Alfa A. Widati, Nuryono Nuryono, Indriana Kartini, Noah D. Martino

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SILICA-METHYLTRIMETHOXYSILANE BASED HYDROPHOBIC COATINGS ON A GLASS SUBSTRATE

Alfa A. Widati1,2, Nuryono Nuryono1, Indriana Kartini1, Noah D. Martino2

ABSTRACT

The preparation of silica-methyltrimethoxysilane nanocomposites and their effect on the glass surface wettability performance were investigated. TEOS, sodium silicate, and rice husk ash were the silica sources used. In this research, rice husk ash was converted into sodium silicate through a reaction with amorphous silica and NaOH. The optimum silica concentration providing the highest hydrophobicity was determined. The silica nanoparticles were characterized by XRD, the hydrophobicity was measured by the water contact angle (WCA) test, the surface roughness was analyzed by AFM, while the transparency was determined spectrophotometrically. The different silica source used affected the hydrophobicity of the coated glass. The nanocomposite using TEOS as a silica source showed the highest WCA (132°), whereas sodium silicate and rice husk ash showed similar WCA (around 116°). Furthermore, WCA was also found affected by the amount of silica. WCA increases with silica amount increase.

Keywords: silica source, mass of silica, hydrophocity, glass surface.

Received 05 January 2017Accepted 20 July 2017

Journal of Chemical Technology and Metallurgy, 52, 6, 2017, 1123-1128

1 Department of Chemistry, Faculty of Mathematics and Natural Sciences Universitas Gadjah Mada, Yogyakarta 55281 Indonesia2 Department of Chemistry, Faculty of Science and Technology Universitas Airlangga, Surabaya 60115 Indonesia E-mail: alfaakustia@gmail.com

INTRODUCTION

A water repellent behavior plays an important role in many applications such as anti adhesion, self cleaning, anti biofouling, corrosion inhibition, and drag reduction [1]. Water repellency is dominated by the solid surface chemical properties and surface morphology. The combination of a low surface energy and a rough surface determines the hydrophobic properties obtained. Methyl trimethoxysilane (MTMS) as an alkylsilane decreases the surface energy of a material. The hydrophobic properties are attributed to the methyl substituent of MTMS, while the methoxy group acts as a coupling agent forming the bonds with the substrate surface. There are many investigations referring to the water repellent properties of MTMS in case of various substrates [2 - 4].

Meanwhile, modification using nanoparticles is used for the preparation of rough surfaces [5]. Silica nanoparticles are easy to synthesize, they are non toxic, they have good transmittance, as well as thermal and chemical stability. They are suitable for water repellent applications. Many researchers have studied the application of SiO2 as a hydrophobic coating [6 - 7]. Silica nanoparticles have been modified by methyl triethoxysilane, 3-aminopropyl triethoxysilane, tridecafluoro-1,1,2,2-tetrahidrooctyldimethylchlorosilane [5, 8 - 10].

There are many studies focused on the preparation of a silica source from natural materials and waste products such as bagasse, fly ash, bottom ash, and rice husk. The latter is rich in silica and its utilization is expected to be cost effective. However, it has not been

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so far studied as a silica source. The study reported is aimed at obtaining a coating of hybrid silica-MTMS on a glass surface following the effect of the silica source on its hydrophobic properties.

EXPERIMENTAL

The hydrophobic coating consisted of: tetraethyl orthosilicate, hydrochloric acid, sodium hydroxide, methanol, ethanol, ammonium hydroxide, and methyltrimethoxy silane (MTMS). All these chemicals except MTMS were purchased from Merck. MTMS was purchased from Sigma Aldrich and used as received without further purification.

The sodium silicate from rice husk was prepared according to the literature with little modification (11 - 12). The rice husk was initially treated by calcination to synthesize the amorphous silica. This was done through heating for 1 h at 190°C, then for 30 min at 300°C and finally for 60 min at 600°C. The rice husk ash was washed with 6M HCl at constant stirring to reach pH 7. Then the mixture was filtered through Whatman No. 41 ashless filter paper. 4M NaOH was added to the residue and the mixture was boiled for 1 h under vigorous stirring to dissolve silica aiming to obtain a sodium silica solution [13]. The slurry was allowed to cool at room temperature and filtered through Whatman No. 41 ashless filter paper. The solution thus obtained is further called called sodium silicate from rice husk ash.

The composite coating solution was prepared by combining SiO2 nanoparticles and MTMS, which was used to prepare the hydrophobic glass. SiO2

nanoparticles were synthesized using the Stöber method [14]. This process used silica of a varying source (TEOS, sodium silicate, sodium silicate from rice husk). MTMS was prepared based on the previous research [15]. The composite of SiO2-MTMS was prepared by mixing 20 mL of of a solution containing 0,002 moles of MTMS with SiO2 in three different mole ratio (0.01 moles, 0.03 moles, and 0.09 moles).

Prior to the immersion, the soda lime glass slide was cleaned with ethanol for 30 min. Without being dried, the glass was immersed in the coating solution varying the time (0 h and 4 h) and withdrawn at a rate of 6 cm/min.

The glass was dried at 80°C. The roughness surface of the coated glass was

observed using Atomic Force Microscopy (AFM, NEOS, N8). Spectrophotometer UV-Vis (Pharmaspec UV-1700, Shimadzu) was used to study the transparency of the glass. The water contact angle with a 10 μL droplet of water was measured using ImageJ software.

RESULTS AND DISCUSSION

The rice husk ash obtained was washed using HCl to dissolve metal oxides such as P2O5, K2O, MgO, Na2O, CaO, and Fe2O3 forming chloride salts [16]. Since silica is insoluble at pH < 10, highly alkaline conditions were required for efficient extraction [17]. 4M NaOH was used to dissolve it in the present study. The corresponding reaction is described by:

Silica nanoparticles are synthesized using Stöber method. The silica precursor is hydrolyzed and condensed using ammonia as a base catalyst and alcohol as a solvent. During this process, hydrolysis of silica generates silanol, which can condense with other silanol to form siloxane linkages following the reactions:

The effect of silica source is studied through XRD analysis. All diffractograms displayed the characteristic peak of silica nanoparticles in 2θ about 20-24° (Fig. 1) in correspondence with previous studies [18 - 19].

The wettability and transparency of the coated glass prepared using different amounts of silica are illustrated in Fig. 2. The water contact angle increases gradually with increase of the amount of silica. This is due to the clustering of the silica nanoparticles on the surface that provides a rough surface. Surfaces of higher roughness tend to roll off the water droplets easily from the surface [20].

The transmittance values of the coated glasses were measured spectrophotometrically. It is found that samples whose coatings contain 0 moles and 0.01

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moles of silica have similar transmittance as compared to other coated glasses. These results are in accord with the film thickness of the glass. The thickness of the films obtained containing 0.09 moles of silica is much greater than that of the others. In other words, samples containing a less amount of silica have thin coatings of higher transparency.

Fig. 3 shows no correlation between the contact angle and the deposition time. The coatings prepared have a similar water contact angle. This result is attributed to glass porosity. Because of the low porosity the adsorbed amount of silica is not affected by the deposition time. Hence, the surface roughness and the contact angle do not differ.

The effect of silica source is followed using 0.09 moles of silica (Fig. 4). TEOS as a silica source exhibits the highest contact angle (132°) when compared to those of the commercial and synthesized sodium silicate (116° and 120°). TEOS as organic silica provides complete hydrolysis leading to ethanol and water as byproducts. However, the commercial and synthesized sodium silicate produce sodium ions as another byproduct during the process. The presence of sodium ion tends to enhance the polarity of surface.

The surface roughness of the silica-MTMS coated glass, measured by the AFM is presented in the Fig. 5.

Fig. 2. a) The UV-Vis spectra; b) the correlation of water contact angle and transmittance as a function of mole of silica in the silica-MTMS coated glass.

Fig. 1. The diffractogram of synthesized silica nanoparticles using: a) sodium silicate from rice husk; b) commercial sodium silicate; c) TEOS as silica source.

Fig. 3. The water contact angle of silica-MTMS coated glass with various deposition time.

a)

b)

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Fig. 4. The image of water droplet on silica-MTMS coated glass with various silica sources.

Fig. 5. The roughness topography of MTMS-silica coated glass using: a) TEOS; (b) commercial sodium silicate; (c) sodium silicate from rice husk ash as a silica source.

(a)

(b)

(c)

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The roughness profile of coated glass using TEOS is higher than that of sodium silicate. This result supports the data referring to the contact angle. A water drop is hampered to penetrate a rough surface of substrates. The contact area between the substrate surface and the water droplet is low, therefore the latter easily rolls off from the surface.

CONCLUSIONSHydrophobic composite coatings were developed

based on silica-MTMS composite. Those containing 0.09 mole of silica were found to have a high contact angle and transparency. The deposition time did not affect the performance of the coated glass. It was found that TEOS exhibited lower wettability than sodium silicate. Furthermore, the composite coating of sodium silicate from rice husk ash showed a performance similar to that of the commercial sodium silicate. The study reported verifies the expected adding value of rice husk as byproduct and waste in the rice, flour, and food industries.

AcknowledgementsThe authors thank the Department of Chemistry,

Faculty of Science and Technology, Universitas Airlangga for the laboratory facilities. This article is dedicated to the memory of Dr. Hamami, M.Si who contributed to the research described.

REFERENCES

1. A.M.A. Mohamed, A.M. Abdullah, N.A. Younan, Corrosion behavior of superhydrophobic surfaces : A review, Arabian J. Chem., 8, 2015, 749-765.

2. K.A. Manjumol, L. Mini, A.P. Mohamed, U.S. Hareesh, K.G.K. Warrier, A hybrid sol-gel approach for novel photoactive and hydrophobic titania coat-ings on alumunium metal surfaces, RSC Adv., 39, 2013, 18062-18070.

3. A.V. Rao, S.S. Latthe, D.Y. Nadargi, H. Hirashima, V. Ganesan, Preparation of MTMS based transparent superhydrophobic silica films by sol-gel method, J. Colloid Interface Sci., 322, 2009, 484-490.

4. W. Bao, F. Guo, H. Zou, S. Gan, X. Xu, K. Zheng, Synthesis of hydrophobic alumina aerogel with sur-

face modification from oil shale ash, Powder Technol., 249, 2013, 220-224.

5. R.V. Lakshmi, P. Bera, C. Anandan, B.J. Basu, Effect of the size of silica nanoparticles on wettability and surface chemistry of sol-gel superhydrophobic and oleophobic nanocomposite coatings, Appl. Surf. Sci., 320, 2014, 780786.

6. M. Farahmandjou, P. Khalili, Study of nano SiO2/TiO2 superhydrophobic self cleaning surface pro-duced by sol gel, Aust. J. Basic Appk. Sci., 7, 6, 2013, 462-465.

7. Jean-Denis Brassard, D.K Sarkar, J. Perron, Fluorine based superhydrophobic coatings, Appl. Sci., 2, 2012, 453-464.

8. J. Lin, H. Chen, T. Fei, J. Zhang, Highly transparent superhydrophobic organic-inorganic nanocoating from the aggregation of silica nanoparticles, Colloids Surf. A, 421, 2013, 51-62.

9. J. Hwang, Y. Ahn, Fabrication of Superhydrophobic Silica Nanoparticles and Nanocomposite Coating on Glass Surfaces, Bull. Korean Chem. Soc., 36, 2015, 391-394.

10. H.M. Shang, Y. Wang, G. Takahashi, G.Z. Cao, Nanostructured Superhydrophobic Surfaces, 40, 2005, 3857-3591.

11. Wibowo, 2006, Pengembangan alat pengolah limbah abu ampas tebu menjadi pozolan, Jurnal Teknik Sipil, 6, 2, 2006, 124-136.

12. D.R. Mujiyanto, N. Nuryono, E.S. Kunarti, Sintesis Dan Karakterisasi Silika Gel Dari Abu Sekam Padi Yang Diimobilisasi Dengan 3-(Trimetoksil)-1-Propantiol, Jurnal Sains dan Terapan Kimia, 4, 2, 2010, 150-167.

13. U. Kalapathy, A. Proctor, J. Shultz, An improved method for the production of silica from rice hull ash, Bioresour. Technol., 85, 2002, 285-289.

14. W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in micron size range, J. Colloid Interface Sci., 26, 1968, 62-69.

15. S. Nagappan, J.J. Park, S.S. Park, Chang-Sik, Ha, Preparation of superhydrophobic and transparent micro-nano hybrid coatings from polymethylhy-droxysiloxane and silica ormosil aeogels, Nano Convergence, 1, 30, 2014, 1-10.

16. N. Yalcin, V. Sevinc, Studies on silica obtained from

Journal of Chemical Technology and Metallurgy, 52, 6, 2017

1128

rice husk, Ceram. Int., 27, 2001, 219-224.17. U. Kalapathy, A. Proctor, J. Schultz, A simple

method for production of pure silica from rice hull ash, Bioresour. Technol., 73, 2000, 257-262.

18. G. Nallathambi, T. Ramachandran, V. Rajendran, R. Palanivelu, Effect of silica nanoparticles and BTCA on physical properties of cotton fabrics, Mater. Res., 14, 4, 2011, 552-559.

19. E. Rafiee, S. Shahebrahimi, M. Feyzi, M. Shaterzadeh, Optimization of synthesis and characterization of na-nosilica produced from rice husk (a common waste material), Int. Nano. Lett., 2, 1, 2012, 1-8.

20. D. Kumar, X. Wu, Q. Fu, J.W.C. Ho, P.D. Kanhere, Z. Chen, Hydrophobic sol–gel coatings based on polydimethylsiloxane for self-cleaning applications, Appl. Surf. Sci., 344, 2015, 205-212.