*1.2.1. Silica*

Rutile TiO<sup>2</sup> particles must be coated with protective layers of SiO<sup>2</sup> and Al<sup>2</sup> O3 through wet chemistry processes in order to decrease their photoactivity, increase weather durability and increase dispersibility in certain media [4, 22–24]. The morphology of the SiO<sup>2</sup> coating layers on the surface of TiO<sup>2</sup> powders can be controlled by adjusting the reaction temperature, pH value of the reaction solution and the SiO<sup>2</sup> loading [4].

Depending on the silica precipitation conditions, very disparate pigment characteristics could be produced. "Fluffy" coating, which provides better spacing and optical efficiency, increases oil absorption and decreases gloss forms at acidic or neutral pH [1]. An example of a "fluffy" coating composed of submicroscopic particles joined together in a gel-like structure is presented in **Figure 2**.

During slow deposition at basic pH, silica can also be formed as a dense, glassy shell that encapsulates the particle and provides the highest durability available. Durability of the silica layer and the amount of energy needed to develop gloss are proportional to the amount of silica in the layer [1]. A dense, glass-like coating is shown in **Figure 3**. Coating is a few nm thick, encapsulating the entire surface of the pigment.

Island-like SiO<sup>2</sup> coating layers can be formed on a TiO<sup>2</sup> surface when the reaction temperature, the pH value and the mole ratio of Na<sup>2</sup> SiO<sup>3</sup> to TiO<sup>2</sup> are low. Continuous and uniform SiO<sup>2</sup> coating layers form in an alkaline pH ranging from 9 to 10. The thickness of the coating

**Figure 2.** Silica surface treatment as a fluffy layer.

layer increases with the increase of the mole ratio of Na<sup>2</sup> SiO<sup>3</sup> to TiO<sup>2</sup> . The SiO<sup>2</sup> coating layers are anchored onto the TiO<sup>2</sup> surface by the Ti─O─Si bonding. Dispersibility of the SiO<sup>2</sup> -coated TiO<sup>2</sup> powders is affected by the morphology of the SiO<sup>2</sup> coating [4].

#### *1.2.2. Coating process of the SiO2 layer*

*1.1.3. Stability towards light and weather*

424 Titanium Dioxide - Material for a Sustainable Environment

**1.2. About surface treatment**

. Formation of extremely reactive radicals (˙

dispersion states as a function of particle size distribution.

of TiO<sup>2</sup>

**Figure 1.** TiO<sup>2</sup>

*1.2.1. Silica*

Rutile TiO<sup>2</sup>

on the surface of TiO<sup>2</sup>

sented in **Figure 2**.

Island-like SiO<sup>2</sup>

SiO<sup>2</sup>

value of the reaction solution and the SiO<sup>2</sup>

thick, encapsulating the entire surface of the pigment.

ture, the pH value and the mole ratio of Na<sup>2</sup>

coating layers can be formed on a TiO<sup>2</sup>

When subjected to intense radiation or weathering, systems with incorporated TiO<sup>2</sup>

particles must be coated with protective layers of SiO<sup>2</sup>

increase dispersibility in certain media [4, 22–24]. The morphology of the SiO<sup>2</sup>

or structural changes. Yellowing, chalking and loss of gloss occur due to photocatalytic activity

coating matrix can be suppressed by doping or coating the surface of the pigment [3, 19–21].

chemistry processes in order to decrease their photoactivity, increase weather durability and

loading [4].

Depending on the silica precipitation conditions, very disparate pigment characteristics could be produced. "Fluffy" coating, which provides better spacing and optical efficiency, increases oil absorption and decreases gloss forms at acidic or neutral pH [1]. An example of a "fluffy" coating composed of submicroscopic particles joined together in a gel-like structure is pre-

During slow deposition at basic pH, silica can also be formed as a dense, glassy shell that encapsulates the particle and provides the highest durability available. Durability of the silica layer and the amount of energy needed to develop gloss are proportional to the amount of silica in the layer [1]. A dense, glass-like coating is shown in **Figure 3**. Coating is a few nm

SiO<sup>3</sup>

coating layers form in an alkaline pH ranging from 9 to 10. The thickness of the coating

to TiO<sup>2</sup>

powders can be controlled by adjusting the reaction temperature, pH

show color

through wet

coating layers

OH, HO2˙), that cause deterioration of the

and Al<sup>2</sup>

surface when the reaction tempera-

are low. Continuous and uniform

O3

Isoelectric point (IEP) of rutile TiO<sup>2</sup> is usually located around pH 3.5 [24, 25]. The rutile TiO<sup>2</sup> surface is negatively charged under conditions where the pH value of the reaction solution is greater or equal to 7. Under conditions where pH values of the reaction solutions are in the range 7–8, Na<sup>2</sup> SiO<sup>3</sup> rapidly hydrolyzes to form a large number of siliceous micelles. The resultant siliceous micelles anchor on the surface of TiO<sup>2</sup> powders via Ti–O–Si bonding to obtain island-like coating layers.

The hydrolysis rate of Na<sup>2</sup> SiO<sup>3</sup> should be lowered by increasing the pH value to 9–10. This results in the formation of small-sized or less aggregated siliceous micelles. Small-sized micelles anchor on the surface of TiO<sup>2</sup> powders initially via Ti─O─Si bonding. Later, the micelles present in the solution and the anchored micelles condense via Si─O─Si bonding resulting in the formation of continuous and uniform coating layers. Over pH 10.5, silicon

**Figure 3.** Silica surface treatment as a dense, glass-like layer.

species exist as single silicate anions, or less aggregated siliceous micelles with very small particle size, which should be more negatively charged. The single silicate anions and the highly negatively charged siliceous micelles do not react with the negatively charged TiO<sup>2</sup> surface due to the strong electrostatic repulsion. Therefore, no SiO<sup>2</sup> coating layers forms on the TiO<sup>2</sup> surface at this high pH value.

Raising the temperature of the reaction affects the covering extent and causes the formation of a dense SiO<sup>2</sup> coating layer on the surface of the rutile TiO<sup>2</sup> powder.
