**2.4.1 The colour characteristics of pigments**

Lightness (l\*) and reflective percent (R) of pigments were measured using goniospectrophotometer colour analysis apparatus and D65 illuminant in different view angles (20º, 45º, 75º, 110º). The results are shown in table 2.3.


Table 2.3. Results of lightness and reflective percent of pigments

The spectral reflectance of the pigment was measured using a Colour Quest Goniospectrophotometer (CE-741-GL-Gretag Macbet Co). The SEM photomicrographs were obtained using an electron microscope (LEO-1455VP) and SEM studies were performed using this microscope equipped with EDX (to determine chemical composition of the observed objects). Appearance and colour of the covered mica flakes were observed by reflectance light microscope (Laborlux 11 pol, Leitz matallux 3). Finally, the phase analysis of the resultant pigment powders was determined by X-ray diffractometer (XRD-Philips-Xpert). Density and particle size of resultant pigment were measured by helium pycnometer

Firstly, a suitable suspension of 2 g milled and sieved mica (20-60 µm) in 40 ml de-ionized water was prepared. The suspension pH was adjusted with dilute hydrochloric acid (5%) to pH which is shown in table 2.1 and then heated to desired temperature according to table 2.1 experiment plane. The mixture was heated up to desired temperature, then 0.0001 % (based on mica wt%) of nonionic surfactant (sorbitan mono-oleate ) was added to this suspension. Then, potassium chlorate aqueous solution (KClO3) was added to

According to the conditions shown in table 2.1, stirring rate, suspension temperature and pH were controlled. In order to coat the mica flake, SnCl2 solution and aluminium chloride (AlCl3) with a constant proportion SnCl2/AlCl3 = 9/1, were added to the mica suspension. The pH of the suspension was kept at desired level by addition of 3% aqueous sodium hydroxide solution. The coating process was stopped after the desired time. Then, the suspension was filtered and the prepared pigments were washed with distilled water until neutral pH was reached and dried at 100ºC. Finally the powders were calcined at 950ºC for

Lightness (l\*) and reflective percent (R) of pigments were measured using goniospectrophotometer colour analysis apparatus and D65 illuminant in different view

number 1 2 3 4 5 6 7 8 reflection(R)% 45 75 82 58 46 57 74 79 lightness(l\*)% 79.1 88.01 93.84 84.76 78.05 84.59 90.02 94.64

number 9 10 11 12 13 14 15 16 reflection(R)% 82 85 74 54 62 79 59 61 lightness(l\*)% 93.20 95.48 88.94 83.25 84.64 90.12 82.30 83.04

(Accupyc 1330) and Master sizer 2000 (Mal 100229) respectively.

**2.3.3 Analysis** 

**2.3.4 Preparation method** 

**2.4 Result and discussion** 

Experiment

Experiment

this suspension as an oxidizing agent.

1h to get the pigment powders with pearlescent luster.

angles (20º, 45º, 75º, 110º). The results are shown in table 2.3.

Table 2.3. Results of lightness and reflective percent of pigments

**2.4.1 The colour characteristics of pigments** 

Doing different analyses such as analysis of variance by using Winrobust software, important parameters were identified. This showed that some of these parameters have less effect in synthesis of the pigments than others. The amount of the effects is shown in figure 2.1.

Fig. 2.1. Effect of synthesis parameters on the a) lightness and b) reflective percent of pigments

Since in the Taguchi method it is possible that an optimum does not exist in the performed experiments in the designed array [2.10-2.11], some operations were then done by Winrobust software to determine the optimized point. Optimized levels of identified parameters for lightness and reflective percent of pigments is shown in table 2.4.


Table 2.4. Optimized levels of parameters determined with Winrobust software

To confirm this statistical method, synthesis is performed at the optimized conditions. If the experimental results equal the results with Winrobust software, it shows that the method is correct. In figure 2.2, normal probability distribution by Winrobust software is shown for the residual amount in which the continuity of the points in this figure is another confirmation for this method [2.10].

Ceramic Coatings for Pigments 247

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.

(a) (b)

(c) (d)

(e) (d)

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

Fig. 2.2. Normal probability distribution of residual amounts

In the following section, according to the presented method and results of experiments, the effect of the parameters has been studied.
