1. INTRODUCTION:
Titanium Dioxide is a multifaceted material in coating applications. It efficiently scatters visible light thereby imparting whiteness, brightness and opacity to paint and coating material. The scattering of light is affected by various factors like refractive index of pigment, pigment volume concentration (PVC) in the coating, degree of dispersion and the distance between pigment particles. The hiding will be the maximum when the pigment particles are separated by distance of one diameter of the pigment.
The concept of spacing is basically rather easy; if it happens, that two or more titanium dioxide pigment particles are not completely dispersed but still have contacts with each other, it can be calculated that the theoretical maximum of white pigment efficiency is not reached. As the scattering volumes are larger than the particles themselves the volumes will overlap, influence each other and are all together decreased therefore. If it is possible to place inert filler particle just between the agglomerated titanium dioxide particles, the pigments are pushed apart so that the scattering volumes are free again. Scattering power recovers to 100% of the theoretical maximum value. Due to the increased efficiency of the pigment the titanium dioxide loading can be reduced respectively [1].
TiO₂ is often poorly dispersed and crowded by the extender and resin particle. Indeed the higher the pigment and extender volume concentration the worse the crowding can become. It is well understood that by reducing the size of extender particles used, the spacing of TiO₂ can be significantly improved. Replacing 6um extender/ filler with a 2 um would, lead to improve TiO₂ spacing, better opacity and to the possibility of reducing TiO₂ levels. However, to further improve the spacing to levels leading to significant improvements in scattering and opacity would ideally require the extender particle size to be an order of magnitude smaller at least [2]. Figure 1 shows the effect on spacing of TiO₂ particle with decreasing particle size of extender.
The particle size and particle shape of Calcium Aluminum Silicate (CaAl2Si2O8) has been found to replace partially the Rutile or Anatase TiO₂ pigments in solvent based coating formulations [3].
This extender is synthesized from naturally occurring mineral “Actinolite”. Actinolite is a Calcium Magnesium Iron Silicate Hydroxide. As compared to other functional fillers Calcium Aluminum Silicate has better wet hiding properties due to its higher refractive index compare to other extenders. There use does not reduce any gloss in the final products due to its lower Oil Absorption Values. These are the prime reasons why it is considered as a better additive to Titanium Dioxide. The properties of CAS filler are shown in the Table: 1
The cost of TiO2 is increasing steadily and thus the cost of paint with minimum operational margin. There are two ways to stabilize this rise in cost, one is to lower the quality of paint by less use of essential ingredients and another way is to use the subsidiary product in the replacement of the primary product either cent percent replacement or partial replacement which gives some cost advantages. Therefore in the present work the TiO2 from the different powder coating formulations were partially replaced with CAS to study the effect on different optical properties.

2. EXPERIMENTAL:
2.1 MATERIALS:
Calcium Aluminum Silicate was procured from M/s Aromax Corporation, Ahmedabad and was used without any further modification. The comparative evaluation of properties of CAS and other conventional fillers & extenders [4] are given in Table: 2.
The Titanium Dioxide used for this study is Rutile (RC–822) grade and was procured from M/s. KMML Ltd., India.
The Saturated Polyester resin (P – 5127) (Make: DSM Coating Resin, Netherlands) and the Bisphenol A type epoxy resin (E–12) (Make: China Hungshan Runfa Chemicals Co. Ltd.) used for the present study were procured from M/s. Vector Agencies, Delhi. The properties of Epoxy resin E – 12 & Polyester resin P - 5127 are shown in table 03.
The Flow modifier (Make: Worlee Chemie, GmbH, Germany) and Matt Hardener (Make: Gadery HK Development Co. Ltd., China) were also procured from M/s. Vector Agencies, Delhi.
Benzoin used for this study was occupied from M/s. Rare Pharma Pvt. Ltd. India.

2.2 PREPARATION OF POWDER COATING [5]:
Preparation of powder coating is a discontinuous process and consisting of various steps. In the present work following method was used to prepare different powder coatings.
All the raw materials were dry blended thoroughly in the ribbon mixer to give a uniform feed to the extruder. Intensive mixing was done to ensure uniform distribution of liquid components such as flow modifier and still retain a free flowing dry pre mix.
The raw material pre mix is fed to the single screw extruder where heat was applied to the extruder barrel melts the resins in the premix. As the molten mass passes through extruder, Intensive shear force exerted by the kneading action of screw resulted in uniform fine dispersion of pigment in the molten resin.
The molted stiff paste of extrudate was passed from the extruder to a pair of water-cooled rolls, which chilled and squeezed the extrudate to a thin band. The band was then further cooled by either a water or air cooled conveyor until it becomes brittle and broken in to small flakes.
The flakes were grinded to a fine powder by a high-speed hammer mill and the powder was sieved using Vibratory sieves (100 to 200 µm) to ensure absence of oversize particles.
Following the above procedure, the powder coatings were prepared in White Glossy Finish, White Matt Finish and Grey Matt Finish. The different compositions are shown in Table 4. The TiO2 Rutile pigment was replaced by Calcium Aluminum Silicate from 5 to 20% by mass in different compositions.

2.3 APPLICATION OF POWDER COATING [6]:
The phosphated mild steel panels (150mm x 100mm x 1.25 mm) were used as a test panels for analysis of gloss, Adhesion, Impact resistance, pencil hardness and salt spray test. The Aluminum test panels (150mm x 100 mm x 0.3 mm) were used for other tests. The different powder coatings prepared in the present work were applied on these panels by Electrostatic Spray application. For contrast ratio measurement, the piece of Morest chart (50 mm x 50 mm) was pasted on the aluminum panel before application because powder coating cannot be applied directly on the nonconductive substrate (Morest chart) by electrostatic spray application. Then the coated panels were heat cured in an oven at the temperature of 180ºC for 20 min.

2.4 CHARACTERISTATION OF POWDER COATING:
Various Optical, Mechanical and performance properties like Contrast Ratio (CIE), Gloss (ASTM D 4449), Color (CIE), Adhesion (ASTM D 3359), Pencil Hardness , Impact Resistance (ASTM D 2794), Flexibility (ASTM D 522) and Salt Spray (ASTM B 117) had been evaluated by Standard methods [7].

3. RESULT & DISCUSSION:
3.1 PARTICLE SIZE & SHAPE OF CAS:
The particle shape of the CAS was analyzed by the Scanning Electron Microscope (SEM) and the particle size was analyzed using Malvern particle size analyzer. The SEM Image and particle size distribution curve of CAS are shown in Figure 2. The shape of the functional fillers was found to be plate like needle structure. This unique particle shape is beneficial for giving “Spacing Effect” between two TiO2 particles to enhance scattering of light. Thus spacing effect of CAS restricts the formation of aggregates of TiO2 particles by placing itself between the two TiO2 particles [8]. The average particle size of Calcium Aluminum Silicate was found to be about 1.5 um and about 97% particles are about 10 um which does not affect the particle size of the final end product. Lower particle size also makes the smoother finish and easy dispensability. Also the surface smoothness of the applied films indicates that there is no any detrimental effect of replacing small particle TiO2 with somewhat higher particles than it.

3.2 OPTICAL PROPERTIES:
The effect of replacement of TiO2 with CAS on prime optical properties of coating like Contrast Ratio, Whiteness Index and Gloss were studied and the results for different compositions of White Glossy Finish, White Matt Finish and Grey Matt Finish are given in Table 5. The results are also shown graphically in Figure 4a, Figure 4b and Figure 4c for the White Glossy Finish, White Matt Finish and Grey Matt Finish respectively.

Contrast Ratio:
The optical property of vital importance is Opacity. This was studied by evaluating Contrast Ratio (K/S Values) [9]. In Calcium Aluminum Silicate based Powder Coatings it was noticed that contrast ratio is almost unaffected by replacement of TiO2 pigment with CAS up to the level of 7% - 20% for different types of powder coatings. This might be attributed to the spacing of the TiO2 pigment particles by Calcium Aluminum Silicate extender particle which increases the scattering volume and hence same hiding can be achieved by lower amount of TiO¬2 pigment. Thus we find that 7% to 8% replacement of TiO2 with Calcium Aluminum Silicate is possible in glossy finish and upto 20% in matt finish, without disturbing the Contrast Ratio of the Paint that can be achieved by the standard coating containing TiO2 as an opacifying pigment.
Gloss:
Gloss is not much affected in any finish of powder coatings by replacing TiO2 with Calcium Aluminum Silicate due to its lower Oil Absorption Value. In glossy finish maximum replacement of TiO2 is possible up to 7% of TiO2 without any detrimental effect on gloss. In the matt finish, where high gloss is not a major criterion, we can replace TiO2 as high as 15 - 20%.
Whiteness Index:
As revealed by Whiteness Indices measured for different powder coatings compositions, Calcium Aluminum Silicate does not affect to much extent to whiteness of the coating when used as replacement of TiO2 because the CAS powder it self has a whiteness index of 96% against Standard (Magnesium oxide).
Color in Grey finish:
In Grey powder coatings, as the replacement of TiO2 with CAS increases the shade will become slightly darker & brighter. So there is a chance of saving the color pigment.

3.3 MECHANICAL PROPERTIES & CORROSION RESITANCE:
The replacement of true pigment i.e. TiO¬2 with other fillers, should not affect the important mechanical properties & corrosion resistance of the coatings in achieving the optical properties. Therefore the effect of replacement of TiO2 with CAS on mechanical properties of coatings like Adhesion, Impact resistance, Flexibility, Pencil hardness and corrosion resistance were studied and the results for different compositions of White Glossy Finish, White Matt Finish and Grey Matt Finish are given in Table 5. All the mechanical properties of coatings are found to be not affected by replacement of TiO2 by CAS. The corrosion resistance of the coatings based on CAS was also found to be equivalent to that of standard coating.

4. CONCLUSION:
The results reveals that Calcium Aluminum Silicate can be successfully used in the Powder Coating systems upto a level of 7 - 8% replacement of TiO2 in white glossy finish and 15 – 20% in any matt finish without significant effect on the quality of final paint products. On the cost factor aspect it was found that there is approximate cost saving of 2.0 to 3.50 INR per kg in glossy finish by having 7% replacement and 4.0 to 6.0 INR per kg in matt finish by having 15% replacement of TiO2 by Calcium Aluminum Silicate.

ACKNOWLEDGEMENT:
The authors express their gratitude to the Executive Director, Sophisticated Instrumentation Center for Applied Research and Testing (SICART), Vallabh Vidyanagar; Director, Institute of Science and Technology for Advanced Studies and Research (ISTAR), Vallabh Vidyanagar and Executive Director, Aromax Corporation, Ahmedabad for providing necessary research, testing and library facilities.

REFERENCES
(1) Hocken Jorg, Sachtleben Chemie GmbH, Journal on Alternative White Pigments and TiO2 Extender for Coatings, Paper and Plastic by Intertech, USA, November 8 –10, 2000, pp. 1 - 10

(2) Dietzu Paul F. “Influence of fine particle size extenders and entrapped air on utilization of TiO2 in paints”, European Coating Show, 2003, Germany

(3) Dighe Ashok, Hirani Hitesh, “Calcium Aluminum Silicate & Magnesium Aluminum Silicate as a partial replacement of opacifying pigment with improved performance” Paint India, June 2005, PP 44 -49.

(4) Payne H.F., “Organic Coating Technology” Vol 2, John Wiley & Sons, Inc., 1961,PP, 773-804

(5) Oil and Colour Chemists Association, Australia, “Surface Coatings” Vol 2, Paint & their Application, Chapman and Hall Ltd., 1984, PP. 598 – 599

(6) Oil and Colour Chemists Association, Australia, “Surface Coatings” Vol 2, Paint & their Application, Chapman and Hall Ltd., 1984, PP. 594 – 595

(7) Sward G.G., “Paint Testing Manual (ASTM Special Technical Publication)” 13th Edition, ASTM.

(8) Dietz and Paul Frederick, Huntsman Tioxide, Germany, “Spacing for Better Effect – Influence of Fine Particle Size Extender and Entrapped Air on Utilisation of TiO2 in Emulsion Paine”, European Coating Journal, UK, 2003, pp.14-20

(9) Patton T.C., “ Paint Flow and Pigment Dispersion”, John Willy & Sons, 2nd Edition,1979 PP. 171 – 180

Physical Properties of Calcium Aluminum Silicate :