ST-JDC- ST-JDC-o de las

White pigments do not absorb visible light; rather, very fine parti- cles of these pigments scatter the entire visible spectrum of light.

Therefore, the observer receives the whole spectrum of visible light and perceives a white color. Maximum light scattering is achieved when the difference between the refractive index of the pigment and the medium is maximum. White pigments have maximum mar- ket share of all the pigments used in the coating industry because they are used not only in white coatings but also in coatings with pastel colors. They are also used in combination with some trans- parent colored pigments when the white pigment contributes to opacity of the coating. White pigments used over the years in the coating industry include titanium dioxide, white lead, zinc oxide, zinc sulfide, lithopone and antimony oxide, of which only titanium dioxide and to some extent zinc oxide are used in modern coatings.

3.3.1.1 Titanium dioxide

Pigment White 6; formula: TiO2

Titanium dioxide is the most important white pigment in today’s coating industry considering its usage in almost all types of pro- ducts. Its high refractive index (and hence opacity), the highest of the white pigments, and its excellent performance properties are the main reasons for its widespread use. More than half of the total titanium dioxide produced is consumed by the coating industry. Titanium dioxide exists in three crystal forms, namely rutile, ana- tase and brookite, of which brookite has no commercial importance

in the industry, while rutile and anatase are commercially produ- ced. The rutile form is thermodynamically more stable than ana- tase; the latter is transformed to rutile above 700 °C. In each type of titanium oxide, the titanium atom in the lattice is surrounded octa- hedrally by six oxygen atoms, and each oxygen atom is surrounded by three titanium atoms, but they differ in the way the octahedra are linked through their corners and edges. The different crystal arran- gements of anatase and rutile are shown in Figure 3.5. The more compact arrangement of the atoms in the rutile form is responsible for its higher refractive index (2.75) and correspondingly higher hiding power compared to the less compact anatase form with its lower refractive index (2.55). Therefore, rutile-type titanium oxide has the predominant market share over anatase. Another apparent difference is that the rutile titanium oxide is slightly yellower than

anatase because it absorbs some visible light in the violet region of the spectrum. The density of rutile titanium oxide is 4.2 g/cm3_, while that of anatase is 3.9 g/cm3_.

Titanium dioxide is extremely stable at high temperatures (melting point 1800 °C). Titanium oxide pigments are chemically very stable. They are insoluble in all liquids with the exception of concentrated sulfuric acid and hydrofluoric acid (and only at 1000 °C). They are also resistant to hydrogen sulfide and other gases generally found in industrial atmospheres.

Titanium dioxide pigments are of commercial importance due to their excellent hiding power resulting from their high scattering power. As discussed in Section 3.2.1.2, hiding power is also dependent on particle size. The optimum particle size of titanium dioxide for high- est hiding power and reducing strength is 20 to 300 nm. The typical primary particle shape of titanium dioxide pigments is nodular. Despite having the highest hiding power and reducing power, one of the primary concerns with titanium dioxide pigments is their photochemical reactivity upon exterior exposure. Titanium dioxide

is a strong absorber of UV radiation. When exposed to exterior conditions, UV irradiation of titanium dioxide excites electrons and leaves positively charged electron holes, which can move to the particle surface, where they react with surface hydroxyl groups and adsorbed oxygen to produce hydroxyl and hydroperoxyl radicals. These radicals cause degradation of binder at the pigment-binder interface that results in chalking. The anatase type has more ten- dency for chalking than rutile. To minimize the problem of chal- king, the crystals are doped with elements such as zinc, aluminum or calcium. Also, surface treatment of titanium dioxide pigment particles by deposition of one or more oxides of silicon, aluminum, zinc or zirconium improves chalk resistance. Surface treatment also affects other properties such as dispersibility and gloss.

Titanium dioxide is manufactured by two processes, the sulfate process and the chloride process. In the sulfate process, ilmenite ore (FeTiO₃) is dissolved in concentrated sulfuric acid and insoluble impurities are removed by clarification, flocculation, sedimentation and filtration. The resulting solution is further purified by crystal- lization to remove ferrous sulfate from titanyl sulfate solution. The titanyl sulfate solution is then hydrolyzed to give hydrated tita- nium dioxide, which is calcined at about 900 to 1100 °C to give the titanium dioxide pigment. Both anatase and rutile-type titanium dioxide pigments are produced using this method.

In the chloride process, rutile ore is used as starting material. A finely ground mixture of ore and coke is reacted with chorine at 900 °C to yield titanium tetrachloride, along with chlorides of other metal impurities. Titanium tetrachloride is a liquid and is purified from other impurities by fractional distillation. Pure titanium tetra- chloride vapor is then reacted with oxygen at 900 to 1400 °C to yield titanium dioxide and chorine, which is recycled. This method is used only for rutile grade titanium dioxide pigment. In both pro- cesses, the titanium dioxide so produced is subjected to subsequent product finishing by processes such as surface treatment, energy milling and size classification.

Titanium dioxide possesses many of the ideal properties required by a pigment for a coating, including excellent hiding power, very

good brightness, a low oil absorption value, soft texture, high che- mical resistance and excellent thermal stability. With this set of properties and the improved durability of modern surface-treated grades, titanium dioxide pigments have replaced most of the tradi- tional white pigments used for coatings. Titanium dioxide pigments are widely used in both solvent-based and water-based exterior and interior paints for decorative as well as industrial segments. They are also used in printing inks.

3.3.1.2 Zinc oxide

Pigment White 4; formula: ZnO

This pigment is a fine brilliant white powder. Historically, it was used as a white pigment, but due to its low refractive index (2.01), it became uneconomical as a white opacifying pigment in comparison to titanium dioxide. It still finds some minor applications in coatings due to certain characteristics. Zinc oxide, being opaque to UV light, retards UV degradation of binders when used in exterior coatings. It is also a good fungistat, and therefore, when used in exterior house paint, it protects the coating from mildew. When used with oleoresinous binder, it has a tendency to produce zinc soaps, which improve hardness, abrasion resistance and moisture resistance. However, its reactivity with acidic media make it unsuitable for use in highly acidic binders. It is also used in anticorrosive coatings in combination with other active inhibitive pigments.

3.3.1.3 Zinc sulfide and lithopone

Pigment White 7; formula: ZnS Pigment White 5; formula: ZnS/BaSO4

Zinc sulfide (ZnS) is another historic pigment that is no longer used widely due to its low refractive index (2.37) compared to titanium dioxide. It has good whiteness with very good chemical resistance. One of the major limitations of this pigment is chalking when used in exterior applications. Lithopone is another such pigment with low refractive index (1.84). It is a coprecipitate of zinc sulfide with barium sulfate (BaSO4). Lithopone as similar properties to zinc sul- fide but less hiding power due to the barium sulfate.

3.3.1.4 Antimony oxide

Pigment White 11; formula: Sb₂O₃

This is another low refractive index (2.09) white pigment. It has good chalk resistance characteristics but its main functional application is in fire-retardant paints.

3.3.1.5 White lead pigments

Pigment White 1; formula: 2PbCO₃·Pb(OH)₂

Despite its low refractive index (1.94), basic lead carbonate pigment was used in paint compositions for many years, but it is no longer used in the modern coating industry due to the toxicity of lead and the availability of more efficient titanium dioxide pigment with almost ten times more hiding power.