zinc oxide materials using Taguchi method

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An optimization of superhydrophobic polyvinylidene fluoride/zinc oxide materials using Taguchi method Adel M.A. Mohamed a,b,∗ , Reza Jafari b , Masoud Farzaneh b a

Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez Canal University, Box 43721, Suez, Egypt Industrial Chair on Atmospheric Icing of Power Network Equipment (CIGELE) and Canada Research Chair on Atmospheric Icing Engineering of Power Networks (INGIVRE) at Université du Québec a Chicoutimi, Québec, Canada b

a r t i c l e

i n f o

Article history: Received 22 August 2013 Received in revised form 3 October 2013 Accepted 4 October 2013 Available online xxx Keywords: PVDF polymer ZnO nanoparticles Superhydrophobic Taguchi method SEM FTIR

a b s t r a c t This article is focused on the preparation and characterization of PVDF/ZnO composite materials. The superhydrophobic surface was prepared through spray coating of a mixture of PVDF polymer and ZnO nanoparticles on aluminum substrate. Stearic acid was added to improve the dispersion of ZnO. Taguchi’s design of experiment method using MINITAB15 was used to rank several factors that may affect the superhydrophobic properties in order to formulate the optimum conditions. The Taguchi orthogonal array L9 was applied with three level of consideration for each factor. ANOVA were carried out to identify the significant factors that affect the water contact angle. Confirmation tests were performed on the predicted optimum process parameters. The crystallinity and morphology of PVDF–ZnO membranes were determined by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The results of Taguchi method indicate that the ZnO and stearic acid contents were the parameters making significant contribution toward improvement in hydrophobicity of PVDF materials. As the content of ZnO nanoparticles increased, the values of water contact angle increased, ranging from 122◦ to 159◦ , while the contact angle hysteresis and sliding angle decreased to 3.5◦ and 2.5◦ , respectively. The SEM results show that hierarchical micro-nanostructure of ZnO plays an important role in the formation of the superhydrophobic surface. FTIR results showed that, in the absence or present ZnO nanoparticles, the crystallization of the PVDF occurred predominantly in the ␤-phase. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction Recent developments in nanotechnology and the demonstration of various quantum size effects in nanoscale particles imply that most of the novel devices of the future will be based on properties of nanomaterials. Surfaces of materials strongly affect functional properties such as mechanical, biological, optical, acoustic and electronic properties of materials, particularly at the micro/nano scale. Surface effects stem from the interplay of surface morphology and surface chemical properties. Superhydrophobic surfaces have attracted a lot of attention because of their unique properties such as self-cleaning, antisticking, and anticontamination [1–3]. In nature, there are many superhydrophobic species such as lotus and tro leaves [1,4,5]. The surfaces of these leaves have

∗ Corresponding author at: Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez Canal University, Box 43721, Suez, Egypt. Tel.: +20 418 543 2943; fax: +20 418 545 5012. E-mail addresses: [email protected], adel [email protected] (A.M.A. Mohamed).

micrometer-scale roughness, resulting in water contact angles up to 170◦ , because air that is trapped between the droplets and the wax crystals at the plant surface minimizes the contact area. In the preparation of artificial superhydrophobic surfaces, the simple and low-cost fabrication approach is very crucial; however, its durability is also very important, in practice, but is rarely considered [6,7]. Several methods have been employed to generate engineering surfaces that can mimic the structure and chemistry of natural superhydrophobic surfaces [8]. As imitations, the artifacts adopt the secret of keeping high water repellence with low surface energy and rough microstructure. Superhydrophobic surface can be created through two-stage process, which used by many researchers. In this two-stage process, they usually create a rough surface and then modify the surface with materials of low surface free energy, such as fluorinated or silicon compounds. This process has been widely used for the fabrication of superhydrophobic surfaces on special solid substrates such as Al alloys and glass. Polymer coatings or layerby-layer deposited particles with both low surface energy and microstructures can be attached to the bulk to achieve superhydrophobic properties [9,10].

0169-4332/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.10.013

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Poly(vinylidene fluoride) (PVDF), which has the molecular unit (CH2 CF2 ), is one of the most popular polymeric materials because of their high mechanical strength, excellent thermal and chemical stabilities, and ease of fabrication into asymmetric hollow fiber membranes. It exhibits interesting electric properties, such as piezoelectricity and ferroelectricity when it exists in special crystalline forms. In addition, the history of PVDF being used as a long-term architectural coating can be traced for more than thirty years, and it has exhibited excellent durability [11,12]. These advantageous properties, coupled with its hydrophobicity, make it an outstanding membrane material particularly for industrial applications, from simple protective coating for pipes and buildings to transducer devices and detectors [13–15]. PVDF exists in at least four main crystalline structures: ␣-, ␤-, ␥-, and ␦-phases [14,16]. The crystalline structure of PVDF influences the polarity of this polymer, where, ␣- and ␤- phases denote nonpolar and polar properties, respectively. PVDF has a good solubility in many common organic solvents such as N,N-dimethylacetamide (DMAc), N,Ndimethylformamide (DMF), and N,N -dimethylacetamide (NMBA) [17–20]. Recently, inorganic nanoparticles such as silica (SiO2 ) [21,22], titanium dioxide (TiO2 ) [23,24], alumina (Al2 O3 ) [25,26], zinc oxide (ZnO) [27], and zirconium dioxide (ZrO2 ) [28] are widely used in polymer materials in order to improve the characteristics of polymer to suit a particular commercial application. The common feature of these modifications is the addition of a higher proportion of inorganic materials. Among these inorganic materials, ZnO particles have received much attention due to their stability; availability; suitable mechanical strength; unique combination of electrical, optical, and piezoelectric properties; as well as good compatibility with organic solvents used to prepare the PVDF solution [27]. Nanoscale particles are different from bulk materials due to their small size as a result of that their increased surface area. Various techniques have been employed to produce superhydrophobic materials with a water contact angles above 150◦ , including chemical vapour transport and condensation (CVTC), pulsed laser deposition (PLD), chemical vapour deposition (CVD), and hydrothermal growth [29–33]. However, most of these techniques have strict conditions (such as poisonous chemicals), expensive materials, and complex processing methods. Therefore, a simple and easy method without high cost problem and the limitation in large-scale superhydrophobic surfaces production should be widely used. The spray coating is a fairly facile and commercially available method for the widest array of applications, which is not specific to a particular substrate and can be easily applied to large surface area; moreover, it does not typically require other complicated and costly application processes [34]. Statistic tools, such as the design of experiments (DOEs), have been taken from the exclusive world of the statistician and brought into the world of manufacturing, aiming at determining how different parameters influence the final properties of the coated materials. For example, Naidu and Gowda [35] employed Taguchi method of obtaining process parameters for optimum coating thickness of teflon in spray-painting application. However, only few reports are available in the literature regarding superhydrophobic PVDF–ZnO nanocomposites prepared by spray coating. Therefore, this study is part of a larger research project which was conducted to provide a better understanding of the effects that the addition of ZnO nanoparticles would have on the hydrophobic properties of PVDF polymer materials using one-step facile spray-coating process. Taguchi method was used to minimize the number of considered experiments, in order to investigate all levels of independent parameters and to filter out some effects due to statistical variations. Confirmation tests with the optimal levels of selected parameters are carried out. The stability and icephobic

properties of the prepared PVDF/ZnO composites will be publish elsewhere. 2. Design of experimental matrix The design of experiments (DOEs) is a statistical approach to the experimental investigation that allows the analysis of the effects of several independent factors and their interaction on a dependent variable. An experimental matrix is implemented and it is composed of control factors at different levels for each run, which is the intensity assumed by each independent variable in a particular experiment [36,37]. Conventional statistical experiment design can determine the optimum condition on the basis of the measured values of the characteristic properties; while Taguchi’s experimental design does this on the basis of the variability of characteristic properties [38]. Taguchi methods have been developed by Genichi Taguchi to supply a systematic approach for conducting experimentation to determine optimum settings of design parameters [39]. The advantages of this method are reduction of effort in conducting experiments, considerable savings in experimental time with decreasing cost, and discovering significant factors in a faster way. Also, the Taguchi method allows for the analysis of many different parameters without a prohibitively high amount of experimentation. In this way, it allows for the identification of key parameters that have the most effect on the performance characteristic value so that further experimentation on these parameters can be performed and the parameters that have little effect can be ignored. Taguchi suggested the use of orthogonal arrays, which are the shortest possible matrix of permutations and combinations. The evaluation of results has been standardized by this method, which can easily be applied by researchers [39–41]. In order to analyze the results, single-to-noise ratio (S/N), where S is the standard deviation of the performance parameters for each array experiment and N is the total number of experiment in the orthogonal array, was used. The S/N ratio characteristics can be divided into three categories: nominal the better, smaller the better, and larger the better. In addition to the S/N ratio, a statistical analysis of variance (ANOVA) can be employed to indicate the impact of selected parameters and estimate the optimal levels of process parameters. In this work, the L9 orthogonal array of the Taguchi method was implemented in order to investigate the effects of the PVDF; ZnO; and stearic acid contents, and spraying distance (independent variables) on the superhydrophobic properties, represented in the water contact angle (dependent variable). Applying the simple factorial design for optimization of the assigned three levels of each parameter, the numbers of permutations would be 34 (degree of freedom = 9–1 = 8). However, the fractional factorial design reduced the number of experiments to 9. Each independent variable was analyzed at three levels. The independent variables, along with their values at selected levels, are given in Table 1. With the aim of taking into account the highest degree of interaction, a full balanced factorial plan was implemented, as shown in Table 2.

Table 1 Process parameters with their different levels of observation. Parameter designation

Variable

A B C D

ZnO (g) PVDF (g) Spraying distance (cm) Stearic acid (g)

Variable level Low

Central

High

0.5 2.5 25 0.15

1 3.75 30 0.25

1.5 5 35 0.35

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Table 2 Control factors for each experimental combination/run and S/N ratio for contact angle. Run

ZnO (A)

PVDF (B)

Spraying distance (C)

Stearic acid (D)

S/N ratio for contact angle (◦ )

P1 P2 P3 P4 P5 P6 P7 P8 P9

0.5 0.5 0.5 1 1 1 1.5 1.5 1.5

2.5 3.75 5 2.5 3.75 5 2.5 3.75 5

25 30 35 30 35 25 35 25 30

0.15 0.25 0.35 0.35 0.15 0.25 0.25 0.35 0.15

41.7656 42.6452 42.4443 43.0458 42.9603 43.3292 44.0170 43.6734 43.5276

3. Experimental procedure 3.1. Materials and sample preparation Poly(vinylidene fluoride) (PVDF, (CH2 CF2 )n ) was provided by ALFA AESAR. Zinc oxide (ZnO) particles with nano-size of >100 nm purchased from Sigma–Aldrich chemical were used as additive for PVDF solutions. N,N-dimethylformamide (DMF, HCON(CH3 )2 , >99%, reagent) and hexanes (C6 H14 ) supplied by Lab. MAT were used as the solvent. The unpolished aluminum substrates before coating were cleaned ultrasonically for 10 min in acetone and for 10 min in distilled water and then blown dry in a stream of air. PVDF was dissolved in DMF at 50 ◦ C and stirred for 1 h to obtain a homogenous solution. Different amounts of ZnO (0.5, 1.0, and 1.5 g) were separately dispersed at 30 ◦ C with a vigorous stirring in a hexane solution of stearic acid until a homogenous dispersion of the ZnO nanoparticles obtained. For nanocomposites, the ZnO dispersed solution was added to the PVDF solution to achieve the desired weight ratio of ZnO to PVDF. Vigorous mixing with a rotating stirrer and light heating occurred to decrease the viscosity and to obtain final complex solution. The complex solution was then sprayed using spray coating method on cleaned Al substrate to produce a flat surface. To be note the dispersion of ZnO in hexane with more than 1.5 g was difficult. 3.2. Characterization techniques The contact angles with distilled water were measured on the upper surface of the coated substrates. The measurements were performed using the sessile droplet method at room temperature. The sessile drop study was carried out using deionised water on dry samples. At least five independent determinations at different sites of one sample were averaged and standard deviations obtained. The cross-section morphology of the PVDF–ZnO membranes was observed using a scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). Before SEM observations, samples were vacuum dried overnight. All samples were sputter-coated with Au. The micrographs of the surface and crosssection of the membranes were taken at various magnifications.

The samples were exposed to EDS of the largest possible surface area to obtain the most general representative spectrum for each sample in order to analyze its chemical elements. Spectra of PVDF samples with 4-cm−1 resolution were obtained at room temperature, using a Fourier transform infrared spectrometer in the range of 4000–400 cm−1 . A total of 16 scans were collected for signal averaging 4. Results and discussion 4.1. DOE analysis Taguchi approach emphasizes the importance of investigation the response variation using the signal-to-noise (S/N) ratio, resulting in minimization of quality characteristic variation due to parameter. The water contact angle was deemed the superhydrophobic characteristics with the concept of the “lager the better”. The signal-to-noise is calculated according to Eq. (1): S/N (dB) = −10 log10

 n   yi2

(1)

i=1

where n is the number of variables and yi is the value of each variable. The units of signal and noise are in decibel (dB) [42]. Fig. 1 shows four graphs, each of which represents the mean S/N ratio, where the horizontal axis corresponds to the ZnO, PVDF, stearic acid, and spraying distance levels. The points in the highest part of the graph (high S/N ratio) indicate superior conditions. The main effect plots shown in this figure indicates that the highest point is the optimum parameter for each factor. The response curves of water contact angle are shown in Fig. 2, which depict the pictorial view of variation of each variable and describe what the effect on the performance of PVDF membranes could be, and when a parameter shifts from one level to another. In order to study the contribution ratio of variable parameters, Pareto ANOVA was performed for water contact angle. The obtained results of sum at variable levels (sum of S/N ratio at each variable) for water contact angles are shown in Table 3. This analysis was carried out for a level of significance of 5%, i.e. for 95% a level of confidence. From Table 3, it can be determined the optimum level

Table 3 Pareto ANOVA for water contact angle. Variables

A

B

C

D

Total

Sum at variable levels 1 2 3 Sum of squares of differences Degree of freedom Contribution ratio Cumulative Contribution ratio Optimum level

126.8551 129.3353 131.218 28.7309 2 83.6101 83.6101 A3

128.8284 129.2789 129.3011 0.4269 2 1.2428 84.8529 B3

128.768 129.219 129.422 0.671 2 1.95331 86.8062 C3

128.254 129.991 129.164 4.5338 2 13.1939 100 D2

387.4084

34.3626 8 100

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A

B

43.5

Mean of SN ratios

43.0 42.5 0.5

1.0 C

1.5

2.50

3.75 D

5.00

25

30

35

0.15

0.25

0.35

43.5 43.0 42.5

Signal-to-noise: Larger is better Fig. 1. Variation of S/N ratio for water contact angles with different parameters.

of control parameters. These levels are a ZnO content of 1.5 g (A3 ), PVDF content of 5 g (B3 ), spraying distance of 35 cm (C3 ), and stearic acid content of 0.25 g (D2 ). It should be noted that the above combination of variable levels A3 , B3 , C3 , and D2 are not among the nine combinations tested for the experimentation. It may be also observed that ZnO content contributes a larger impact on the water contact angle, as depicted in Fig. 3. From the contribution ratio shown in Fig. 3, the most significant variables which affect the water contact angle are ZnO content (A) and stearic acid content (D), while two other factors are less effective (see Table 4). Confirmation test was used to verify the estimated results with the experimental results. Generally, if the optimal combination of parameters and their levels coincidently match with one of the experiments in the design of experiments, then in this case the confirmatory test is not required. Estimated value of the water contact angle at optimum condition was calculated by adding the average

Table 4 ANOVA for water contact angle (WCA). Source of variation

Sum of squares of differences

Degree of freedom

Mean square

Adjust of squares of differences

A B C D

28.7309 0.4269 0.671 4.5338

2 2 2 2

14.36545 0.21345 0.3355 2.2669

28.7309 0.4269 0.671 4.5338

Total

34.3626

8

performance to the contribution of each parameter at the optimum level using Eqs. (2) and (3) [42]: Yopt = m + (mAopt − m) + (mBopt − m) + (mCopt − m) + (mDopt − m) (2)

A

B

160 150 140 130

CA

120 0.50

0.75

1.00

1.25

1.50

3

C

160

4

5

D

150 140 130 120 25.0

27.5

30.0

32.5

35.00.15

0.20

0.25

0.30

0.35

Fig. 2. Response curve of water contact angles.

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Percentage contribution of water contact angle

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90

5

83.6101

80 70 60 50 40 30 20 13.1939

10 0

A

1.2428

1.9533

B

C

D

Control variables Fig. 3. Percentage of contribution ratio of control variables of water contact angle.

T m= n

(3)

where m is the average performance, T is the total of average total water contact angle for each experiment, n is the total number of experiments and mAopt is the average total water contact angle for parameter A (ZnO) at the its optimum level, mBopt is the average total water contact angle for parameter B (PVDF) at the its optimum level, mCopt is the average total water contact angle for parameter C (spraying distance) at the its optimum level and mDopt is the average total water contact angle for parameter D (stearic acid) at the its optimum level. Confirmation test required was applied because the optimum combination of parameters and their levels, i.e. A3 B3 C3 D2 did not correspond to any experiment of the orthogonal array. The value of water contact angle obtained from the experiment was compared with the estimated value as shown in Table 5. It can be observed that the difference between estimated and experimental results is found to be 0.3◦ . This means that the experimental result is strongly correlated with the estimated result. 4.2. Contact angle analysis Contact angles are the characteristic constants of liquid/solid systems and provide valuable information on the surface energies of solids. The sliding angle of the best conditions, defined as the angle when a water droplet of certain volume begins to slide down an inclined plane, was also measured. The maximum water contact angle (WCA) that can be reached by coating fluorinated methyl groups onto a flat solid surface is only 120◦ , which can be hardly called superhydrophobic [43]. Therefore, ZnO nanoparticles are introduced into the solid surface to achieve superhydrophobicity. The average water contact angles and contact angle hysteresis Table 5 Results of confirmation experiment. Level

Water contact angle (◦ ) S/N ratio (dB)

Optimal condition Estimation

Experiment

Difference

A3 , B3 , C3 , D2

A3 , B3 , C3 , D2

–

160.79 44.1746

160.45 44.1746

0.34

of the PVDF–ZnO composites are depicted in Fig. 4. The images of water droplet locating on the Al substrates coated with PVDF membranes are seen in Fig. 5. The water droplet (WCA) on unpolished Al substrate was around 74◦ ± 2.2◦ before and 105◦ ± 4.7◦ after coating with pure PVDF, as shown in Fig. 5a and b. It can be observed that, the water contact angles increased dramatically as the ZnO weight fraction increased up to 37.5 wt.%, P7 sample, and above this point, it decreased slightly. The maximum water contact angle was 159◦ , however, the contact angle hysteresis and sliding angles were 3.5◦ and 2◦ , respectively, showing extremely low adhesive force between the prepared PVDF nanocomposite substrate and water droplet. This indicates that the ZnO addition increases the superhydrophobicity of the PVDF. The observed increase of water contact angle and decrease of contact angle hysteresis with the addition of ZnO may be explained as follows. When the level of ZnO nanoparticles is 17%, P1 sample, the PVDF nanocomposite starts to form a compact film with very few nano-sized asperities, which achieve a contact angle of about 122◦ . As the level of ZnO addition increased, the roughness of the film was increased with the increasing number of nano-size asperities produced on the surface. These nano-sized asperities help to trap air in between the water droplet and the space between the asperities, such that the water droplet stands on them with small liquid–solid contact area. This result is somehow related with the famous Cassie–Baxter theory. The WCA of air was generally regarded as 180◦ . In Cassie–Baxter’s theory, the liquid droplet will not completely contact with the whole solid surface due to the Yong–Laplace pressure between the interfaces, and thus the asperities trap air inside, forming a stable solid–air–liquid three phases interface. The relationship between the WCA on a flat surface () and a rough surface (  ) composed of a solid and air can be described according to Eq. (4) [44–46]. cos(  ) = f1 cos() − f2

(4)

In this proposed equation, f1 and f2 were ratios of solid surface and air in contact with liquid, respectively. Given the WCAs of the flat pure PVDF film (∼105◦ ) and the PVDF nanocomposite (159◦ ), f2 was calculated to be 0.91, which indicated that the achievement of superhydrophobicity by PVDF nanocomposites was mainly a result of the air trapped in the rough hierarchical micro-/nanostructures of ZnO. As a result, the larger proportion of trapped air would induce the bigger CA as soon as the three phases stably existed. The water contact angle hysteresis (WCAH) is one important factor in

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180

100

160

90 80

o

o

Water Contact Angle ( )

140

Contact Angle Hysteresis ( )

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6

70

120

60 100 50 80 40 60

30

40

20

20

10

0

0 P1

P2

P3

P4

P5

P6

P7

P8

P9

PVDF-ZnO Concentration Fig. 4. Water contact angle (WCA) and contact angle hysteresis (WCAH) of PVDF–ZnO composites.

determining surface hydrophobicity as well as the static WCA. The WCAH of PVDF was decreased from 37◦ to 3.5◦ with increasing the weight ratios of ZnO/PVDF from 1/6 to 3/8 (Fig. 4), allowing water droplets to roll off easily from the surface. As a result, the hydrophobicity of nanocomposite increased with increasing the ZnO component in the PVDF polymer.

4.3. Morphology of PVDF membrane The microstructure of the hydrophobic surface shown in Fig. 6a–c at different magnification indicates that the coating film mainly consists of PVDF polymer and reveals sponge-like structures at high magnification. It should be noted that the hydrophobic sample has a uniform surface structure and minute roughness. Some large microvoids beyond these structures are also observed. The composition of pure PVDF polymer observed in Fig. 6c was verified by the energy dispersive X-ray spectroscopy

(EDS), as seen in Fig. 6d. The EDS results showed that PVDF material contains mainly two peaks of fluorine (F) and carbon (C). An obvious change of morphology was obtained with the addition of ZnO nanoparticles which represented with white spots in Figs. 7 and 8. The composite coatings showed aggregates of ZnO particles, as observed in Fig. 7a. Aggregate size and distribution increased with increase in ZnO concentration, as shown in Fig. 8b compared to Fig. 7b. ZnO particles revealed a variety of morphologies and irregular shapes (nanotowers, deformed spherical particles, elongated rods and blocks with sharp edges), which constructed the micronanoscale structured roughness surfaces, as seen in Figs. 7c and 8c. This micronanoscale structure plays an important role in the superhydrophobicity of the films with static WCA of 159◦ and slide angle of 2◦ . The EDS spectrums in Figs. 7d and 8d show the main chemical elements of the samples containing ZnO/PVDF in weight ratios of 2/7 and 3/8, respectively. These elements were identified as carbon and fluorine originating from the PVDF structure, and as zinc (Zn) and oxygen (O)

Fig. 5. Optical images of the water contact angles on (a) Unpolished Al substrate, (b) Pure PVDF, (c) P1 sample (ZnO/PVDF ratio = 1/6), (d) P4 sample (ZnO/PVDF ratio = 2/7), and (e) P7 sample (ZnO/PVDF ratio = 3/8).

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Fig. 6. (a-c) SEM images of pure PVDF materials with different magnification view and (d) EDS analysis corresponding to point 1 in c.

originating from the incorporated ZnO. The elemental analysis using EDS for the coated surfaces are shown in Table 6. 4.4. FTIR analysis The advantage of using a Fourier transform infrared instrument is its multiplex advantage, i.e. measuring all spectra elements at the

same time, high throughput and inherent spectral accuracy. Alfa (␣) phase is one of the most common structure for PVDF polymer, being normally obtained by melt crystallization at temperatures below 160 ◦ C. However, ␤ phase has aroused more technological interest, for providing the best piezoelectric properties [47–49]. Gregorio et al. [50] reported that ␣ and ␤ phases have a characteristic infrared absorption at 763 and 840 cm−1 , respectively. Fig. 9 shows the IR

Fig. 7. (a-c) SEM images of PVDF composites containing 28.5 wt% ZnO, P4 sample, at different magnifications and (d) EDS analysis corresponding to point 1 in b.

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Fig. 8. (a-c) SEM images of PVDF composites containing 37.5 wt% ZnO, P7 sample, at different magnifications and (d) EDS analysis corresponding to point 1 in b.

Fig. 9. FTIR spectra of PVDF membranes with various ZnO/PVDF weight ratios: (a) 0/1, (b) 1/6, (c) 2/7, and (d) 3/8.

Table 7 FTIR peak assignments for PVDF–ZnO nanocomposites.

Table 6 Surface elements analysis using EDS. Element

Pure PVDF

Weight% CK OK FK Zn K

47.32 52.68

Total

100.00

P4 sample (ZnO/PVDF = 2/7) Atomic% 58.70 41.30

Weight% 28.47 11.84 14.37 45.33 100.00

Atomic% 51.98 16.23 16.59 15.21

P7 sample (ZnO/PVDF = 3/8) Weight% 33.63 7.43 11.84 47.10 100.00

Atomic% 60.76 10.08 13.53 15.64

Peak position Functional groups and crystallites

Phase ZnO

430

Zn-O band

510 840 1070–1410 2985 3025

CF2 bending CH2 rocking Fluorocarbon absorption (C F) ␤-PVDF Symmetric stretching vibrations of the CH2 group Asymmetric stretching vibrations of the CH2 group

2847 2915

Symmetric C H stretching modes of CH2 groups Asymmetric C H stretching modes of CH2 groups

Stearic acid

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spectra of the films with different ZnO contents as well as the PVDF polymer. The wavenumbers of the FTIR peaks allocated to several groups and crystallites were depicted in Table 7. In Fig. 9a, sharp characteristic peaks at 480, 510 (CF2 bending), and 840 (CH2 rocking) cm−1 are observed in the pure PVDF sample, whereas there are no peaks at 531, 612, 766, 795, 855, and 976 cm−1 , which are typical of the ␣ phase, and at 430 cm−1 , which is the characteristic absorption position of the ␥ phase. The FTIR results, therefore, showed that the PVDF material was composed of the ␤ phase. These results are in agreement with those obtained in previous works [51,52]. In addition, the constant band absorption region from 1410 to 1070 cm−1 corresponds to the fluorocarbon absorption (C-F), while the asymmetric and symmetric stretching vibrations of the CH2 group in the PVDF sample were observed at 3025 and 2985 cm−1 , respectively. Compared with the spectrum of the pure PVDF film (Fig. 9a), fourth new bands were detected in the spectrum of Fig. 9b. The two peaks at wavenumbers 2915 and 2847 cm−1 were assigned to the asymmetric and symmetric C H stretching modes of CH2 groups of stearic acid, respectively. The intensity of the two peaks of stearic acid was varied depending on the amount of stearic acid added in the nanocomposite samples, as shown in Fig. 9b–d. However, the other two peaks at 1535 and 430 cm−1 were attributed to the stretching vibration of Zn O band [53,54]. The intensity of ZnO peaks increased and became clearly visible with the highest levels of ZnO used, as shown in Fig. 9d. 5. Conclusion In light of above analysis, the following conclusions may be drawn: 1. The optimum level of parameters used in this study to obtain good superhydrophobic properties of PVDF membranes are a ZnO of 1.5 g and stearic acid of 0.35 g. 2. PVDF–ZnO composites were successfully prepared by using spray coating methods. This method may even be applicable to a large scale process and has the potential to be an economical route for industrial applications. 3. The superhydrophobicity of PVDF surfaces was attributed to the low surface energy of PVDF and the combination of hierarchical micro- and nanostructures of ZnO inherent in the polymer. 4. An increase in ZnO concentration in PVDF matrix allowed a substantial increase in water contact angle from 122◦ to 159◦ , however, the contact angle hysteresis and sliding angle decreased to 3.5◦ and 2◦ , respectively. 5. The dispersed ZnO particles in the PVDF matrix have different morphologies, e.g. nanotowers, deformed spherical particles, elongated nanorods and blocks with sharp edges, all play an important role in the improvement of superhydrophobicity of PVDF membranes. 6. In the absence or present ZnO nanoparticles, the crystallization of the PVDF occurred predominantly in the ␤-phase Acknowledgments The authors would like to express their grateful acknowledgment for financial and in-kind support received from the National Sciences and Engineering Research Council of Canada; and from Hydro-Quebec Company. Thanks are also due to Mr. Pierre Camirand at the CIGELE Laboratory, UQAC for his assistance with sample preparation.

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Please cite this article in press as: A.M.A. Mohamed, et al., An optimization of superhydrophobic polyvinylidene fluoride/zinc oxide materials using Taguchi method, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc.2013.10.013