FUNCTIONAL SOL-GEL COATINGS ON ENAMEL SURFACES
Gamze Gül Avci, Volkan Günay
TUBITAK-Marmara Research Center-Materials Institute - PO Box 21, 41470 Gebze/Kocaeli/Turkey

Introduction
Sol-gel is a versatile process for coating various substrates such as metals, glass, plastics, ceramics, paper, textiles etc. with functional properties. The process is known for almost 150 years but it has been only about 30 years the first patent is prepared and only in recent years commercialisation of products has been started. Sol-gel process is ideal for the synthesis of organic-inorganic materials, composites, fibres, powders or coatings at low processing temperatures. After the synthesis of organic-inorganic hybrid contact lenses via sol-gel [1] a variety of hybrid materials have been fabricated such as hard coatings on optical devices [2], chemical sensors [3], corrosion resistant [4] coatings and so on.
Work on sol-gel coatings has widened considerably in a very short period of time and various types of functional coatings have been developed. An example of functional coating is the easy clean type which can be formulated as hydrophilic, hydrophobic or photo catalytic depending on the desired characteristics. Hydrophilic surfaces with high wettability has a very low contact angle due to the reduced surface tension where hydrophobic surfaces with contact angles of about 100° show low wettabilities. Hydrophilic surfaces attract water rather than repel as in hydrophobic surfaces. Low or high wettability of the surface help the dirt, particles or even bugs to be easily removed from the surface. Hydrophilic and photo catalytic effects can be formed with TiO2 in the anatase form by exposing the coated layer to UV light.
In this paper properties of hydrophilic, hydrophobic and photo catalytic coatings fabricated on enamel coated steel sheets are investigated by contact angle measurements, Rhodamin B tests, UV spectrometer measurements and microstructures by SEM.

Experimental
Hydrophilic and photo catalytic films were fabricated on enamel coated steel sheets via sol-gel method by using Ti-Alkoxides as the TiO2 source and heat treated at 400-500 °C to convert the amorphous gel to crystalline TiO2 in the anatase form. In these coatings, Cu, Ag and Zn-oxides were added to increase the photo catalytic activities. These coatings can be used in window glasses as well and certain optical properties were also required with photo catalytic properties.
For the synthesis of hydrophobic coatings a solution was prepared by mixing 0.01 mol fluorsilane with 0.1 mol TEOS and 0.15 mol HCl.
Photo catalytic surfaces were also formed by applying a coating solution to the enamel surfaces prepared by mixing TiO2 powder (P25 Degussa) with TEOS and MTEOS in different amounts with synthesis of hydrophobic and photo catalytic coatings the applied solutions were heat treated between 120-180 °C for 30 minutes. The samples were prepared by the well-established method of dip coating. Before coating all the samples were cleaned from oil or dirt by wiping with a soft tissue wetted with alcohol.

Characterization of the samples
The contact angles of the coatings were measured by Kernco contact angle meter and compared between the coated and uncoated samples. The change of the contact angle values in acidic and alkaline medium for the hydrophobic coatings were determined in 1 % HCl and 1 % NaOH. The effect of UV radiation on contact angle was also determined. Rhodamin B tests for the photo catalytically coated enamel surfaces were carried in 2 ppm Rhodamin B solution. The samples were placed in the Rhodamin B solution and irradiated in a commercial solarbox at 900W/m2. The changes in absorption of the Rhodamin B solution with and without the coated samples were measured by a UV-VIS spectrometer at 553 nm, the absorbents wavelength of Rhodamin B. The canning electron micrographs were obtained with a SEM, Jeol JXA 840A microscope.

Results
Hydrophilic and photo catalytic coatings

Coating thickness can be controlled in the dip-coated samples by the number of coatings applied on the surfaces. This type of coatings was also reflective and this gives enamel surfaces very shine appearances. The photo catalytic properties were also improved by the additions of certain oxide additives such as Cu, Ag and Zn as around 1wt% in the coatings. Also, the heat-treatment at around 500 oC or above affects the crystalline structure and the crystallite sizes of TiO2, which in turn affect the photo catalytic properties of the coatings.

Hydrophobic coatings
The contact angle of the enamel surface coated by hydrophobic sol was measured by dropping 0.5 μl water on the surface and it was found to be 98 degrees. After applying abrasion test using a scotch bright sponge for 50, 100, 150, 200, 250 and 300 times the change in the measurements are given below.

Fig.1 The change of contact angle with the abrasion number.

The change of contact angle measured on samples treated with 1 % HCl for 48 hours are given in Fig. 2. The measurements were taken every 6 hours. After 24 hours the drop of contact angle value increases but at the end of 48 hours the change in HCl is less than the change for the sample treated with NaOH for 48 hours.

Fig. 2 The change of contact angle in 1 % HCl.

Fig. 3 The change of contact angle in 1 % NaOH.

In Fig. 4 the change in contact angle measurements are given after irradiating the hydrophobiccoated enamel surfaces with Xenon lamp in solar box for 7 days at 940 W/ m2. The measurements are taken every 24 hours, which show that there is not a considerable change in the hydrophobic effect at the end of the duration period of the test.

Fig. 4 The change of contact angle with UV irradiation.

Photo Catalytic Coatings
When light is absorbed by titanium dioxide (TiO2), electrons and holes are produced in the materials. These holes move to the surface of the material, react with water to form very reactive (•OH) ions and decompose the adsorbed substances with their high oxidation capacity. This phenomenon is called photo catalytic reaction. Titanium dioxide shows its photo catalytic effect only when irradiated by short-wavelength light, as ultraviolet rays.
TiO2 + hν -> h+ + e-
H2O + h+-> •OH + H+
On the other hand; oxygen in air acts as an electron acceptor to form super-oxide ion O2-
O2 + -> e- O2-
Super- oxide ions are very reactive ions that can oxidize the organics and bacteria [5].
In this work photo catalytic effect of TiO2 (P25) was measured against Rhodamin B. The results are given in Table 1. TiO2 sol also prepared by the addition of Ag ions as AgNO3 was determined for its photo catalytic activity. The absorbance results of Rhodamin B show the increasing photo catalytic effect of TiO2 with the addition of Ag ions.
The absorbance values of Rhodamin B solution before irradiation was 0.432. After irradiation there was an ignorable decrease measured as 0.387. Comparing the absorbance results of Rhodamin B solution before and after UV radiation without any photo catalytic means show almost no change in the values. Placing an enamel sample coated with photo catalytic material in the Rhodamin B solution decreases the absorbance values of the solution to almost 0 levels meaning that the sensitive organic structure of Rhodamin B is decomposed by the photo catalytic effect of the coating.

Table 1. Absorbance values of Rhodamin B after UV irradiation with different amounts of TiO2 (P25) powder
% P25/ TEOS/ MTEOS
Absorbance
% 50 P25/ TEOS/ MTEOS
0.370
% 60 P25/ TEOS/ MTEOS
0.126
% 70 P25/ TEOS/ MTEOS
0.223
% 50 P25/ TEOS/ MTEOS/ % 1.5Ag
0.031
% 60 P25/ TEOS/ MTEOS/ % 1.5 Ag
0.051
% 50 P25/ TEOS/ MTEOS/ % 3 Ag
0.008
% 60 P25/ TEOS/ MTEOS/ % 3 Ag
0.004
% 70 P25/ TEOS/ MTEOS/ % 3 Ag
0.011

Fig. 5 SEM micrograph of TiO2 (P25)
film on enamel surface.
Fig. 6 SEM micrograph TiO2 (P25)/Agfilm on enamel surface.

Fig. 5 shows the scanning electron micrograph of TiO2 coating with only P25 and Fig. 6 shows P25 with the addition of Ag ions in the form of AgNO3. The texture and morphology of the TiO2/Ag in comparison with TiO2 can be seen. The pure TiO2 layer is in granular morphology while TiO2/Ag sample has a smoother surface. This means the TiO2/Ag layer has smaller particle size and correspondingly gives higher exposed surface, which increases the photo catalytic activity of the coating layer.

Conclusions
Sol-gel coatings on any surfaces especially on glass surfaces have been used to obtain functional coatings. This is a chemical route in the coatings and has many advantages compared to the other coating techniques. It is simple and can be applied by using various techniques at room temperature in ordinary atmospheric conditions.

Hydrophilic, hydrophobic and photo catalytic coatings were prepared on enamel surfaces by solgel method. The properties of the films were investigated showing that easy-clean surfaces through the mechanisms of hydrophilicity, hydrophobicity and photo catalytic activity can be formed on enamel substrates. The durability of the properties and the degradation time of the coatings have to be investigated for the determination of the life of the properties.

References
[1] G Philipp, H. Schmidt: J.Non-Cryst. Solids 63 (1984), 283.
[2] H. Schmidt. H.Wolter: J.Non-Cryst. Solids 121 (1990), 428.
[3] C.Dave,B.Dunn: Anal.Chem. 66 (1994), 1120.
[4] M.Atik, M.Aegerter: J.Non-Cryst. Solids 483 (1992), 147.
[5] W.Barthlott , C.Neihuis: Planta,1 (1997), 202.

 

 

 

 

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