsstillcouldbeusedinoil-waterseparation.

Fig.7　WCAchanges(a)and separation efficiency(b)after abrasion under different masses(m)

Fig.8　SEM images of outermost fiber(a)and deeper fiber(b) after abrasion under a mass of 1000 g

Besides that,the fabricated fabric showed good stability in separation cyclic test.Gasoline-water mixture was used in oilwater separation experiment.After each separation cycle,the coated fabric was dried in an oven at 60°C for 40 min.As shown in Fig.9(a),the separation efficiency maintained above 99%after 50 separation cycles,which indicated good stability and recyclability.Moreover,considering the practical situation,the fabric should be robust when working in strong acidic or alkaline solutions and ultraviolet irradiation,so the evaluation of the stability in described environment is very significant.The coated fabric was steeped into acidic(HCl,pH=2)and alkaline(NaOH,pH= 12)solution for 7 days and was illuminated by an ultraviolet light for 7 days.Fig.9(b)showed the results of water contact angles slight variation after exposure with the separation efficiency above 99.5%all the time.It was obvious that the coated fabrics maintained the superhydrophobic property.Through these tests,it is convincing that as-prepared fabric has a stable durability even under such demanding conditions.

Fig.9　Effect of cycle times on the separation efficiency(a)and variation of the WCAand separation efficiency of the modified fabrics exposed to acidic,alkaline solutions,and UV irradiation(b)

4　Conclusions

In summary,we have explored a feasible method to fabricate superhydrophobic polyester fabrics with commercially available silica nanoparticles and methyl MQ silicone resin.The modified fabric has a micro-nano hierarchical structure and shows excellent superhydrophobicity.Particularly,the obtained fabric shows good stability in extreme environment such as mechanical abrasion, acidic and alkaline attack,and UV irradiation.Moreover,the superhydrophobic fabrics can efficiently separate oil from water with high separation efficiency above 99%,which have good stability and recyclability.Importantly,it is obvious that this rapid and simple method has the strong potential for industrial production.Durable superhydrophobic fabric will provide more possibilities of applications in the future.

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Feasible Fabrication of a Durable Superhydrophobic Coating on Polyester Fabrics for Oil-Water Separation

LI YangWANG Jia-Dao*FAN Li-NingCHEN Da-Rong
(State Key Laboratory of Tribology,Tsinghua University,Beijing 100084,P.R.China)

Adurable superhydrophobic coating on polyester fabrics has been fabricated by a simple solutionimmersion method in a solution consisting of a methyl MQ(M:mono-functional silicon―oxygen unit R3SiO1/2, Q:tetra-functional silicon―oxygen unit SiO2)silicone resin and hydrophobic silica nanoparticles.After coating, the microstructured fibers were wrapped by compact hydrophobic nanoparticles that could lower the surface energy of the fibers.Therefore,the obtained fabric exhibited an excellent superhydrophobic property with a water contact angle of 156°and a sliding angle of 5°.It is worth mentioning that the as-prepared fabric was proved to be able to withstand extreme environmental conditions such as mechanical abrasion,acidic and alkaline attack,and UV irradiation.The practical application of the modified fabric for oil-water separation was also demonstrated with a high separation efficiency above 99%.This feasible fabrication method paves the way for using the superhydrophobic fabric on a large scale.

Superhydrophobicity;Polyester fabric;Nanoparticle;Durability;Oil-water separation

1　Introduction

Nowadays,functional materials have been popular all over the world because of their outstanding performances.As one of them, superhydrophobic materials inspired by lotus leaf have been comprehensively studied and gradually applied in many areas of daily life and industrial production in recent decades1－4.Specifically,superhydrophobic surfaces with water contact angle(WCA) higher than 150°and a sliding angle(SA)lower than 10°have many excellent properties,like self-cleaning,anti-icing,and contamination prevention.Besides creating superhydrophobic surfaces on metal5,6,glass7,8,and paper9,10,fabrics with water repellence are supposed to be fabricated for water resistant apparel, oil-water separation,and so on11－14.There are a large number of reports on fabrication methods of superhydrophobic fabrics,in-cluding layer by layer electrostatic assembly15,nanopartical deposition16,17,dip-coating18,vapor phase deposition19,electrospinning20,and so forth21－23.Nevertheless,many fabricated superhydrophobic surfaces have weak adhesion with substrates and can be easily destroyed by external force and extreme environmental conditions,which limits the further application and development. Therefore it is urgent and important to improve the robustness of superhydrophobic fabrics.

November 13,2015;Revised:January 12,2016;Published on Web:January 13,2016.*Corresponding author.Email:jdwang@mail.tsinghua.edu.cn;Tel:+86-10-62796458. The project was supported by the National Natural Science Foundation of China(51375253,51321092).

O647

10.3866/PKU.WHXB201601131

国家自然科学基金(51375253,51321092)资助项目