**2.4.2.2 Reaction temperature**

By observing brightness and reflectance versus reaction temperature based on Taguchi method and the use of Winrobust software it can be observed that the maximum of brightness and reflectance is related to reaction temperature of about level 2 (figure 2.6).

Fig. 2.6. Effect of parameter temperature on the a) lightness and b) reflection , at different levels

According to SEM analysis observed in figures of 7-a, 7-b and 7-c, when the reaction temperature is low (T = level 1), approximately no SnO2 coating is formed on the mica flakes, thus lightness and reflectance are small (figure 2.7-a). Lightness and reflectance increase with temperature increasing until level of 2. SnO2 particles coated on the mica flakes are uniform at temperature of level 2 (figure 2.7-b). Increasing temperature causes the flocculation of the colloid particles, the particles become larger, and the membrane on substrate surface becomes loosen and SnO2 particles irregularly deposit on the mica flakes (figure 2.7-c).

Fig. 2.7. SEM images of pigments synthesized at different levels of reaction temperature

As the hydrolysis process is a precipitation reaction, precipitation decreases with temperature decreasing. This seems to be due to the solubility of tin hydroxide increasing with temperature increasing. So, the precipitate reduces and the coating becomes uneven.

Ceramic Coatings for Pigments 251

By observing brightness and reflectance versus reaction time based on the Taguchi method, it can be observed that the maximum of lightness and reflectance are related to reaction time

(a) (b) Fig. 2.10. Effect of time parameter on the a) lightness and b) reflection, at different levels

As SEM analysis show in figures 2.11-a and 2.11-b, when reaction time is low a few coating of SnO2 are formed on the mica flakes, thus lightness and reflectance are decreased (figure 2.11-a). While increase of reaction time, until level of 2, a uniform SnO2 coating is formed on the mica flakes and thus, lightness and reflectance increase as shown in figure 2.13-b. However, increasing time more than level 2 causes decreasing of lightness and reflectance, because the SnO2 coatings formed on the mica flakes are

Fig. 2.9. SEM image of pigment synthesized at great concentration

**2.4.2.4 Reaction time** 

separated gradually.

of about level 2 (Fig. 2.10).

Also increasing temperature causes the weakening of hydrogen bonding between SnO2 particles and mica flakes [2.13].
