**2.4.2.1 pH**

By plotting brightness and reflectance versus pH based on Taguchi method and using of Winrobust software it can be observed that maximum brightness and reflectance is related to pH of about level 3 (figure 2.3).

Fig. 2.3. Effect of parameter pH on the a) lightness and b) reflection , in different levels

To follow and conform the effect of these changes, SEM and EDS analysis were performed on pigments synthesized at different levels of pH. The results are shown in figures 2.4, 2.5 and 2.6. According to figure 2.4-a, SnO2 particles have precipitated on mica flakes uniformly in sample 9

In the following section, according to the presented method and results of experiments, the

By plotting brightness and reflectance versus pH based on Taguchi method and using of Winrobust software it can be observed that maximum brightness and reflectance is related

(a) (b)

To follow and conform the effect of these changes, SEM and EDS analysis were performed on pigments synthesized at different levels of pH. The results are shown in figures 2.4, 2.5 and 2.6. According to figure 2.4-a, SnO2 particles have precipitated on mica flakes uniformly in sample 9

Fig. 2.3. Effect of parameter pH on the a) lightness and b) reflection , in different levels

Fig. 2.2. Normal probability distribution of residual amounts

**2.4.2 The factors that affect the lightness and reflection of pigment** 

effect of the parameters has been studied.

to pH of about level 3 (figure 2.3).

**2.4.2.1 pH** 

which its pH is optimum (pH = level of 3). Figure 4-b (EDS analysis of sample 3) shows a great amount of tin on the mica flakes. In sample 6, only a small quantity of SnO2 particles have been precipitated on mica flakes that its pH is less than optimum pH because hydrolysis process has not been performed completely . The results are shown in figures of 2.5-a and 2.5-b but in sample 15 a great amount of SnO2 particles are not agglomerated on the mica flakes uniformly. The results are illustrated in figures 2.6-a and 2.6-b. It seems that due to the progress of hydrolysis the homogeneity of the SnO2 film can vary and the hydrolysis rate is very fast.

Fig. 2.4. SEM and EDS Images of pigments synthesized at different levels of pH

Ceramic Coatings for Pigments 249

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).

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

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

(a) (b) (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.

**2.4.2.2 Reaction temperature** 

levels

(figure 2.7-c).

Therefore, the uniformity of SnO2 coated on mica flakes depends on the hydrolysis rate of SnCl2 solution. The equation of hydrolysis (Equation 1) and rate of hydrolysis (Equation 2) can be considered as follows:

$$\left[M\left(OH\_2\right)\_N\right]^{Z+} + hH\_2O \rightarrow \left[M\left(OH\right)\_h\left(OH\_2\right)\_{N-h}\right]^{\left(Z-h\right)^+} + hH^+\_{\text{Soluted}}\tag{1}$$

$$hh = \left[\frac{1}{1 + 0.41pH}\right] \left[\left(1.36Z - N\right)\left(0.236 - 0.08pH\right) - \frac{2.621 - 0.02pH - X\_m^\*}{\sqrt{X\_m^\*}}\right] \tag{2}$$

Where "h" is the hydrolysis rate, "Z" is the charge of M cation, "N" is the coordination number of M, "X\*m" is electronegativity of M. According to the equation (2), if the pH level is less than the optimum level, the hydrolysis process is not performed and SnO2 particles do not precipitated, because the hydrolysis rate is negative. Therefore, brightness and reflectance will decrease. On the other hand, if the pH level is more than the optimum level, the hydrolysis rate increases with pH increasing. Therefore, agglomerated particles are initially formed in suspension and then on the mica flakes. These agglomerated particles cause irregular scattering of light which in turn decrease the brightness and reflectance [2.12, 2.13, 2.14].

Furthermore, deposition of hydrolysed particles on the mica flakes depends on the electrical charges of hydrolysed particles and mica flakes. On the other hand, the electrical charge of particles in the suspension depends on pH of the media. Therefore SnO2 particles can precipitate on the mica flakes in a special pH range. As the electrical charge of mica flakes is negative in the suspension, most of the deposition is performed at a pH less than 3.5. This is because the charge of SnO2 particles are positive at this pH. The variation of electrical charges can be seen in figure 2.5.

Fig. 2.5. Variation of zeta potential with pH for SnO2. Isoelectric point of SnO2 is 3.5 and optimum amount of positive charge of SnO2 particles is pH 2.5 [2.15]

Therefore, the uniformity of SnO2 coated on mica flakes depends on the hydrolysis rate of SnCl2 solution. The equation of hydrolysis (Equation 1) and rate of hydrolysis (Equation 2)

*Z Z h*

<sup>1</sup> 2 621 0 02 1 36 0 236 0 08

Where "h" is the hydrolysis rate, "Z" is the charge of M cation, "N" is the coordination number of M, "X\*m" is electronegativity of M. According to the equation (2), if the pH level is less than the optimum level, the hydrolysis process is not performed and SnO2 particles do not precipitated, because the hydrolysis rate is negative. Therefore, brightness and reflectance will decrease. On the other hand, if the pH level is more than the optimum level, the hydrolysis rate increases with pH increasing. Therefore, agglomerated particles are initially formed in suspension and then on the mica flakes. These agglomerated particles cause irregular scattering of light which in turn decrease the brightness and reflectance

Furthermore, deposition of hydrolysed particles on the mica flakes depends on the electrical charges of hydrolysed particles and mica flakes. On the other hand, the electrical charge of particles in the suspension depends on pH of the media. Therefore SnO2 particles can precipitate on the mica flakes in a special pH range. As the electrical charge of mica flakes is negative in the suspension, most of the deposition is performed at a pH less than 3.5. This is because the charge of SnO2 particles are positive at this pH. The variation of electrical

Fig. 2.5. Variation of zeta potential with pH for SnO2. Isoelectric point of SnO2 is 3.5 and

optimum amount of positive charge of SnO2 particles is pH 2.5 [2.15]

. . . .. .

*pH X <sup>h</sup> Z N pH pH <sup>X</sup>* 

*Solvated <sup>N</sup> <sup>h</sup> N h*

(1)

\*

*m*

(2)

\*

*m*

 2 2 2

*M OH hH O M OH OH hH*

can be considered as follows:

[2.12, 2.13, 2.14].

charges can be seen in figure 2.5.

1 0 41
