SYNTHESIS AND CHARACTERIZATION OF NEW SUPER-HYDROPHOBIC NANOCOMPOSITE AS SELF-CLEANING GLASS

SYNTHESIS AND CHARACTERIZATION OF NEW SUPER-HYDROPHOBIC NANOCOMPOSITE AS SELF-CLEANING GLASS

Reham H. Mohamed⁽¹⁾* ; Mahmoud R. Noor El-Din⁽¹⁾ ;Rasha A. El-Ghazawy⁽¹⁾ ; Mohamed S. Selim⁽¹⁾ and N. O. Shaker⁽²⁾

⁽¹⁾Egyptian Petroleum Research Institute (EPRI), 1Ahmed El-Zomor St., Nasr City, 11727, Cairo, Egypt.

⁽²⁾Chemistry Department, Faculty of Science, Al-Azhar University, Cairo, Egypt

*Corresponding author: h_reham5@yahoo.com

*To whom correspondence should be addressed

Key Words: Superhydrophobic surfaces, nanocomposit, Sol–gel process, Contact angle.

ABSTRACT

Self-cleaning surfaces are developed through the use of superhydrophobic and water-repellent surface for glass coating applications. In the present study, anew copolymer of styrene-vinyl acetate (PS-VAc) has been synthesized. Furthermore, SiO₂-NPs ranging from 90-100 nm were synthesized via sol-gel method. To increase the hydrophobicity, silane coupling agent namely dodecyl triethoxysilane (DTES), is used to prepare a hydrophobic modified SiO₂-NPs with different concentration (1%, 3% and 5%). New super-hydrophobic nanocomposite are prepared through incorporation of the modified SiO₂-NPs inside the prepared copolymer matrix donated as D-composites. We Investigated SH films using many techniques such as Fourier transform infrared spectroscopy)FT-IR), transmission electron microscopy (TEM), and dynamic light scattering (DLS), contact angle and Thermogravimetric analysis. Increasing the concentration of D-composite in the coated glass film improves the repulsion force between the water droplet and the coated film, the final contact angle of the coated film increases from 108.89 ±20^o (hydrophobic in nature) to 150^o (superhydrophobic in nature) with increasing the concentration of D-composite from 1 to 5 wt., %, respectively. From the obtained results, it is revealed that D-composites have distinguishable results to use it as a good water-repellent glass surface. Thus, a promising self-cleaning nanocomposite material was approved with superhydrophobicity and economic savings for glass surfaces.

1. INTRODUCTION

Self-cleaning surfaces are capable of repelling contaminants, including solid particles, organic liquids. The most common principle of a super-hydrophobic self-cleaning surface is Lotus effect,induced by surface roughness. Surfaces with a water contact angle greater than 150◦are called “super-hydrophobic” are of interest for numerous applications, including self-cleaning coating, anti-corrosion, biotechnology, low friction coatings.

To achieve super-hydrophobicity, a surface must have two physical properties: a surface roughness and a low surface energy. Thetypical size of roughness detail in super-hydrophobic surface is micro and nano scales. Various materials are used to make super-hydrophobic coatings and can be divided into two series: causing arough surface with a low surface energy material and modifying arough surface by a material which have a low surface energy ^[1-4].

Organosilanes are used as efficient coupling agents extensively used in composites and improving the adhesion of various coatings. Functional trialkoxysilanes, R´ıSi(OR)3, are common graffiting agents, used in numerous industrial application in order to increase adhesion between organic and inorganic substrate.

The Wenzel model ^[5] and the Cassie-Baxter model ^[6] are most frequently used to account for experimental results. The Wenzel model describes wetting behavior where a liquid droplet on a rough surface is in intimate contact with surface asperities, while the Cassie model describes states where the droplet sits on a solid–air composite surface ^[7].

Super-hydrophobic surface can be prepared from a variety of methods ^[8]. However, sol–gel process has been used to fabricate super-hydrophobic surfaces ^[9]. Sol–gel techniques for a long time have been used for preparing a range of materials which have a wide range of physicochemical properties.

The main objective of this research is to prepare a new super-hydrophobic coated film for a glass surface with a highly water-repelling surface, and highly contact angles. To achieve this goal, styrene-vinyl acetate copolymer with high molecular weight has been synthesized through new emulsification method namely emulsion phase inversion concentration (EPIC) technique.

Furthermore, SiO₂-NPs ranging from 90-100 nm were synthesized via sol-gel method. To increase the hydrophobicity of the prepared SiO₂-NPs, saline coupling agent namely dodecyl triethoxysilane, was used to prepare a hydrophobic modified SiO₂-NPs. New super-hydrophobic nano-composites with highly contact angles of 150.9± 0.15^o are prepared through incorporation of the modified SiO₂-NPs inside the prepared copolymer matrix.

2. MATERIALS AND METHODS

2.1   . Materials

Analytical grade of styrene and vinyl acetate monomers denoted as (S) and (VAc), respectively were purchased from Fluka, Germany. A technical grade of ammonium per sulfate as initiator was supplied from Sigma-Aldrich, Germany. Three technical grades of non-ionic and anionic surfactants namely; Polyethylene glycol dodecyl ether (sold as Brij L4) with hydrophilic-Lipophilic Balance (HLB) value of 9.7 and poloxamer 188 nonionic surfactant having HLB value of 29. Atechnical grade of sodium bis(2-ethylhexyl) sulfosuccinate (sold as AOT) as anionic surfactant with HLB value of 40 was purchased from Acros- Organics, Belgium. The previously surfactants were denoted as (B), (P) and (A),respectively.Tetraethylor-thosilicate (TEOS) (97%,Sigma–Aldrich,Germany), Dodecyl triethoxysilane (DTES) (97%, Sigma–Aldrich,Germany),ammonia(NH₄OH)(28%, Merck, Germany).Triethylamine(TEA) and Deionized water was used.

2.2   .Methods

2.2.1.      Purification of monomers

Styrene (S) and vinyl acetate (VAc) monomers were firstly washed with a 5% NaOH aqueous solution to remove the inhibitor and then with distilled water to remove the excess of NaOH. Styrene was dried over anhydrous Na₂SO₄, while VAc was dried over anhydrous CaCl₂ then distilled under vacuum and used immediately ^[10].

2.3.      Preparation of styrene-vinyl acetate co-polymer (P-SVAc)

Oil-in-water nano-emulsion polymerization was prepared using a new technique namely Emulsion Phase Inversion Concentration (EPIC) ^[11]. Surfactants [Brij L4 (B), Poloxamer (P) and AOT (A)] were mixed with adjusted proportion to satisfy the proper HLB_BPA value for optimum emulsification conditions ^{[12, 13]}.

A 1-liter reactor equipped with a condenser and magnetic agitator was immersed in a thermostated bath held at 40°C. First, water (300 ml), VAc ( 0.43 mole), triethylamine (8 g), mixed surfactants (BPA) (0.5 wt. %) based on total weight of mixture,ammonium persulfate (0.687 g), and sodium bisulfite (0.416 g) were added simultaneously and purged with nitrogen. After 1 hour.an additional amount of styrene (0.17 mole) was added dropwise over a period of 30 min. After two more hours, ammonium persulfate (0.065 g) and sodium bisulfite (0.040 g) were added and the reaction was continued for another hour ^[14].The copolymer was precipitated according to a procedure based on solubility trials in various solvents. The separation process of (S-VAc) copolymer is depicted in Fig. 1.The reaction scheme of the prepared copolymer as shown in   Scheme 1.

Fig. 1: Precipitation order for separating poly (S-VAc) co-polymer

Scheme 1: Chemical reaction of (S-VAc) copolymer

2.4.      Preparation of silica nanoparticles (SiO₂-NPs)

      SiO₂ nanoparticles were prepared via hydrolysis andcondensation reactions of tetra ethyl-orthosilicate (TEOS) as coupling agent, which was catalyzed by ammonia solution ^{[15, 16]} according to eqs. 2-4: -

Si(OC₂H₅)₄ + H₂O → Si(OC₂H₅)₃OH + C₂H₅OH                     eq. (2)

≡Si–O–H + H–O– Si≡ → ≡ Si–O–Si≡ + H₂O                            eq. (3)

≡Si–O–C₂H₅ + H–O–Si≡  →≡Si–O–Si≡ + C₂H₅OH               eq. (4)

In a typical procedure ^[17], Tetraethyl-orthosilicate (TEOS) (6.3ml) was drop-wisely added into a homogenous mixture of ethanol (142.8 ml) and water (19.5 ml). The mixture is stirred at constant rotation rate of 600 rpm for 2.5 hours, and working temperature of 45 ± 2 °C. A mixture of ammonia solution (3.15 ml) and ethanol (4.0 ml) is subsequently added to the reactants with feeding rate of 1 ml\3 minutes. Afterwards, the mixture is left to agitate for 3 hours. The solution is allowed to settle overnight at room temperature to obtain SiO₂-NPs. The precipitated SiO₂-NPs are centrifuged at high speed of 10000 rpm for 5-10 minutes. The produced SiO₂-NPs is purified by washed it with ethanol for three times, and dried under vacuum at temperature of 25^oC. The yield of SiO₂- NPs is approximately 2 g.

2.5.      Preparation of hydrophobic silica

Alkoxysilane was used to functionalize the SiO₂ NPs by sol–gel process.0.5 g SiO₂-NPs was dispersed in 10 ml toluene. Thereafter, the mixture was sonicated via Q500 ultra-sonication for 30 minutes.0.5 ml of either dodecyl triethoxysilanewas separately added to the dispersed SiO₂-NPs.The mixture was kept at 50 ^○C under ultrasonication (in an ultrasonic bath) for 4 h.

Subsequently, the mixture was dried in air at room temperature.The modified SiO₂-NPs are dispersed in methanol solution with sonication for 30 minutes. Afterwards, the suspension was centrifuged at 10000 rpm for 35 minutes to precipitate the modified SiO₂-NPs. The methanol supernatant was decanted. The obtained hydrophobic SiO₂-NPs were dried under vacuum at 30 ^oC for 24 hours.

2.6.      Preparation of (copolymer-modifide SiO₂-NPs) nanocomposites.

Modified SiO₂-PSVAc nanocomposites denoted as D-composite was prepared as per the method reported by LaidaCanoa^[18]. Briefly, 0.5 g of P-SVAc was dissolved in 50 ml benzene under magnetic stirring for 30 min. Different concentration (1, 3 and 5 wt.,%) of modified SiO₂ NPs was dispersed in 50 ml benzene to form modified SiO₂ NPs solution. Modified SiO₂ NPs solution was dropwise added to PSVAc solution with stirring for 2 hours at room temperature.

Different concentration of nanocomposites are separately sprayed using gun-sprayer (TCP Global Brand HVLP gun- spray) under a static pressure of 50 psi. Annealing of the coatings was performed at 80 ºC for 8 hours. Fig. 2 shows the schematic diagram of the coating process.

Fig. 2: Scheme for preparation of H-composites and coating process on glass.

1. Characterization of the prepared compounds

The chemical composition of the prepared samples was elucidated using different techniques as follows:-

1. Fourier Transmittance Infra-Red (FT-IR): The chemical structures of the prepared  copolymer (PS-VAc),  SiO₂-NPs and modified SiO₂ were characterized using a Nicolet NICOLET IR 100 FT-IR spectrometer (USA).
2. Thermo-gravimetric analysis (TGA): TGA measurements of copolymer and H-composite were performed using simultaneous SHIMADZU DTG-60H (Japan).
3. Differential scanning calorimeter (DSC): DSC analysis was conducted using SHIMADZU DSC-60 (Japan). 8-10 mg of copolymer and H-composite samples were performed with temperature ranging from -140 to 600 ^οC.
4. Dynamic scattering (DLS): light The particle size of the prepared P(S-VAc) copolymers and SiO₂ NPs were measured at 20°C using dynamic light scattering (Malvern Zetasizer ZS, Worcestershire, U.K.).
5. Transmission electron microscopy (TEM): The size, shape and the aggregation of the prepared SiO₂-NPs were analyzed using JEOL 120/JEOL 200 TEM (Carl Zeiss NTS).

4.Evaluation

The method were used to evaluate ofthe coated surface glass as the following:-

1. Contact angle measurement: Contact angles for uncoated and coated glass surface by copolymer, SiO₂-NPs, as and D-composites at different concentration of the modified SiO₂-NPs as 1, 3 and 5 wt. %, and solvent type of DI-water were determined using Theta optical tensiometer, Attention-Biolin Scientific Company, Finland.

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   RESULTS AND DISCUSSION

5.1.FT-IR spectroscopy analysis:

FTIR analysis was applied to investigate their chemical bonds within the range of 500–4000 cm⁻¹. As shown in Fig. 3a strong absorption band at λ= 2930.02 cm^-1 and 2850.83 cm^-1 are referred to aliphatic C-H stretching vibration absorption bands. The absorption peaks at λ=3030.07 cm^-1 iscorresponded to the existence of aromatic C-H stretching vibration bands that belong to benzene ring. In-plane stretching vibration bands at λ=1600.9, 1490.3 and 1450.3 cm^-1, and out-of-plan vibration band at λ=755.08 cm^-1 are represented to C=C stretching, and bending vibrations in the aromatic ring, respectively. On the other hand, the observation of absorption peak at λ of 3450.03 cm^-1 indicates to the presence of (OH) group. The presence of hydroxyl group may be come from the presence water in trace amounts that resulted from the purification step.Meanwhile,the absorption peaks at λ of 1740, and 1250 cm^-1 are appeared. These peaks are assigned to (C=O), and (C-O) groups of the vinyl acetate, respectively.

Fig. 3: FT-IR spectrum of (PS-VAc) copolymer.

5.2.FT-IR spectroscopy analysis of SiO₂-NPs and modified SiO₂-NPs

Figs.4 and 5,show the chemical structure of the prepared SiO₂-NPs, and modified SiO2-NPs is verified by FT-IR spectroscopy. Fig.4 presentsthe FT-IR spectrum of the prepared SiO₂-NPs. In Fig.4,  The broad absorption band at around 3440 cm⁻¹and sharp band at 1650 cm⁻¹ were respectively corresponded to the O-H stretching and bending absorption.The dominant bands at 610, 796, 1130 cm⁻¹ are related to the stretching and bending vibrations of the silicon-oxygen bond, namely, the asymmetric vibrations of Si-O-Si (1130 cm⁻¹), the symmetric stretching vibrations of O-Si-O (796 cm⁻¹), and the symmetric stretching vibrations of Si-OH (610 cm⁻¹) ^[134]_. Meanwhile,The absorption peaks at 2930 and 2870 cm^–1 correspond to the –CH₃ and –CH₂ groups of dodecyltriethoxysilane increased in intensity as in Fig.5,  showing that the long chain alkyl has bonded to the surface of nano-SiO₂ particles. This confirms the modification of the silica nanoparticles by displacing the polar and hydrophilic groups’ OH with non-polar bonds like Si-C and C-H leads to increase in the hydrophobicity of the silica films.

Fig. 4: FT-IR spectrum of SiO₂-NPs.

Fig. 5: FT-IR spectrum of modified SiO₂-NPs.

1. Particle Size Investigation of polymer and SiO₂ NP

Dynamic light scattering (DLS) can be considered as a main tool to understand and verify models pertaining to the dynamics of polymers in dilute solution. It determines the size and hydrodynamic radius of polymer molecules in solution.DLS is a noninvasive technique to measure the size and size distribution of NPs ^[19]. The size distribution profiles of the synthesized NPs dispersed in DI water were found to have the average particle size of 88.53 nm as shown in Fig. 6, the polydipersity index (PDI) of Polymer nano-particle was 0.248 which indicates that particle are polydisperse in nature.

Fig. 6: Z_aveand PDI of the prepared copolymer at BPA concentration of 0.50 wt.,%,and 25 ^oC.

The hydrodynamic size distribution (Z_avag) and the PDI of the prepared SiO₂-NPs are measured by DLS as shown in Fig. 7.The average particle size (Z_avag) of the prepared SiO₂ nanoparticles with the range of 91 to 100 nm was obtained. Increasing the value of PDI to 0.226 refers to the formation of slightly heterogeneous SiO₂ nanoparticles in the suspended solution. The slight agglomeration of the SiO₂ nanoparticles results in the entanglement between the different sizes of the dispersed SiO₂ nanoparticles ^[161].

Fig. 7:Z_avag, and PDI of the prepared SiO₂ nanoparticles.

1. 249Egypt. J. of Appl. Sci., 34 (11) 2019

   TEM of SiO₂nano-particle

The TEM techniques was applied to verify the morphology, shape, surface state, and size of the products. From the TEM image in Fig. 8, we can see that the SiO₂ NPs are monodisperse, spherical, smooth anduniform in size. The TEM image characteristics show that the SiO₂ nanoparticles in shape and the particle size was 100 nm .

Fig. 8: TEM images of SiO₂ nanparticles with avarage size ranging from 91 to 100 nm.

1. Characterization of modified SiO₂-NPs

Modification of silica surface with silane coupling agents is one of the most effective techniques available to perform superhydrophobic materials of SiO₂-NPS ^[21]. Silane coupling agents as general formula of [Si(OR)₃R] have the ability to bond inorganic materials such as silica nanoparticles to organic resins. In general, Si(OR)₃ portion of the silane-coupling agents combines with the inorganic reinforcement, while the organo-functional groups (R) couples with the resin backbone. In our study, silane coupling agents as dodecyl triethoxysilane with carbon chain length of 12 is adopted as precursory modifiers to enhance the hydrophobic surface properties of Si₂O-NPs. This type of silane agents play a major role in the enhancement of the reactivity between silanes and copolymer matrix. (PSVAc-modifideSi₂O-NPs) composites are fabricated by interaction of the organic functional and alkoxy groups in the modified SiO₂-NPs through strong covalent bonds with the polymeric matrix of copolymer ^[22].

As a result, the physic-chemical properties of the prepared composites improve with surface treatment of the Si₂O-NPs by the silane coupling agent. This may be leads to improve the interfacial adhesion force between the prepared composites and the glass surface. The surface wettability (water repellent) of the coated film on the glass surface using and D-composites compared to T-(presence of SiO₂NP in copolymer) composite is studied from the points of view of contact angle as follows: -

8.1.