**2.1 Introduction**

For a long time beautiful and deep pearlescent pigments have attracted human attention and have been used in many cases [2.1]. These pigments consist of thin transparent small

Ceramic Coatings for Pigments 243

Muscovite mica in bulk state was milled for 1 hour and then sieved and was used as the base. SnCl2 (analytical reagent) was used to produce the metal oxide hydrate on mica flakes and the precipitation was performed by the presence of KClO3 (analytical reagent) as the oxidizing agent. Sorbitan mono-oleate (analytical reagent) was used as the non-ionic surfactant. Also, NaOH (chemical reagent) and HCl (chemical reagent) were used for

The experiments were conducted by standard orthogonal array. The selection of the orthogonal array is based on the condition that the degrees of freedom for the orthogonal array should be greater than or equal to sum of those lightness and reflective percent parameters [2.6-2.9]. In the present investigation, an L16 orthogonal array was chosen, which has 5 rows and 16 columns as shown in Table 2.1. Table 2.2 indicates the factors and their level. The experiment consists of 16 tests (each row in the L16 orthogonal array) and the columns were assigned with parameters. The first row was pH, second row was reaction temperature (T), third row was concentration of SnCl2 solution (C), fourth row was reaction time (t) and fifth row was stirring rate (R). The response to be studied was the lightness and reflective percent with the objective of the bigger the better. The experiments were conducted by orthogonal array with level of parameters given in each array row. The l test

number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 pH 1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4 T 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 C 1 2 3 4 2 1 4 3 3 4 1 2 4 3 2 1 t 1 2 3 4 3 4 1 2 4 3 2 1 2 1 4 3 R 1 2 3 4 4 3 2 1 2 1 4 3 3 4 1 2

pH 1 2 2.5 3 T (°C) 60 65 70 75 C (g/lit) 10 20 40 60 t (h) 5 7 8.5 10 R (r.p.m) 100 200 300 400

Levels 1 2 3 4

**2.3 Experimental** 

adjustment of pH.

Experiment

Parameters

**2.3.2 Design of experiments** 

results were subject to the analysis of variance.

Table 2.1. Orthogonal array L16(45) of Taguchi [2.4]

Table 2.2. Process parameters with their values at four levels

**2.3.1 Raw materials and reagents** 

flat surfaces with high reflective index. They reflect most of the radiant light and transmit a bit. Simultaneous reflection of light from small parallel surface layers of pigments causes the effect of deepness and brightness, such as exhibited by a pearl [2.2]. Each layer regularly reflects a part of the light and transmits the rest. The transmissible light is again reflected by other layers. Therefore, the manifestation of lustrous and interfering colours are revealed due to reflection from the interface between layers. This phenomenon is observed in natural pearl, fish scale, pearl body, birds feather, the butterfly wing, etc .

Pearlescent pigments are synthesized via two main methods. In the first, single crystals such as BiOCl or polycrystals like TiO2 are considered as bright and light materials due to their special structure. In the second, the pearlescent state is formed through the coating of materials with a high refractive index, (mainly metal oxide) on a transparent substrate like mica. This group is more important because of higher mechanical stability and brightness [2.3].

One of the most important kinds of these pigments is TiO2 coated mica which due to its high refractive index of TiO2, has high light resistance, low cost, good chemical and heat resistance and nontoxicity has many applications in different industries.

However, a new kind of these pigments is gold pearlescent pigment of mica-tin dioxide which is obtained through coating mica platelets by tin dioxide. This chapter section examines mica-tin dioxide and uses muscovite mica as the substrate and is prepared by laying a particle layer of tin dioxide on its surface. Due to the absorption and reflection of light in this layer, the pigment is termed a colouring pearlescent pigment. This pigment is widely used in many fields such as glass, glaze, automotive, plastics, cosmetics, etc.

The main purpose of this study was to prepare pearlescent pigment and to find the optimum values of process parameters which affect its properties using the Taguchi statistical method.
