Ceramic Coatings for Pigments

Special effect pigments, which can be natural or synthetic, show outstanding qualities of luster, brilliance and iridescent colour effects based upon optically thin layers [1.1–1.4]. This visual impression develops by reflection and scattering of light on thin multiple layers. In nature this is not limited to pearls and mussel shells alone; there are a multitude of birds, fish, precious stones and minerals, even insects, that demonstrate a luster effect. Experiments to understand the optical principles of natural pearl luster demonstrate that the brilliant colours are based upon structured biopolymers and upon layered structures, which are developed by biomineralization. Figure 1.1 illustrates the various optical principles of conventional pigments (A) (absorption pigments), metal effect pigments (B), and pearls (C) and pearl luster pigments (D), the most important group of special effect pigments. In the case of absorption pigments, the interaction with light is based upon absorption and/or diffuse scattering. A completely different optical behavior can be observed with the group of effect pigments including pearl luster and metal effect pigments. Metal effect pigments consist of small metal platelets (for example aluminum, titanium, copper), which operate like little mirrors and almost completely reflect the incident light.


Introduction
Special effect pigments, which can be natural or synthetic, show outstanding qualities of luster, brilliance and iridescent colour effects based upon optically thin layers [1.1-1.4]. This visual impression develops by reflection and scattering of light on thin multiple layers. In nature this is not limited to pearls and mussel shells alone; there are a multitude of birds, fish, precious stones and minerals, even insects, that demonstrate a luster effect. Experiments to understand the optical principles of natural pearl luster demonstrate that the brilliant colours are based upon structured biopolymers and upon layered structures, which are developed by biomineralization. Figure 1.1 illustrates the various optical principles of conventional pigments (A) (absorption pigments), metal effect pigments (B), and pearls (C) and pearl luster pigments (D), the most important group of special effect pigments. In the case of absorption pigments, the interaction with light is based upon absorption and/or diffuse scattering. A completely different optical behavior can be observed with the group of effect pigments including pearl luster and metal effect pigments. Metal effect pigments consist of small metal platelets (for example aluminum, titanium, copper), which operate like little mirrors and almost completely reflect the incident light.
Pearl luster pigments simulate the luster of natural pearls. They consist of alternating transparent layers with differing refractive indices. The layers consist of CaCO 3 (high refractive index) and proteins (low refractive index). This difference in refractive indices, arising equally on the interface between an air/oil film or oil film/water, is a prerequisite for the well-known iridescent colour images in these media. Small highly refractive platelets of pearl luster pigments align themselves parallel in optically thin systems such as paints, printing inks, or plastics. Interference effects develop when the distances of the various layers or the thicknesses of the platelets have the right values. Synthetic pearl luster pigments are either transparent or light-absorbing platelet shaped crystals. They can be monocrystalline, as in Pb(OH) 2 . 2PbCO 3 and BiOCl, or possess a multi-layered structure in which the layers have differing refractive indices and light absorption properties. The use of pearls and nacreous shells for decorative purposes goes back to ancient times (e.g., in Chinese wood intarsia). The history of pearl pigments dates back to 1656, when French rosary maker Jaquin isolated a silky lustrous suspension from fish scales (pearl essence) and applied this to small beads to create artificial pearls.  It took more than 250 years to isolate the pearl essence material (guanine platelets) and understand the pearl effect. Attempts were made to create synthetic pearl colours as organic or inorganic, transparent, highly refractive coatings and pearl pigments as crystalline platelets. From 1920 onwards, hydroxides, halides, phosphates, carbonates, and arsenates of zinc, calcium, barium, mercury, bismuth, lead, and other cations were produced for this purpose. Only the traditional natural pearl essence, basic lead carbonate and bismuth oxychloride, is still of importance. The strong demand for pearl effects came from the growing coatings and plastics industries, which wanted to improve the acceptance and popularity of their products. Furthermore, pearl luster pigments also allowed artists and designers to create new visual effects similar to those found in nature. The breakthrough for pearl luster pigments came with the invention of mica coated with metal oxides. Mica-based pearl luster pigments now account for >90% of the world market. Important manufacturers of pearl luster pigments are Merck KGaA, Germany (with overseas subsidiaries EMD Chemicals Inc., USA and Merck Ltd., Japan) and Engelhard Corp., USA. Of special interest are pigments which are toxicologically safe and which can be produced by ecologically acceptable processes. The total market for effect pigments can be estimated to be about 50,000 tons per year. Half of this amount can be calculated to be special effect pigments, the other half to be metal effect pigments.

Optical principles of pearl luster and interference pigments
The physical background of optical interference effects has been the subject of many publications [1.1-1.4, 1.6-1.9]. The optical principles of pearl luster (interference) pigments are shown in Figure 1.2 for a simplified case of nearly normal incidence without multiple reflection and absorption. At the interface P 1 between two materials with refractive indices n 1 and n 2 , part of the beam light L 1 is reflected (L 1 ) and part is transmitted (i.e., refracted) (L 2 ). The intensity ratios depend on n 1 and n 2 . In a multilayer arrangement, as found in pearl or pearl luster and iridescent materials (Figure 1.1D), each interference produces partial reflection. After penetration through several layers, depending on the size of and difference between n 1 and n 2 , virtually complete reflection is obtained, provided that the materials are sufficiently transparent.   In pigments that simulate natural pearl effects, the simplest case is a platelet shaped particle with two phase boundaries P 1 and P 2 at the upper and lower surfaces of the particles, i.e., a single, thin, transparent layer of a material with a higher refractive index than its surroundings. For small flakes with a thickness of ca. 100 nm, the physical laws of thin, solid, optical films apply.

Pigment type Examples
Multiple reflection of light on a thin solid f i l m w i t h a h i g h r e f r a c t i v e i n d e x c a u s e s interference effects in the reflected light and in the complementary transmitted light. For the simple case of nearly perpendicular incidence, the intensity of the reflectance depends on the refractive indices (n 1 , n 2 ), the layer thickness (d), and the wavelength (λ): With given n 1 and n 2 the maximum and minimum intensities of the reflected light, seen as interference colours, can be calculated and agree well with experimental results. Values for the refractive indices of the most important materials for pearl luster pigments are shown in Table 1 In practice, platelet crystals are synthesized with a layer thickness d calculated to produce the desired interference colours (iridescence). Most pearl luster pigments now consist of at least three layers of two materials with different refractive indices.
Thin flakes (thickness ca. 500 nm) of a material with a low refractive index (mica, silica, alumina, glass) are coated with a highly refractive metal oxide (TiO 2 , Fe 2 O 3 , layer thickness ca. 50-150 nm). This results in particles with four interfaces that constitute a more complicated but still predictable thin film system. The behavior of more complex multilayer pigments containing additional, thin, light-absorbing films can also be calculated if appropriate optical parameters are known.
Colour effects depend on the viewing angle. Pearl luster pigment platelets split white light into two complementary colours that depend on the platelet thickness. The reflected (interference) colour dominates under regular (maximum) reflection, i.e., when the object is observed at the angle of regular reflection. The transmitted part dominates at other viewing angles under diffuse viewing conditions, provided that there is a non-absorbing (white) or reflecting background.
Variation of the viewing angle therefore produces a sharp gloss (reflectance) peak, and the colour changes between two extreme complementary colours. The resulting complex interplay of luster and colour is measured goniophotometrically in reflection and at different angles. A pearl luster pigment is characterized by a minimum of three L * a * b * data sets (CIE L * a * b *system) measured under different conditions (e.g., 0°/45° black background, 22.5°/22.5° black background, 0°/45° white background). An analysis of these data specifies a pigment on the basis of its hiding power, luster, and hue [1.1, 1.10, 1.11, 1.12].

Application of the Taguchi method to develop a robust design for the synthesis of mica-SnO 2 gold pearlescent pigment 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 www.intechopen.com 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 TiO 2 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 TiO 2 coated mica which due to its high refractive index of TiO 2 , 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 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.

Taguchi techniques
The Taguchi technique is a powerful tool for the design of high quality systems developed by Taguchi between 1950Taguchi between to 1960]. It provides a simple, efficient and systematic approach to optimize designs for performance, quality and cost. The methodology is valuable when design parameters are qualitative and discrete. Taguchi parameter design can optimize the performance characteristics through the setting of design parameters and reduce the sensitivity of the system performance to source of variation [2.6-2.7]. This technique is multi-step process, which follow a certain sequence for the experiments to yield an improved understanding of product or process performance. This design of experiments process is made up of three main phases: the planning phase, the conducting phase and analysis interpretation phase. The planning phase is the most important phase and one must give a maximum importance to this phase. The data collected from all the experiments in the set are analysed to determine the effect of various design parameters. This approach is to use a fractional factorial approach and this may be accomplished with the aid of orthogonal arrays. Analysis of variance is a mathematical technique, which is based on a least square approach. The treatment of the experimental results is based on the analysis of average and analysis of variance [2.8-2.9]. www.intechopen.com

Raw materials and reagents
Muscovite mica in bulk state was milled for 1 hour and then sieved and was used as the base. SnCl 2 (analytical reagent) was used to produce the metal oxide hydrate on mica flakes and the precipitation was performed by the presence of KClO 3 (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 adjustment of pH.

Design of experiments
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 the columns were assigned with parameters. The first row was pH, second row was reaction temperature (T), third row was concentration of SnCl 2 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 results were subject to the analysis of variance.

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

Preparation method
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 According to the conditions shown in table 2.1, stirring rate, suspension temperature and pH were controlled. In order to coat the mica flake, SnCl 2 solution and aluminium chloride (AlCl 3 ) with a constant proportion SnCl 2 /AlCl 3 = 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 1h to get the pigment powders with pearlescent luster.

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 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].

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.

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). 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 SnO 2 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 SnO 2 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 SnO 2 film can vary and the hydrolysis rate is very fast.
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 SnO 2 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 SnO 2 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 SnO 2 particles are positive at this pH. The variation of electrical charges can be seen in figure 2.5.

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). 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.
Also increasing temperature causes the weakening of hydrogen bonding between SnO 2 particles and mica flakes [2.13].

Concentration of SnCl 2 solution
By observing brightness and reflectance versus concentration of SnCl 2 solution, based on Taguchi method and the use of Winrobust software it can be observed that the maximum of brightness and reflectance is related to concentration of about level 3 (figure 2.8). It can be seen that with the increase in the concentration of SnCl 2 solution, to a level higher than 3 causes the lightness and reflectance of the pigment to decrease.
The relationship between the intensities of reflected light, transmitted light, scattered light, absorbed light and the intensity of incident light is I = S+T+D+A [2.16], where S, T, D and A represent the strength of reflecting light, transmitting light, scattering light and absorbing light respectively. These are function of tin chloride concentration. When I is fixed, the thicker the membrane layer, the larger the value of A, while the smaller the value of S and T. As T decreasing, there is less incident light traveling through the coating layer and arriving at the surface of the mica substrate. Thus, the intensity of reflecting light generated at the surface decreases, the reflection and interference action becomes weaker, and the lightness decreases.  The stirring rate affects the densification of the membrane layer directly. If the stirring rate is too low, the reaction solution can not form sufficient turbulence [2.17], then the microscopic mixture is uneven, and the generated crystal particles of SnO 2 are of various sizes. On the other hand, if the stirring rate is too high, it will affect the rate of growing of the crystal nucleus, with the result that some tiny colloidal micro-particles enter the solution through the filter paper rather than depositing on the surface of mica flakes, which will cause the light to scatter. Therefore, the stirring rate in this reaction should be of level 2.

Optimized sample
After performing experiments designed by Taguchi method and analysing results with Winrobust software the optimized sample was provided using the following condition: pH = 2.5, T = 65°C, C = 40 g/lit, t = 7 h, R = 200 r.p.m .
The XRD pattern of this sample shows the hydroxide phase before calcination and Casiterite SnO2 phase after that.
According to SEM results, SnO 2 particles are coated on mica flakes almost regularly and uniformly (figure 2.13-a). According to the laser beam diffraction technique the mean particle size of pigments is about 65 micron. The density of this pigment is 3.3 g/cm 3 and the lightness and reflective percent are 97% and 89% respectively in which case these amounts of l * and R are equal to the Winborust software results. Consequently choosing to use the Taguchi method and L 16 algorithm is correct for this work. According to table 2.5 and figure 2.14 parameters l * and R in the different angle views have different values, and also these values are higher in smaller angle views. Therefore the optimized sample has lightness and reflection [2.21, 2.22]. In addition since the maximum value of reflection is in the range of yellow light wavelength (570-600 nm), the pigment is yellow. So it is possible to say that the optimized pigment is gold pearlescent pigment.