Complex Inorganic Colored Pigments

Introduction

An important class of high-performance pigments are known as complex inorganic color pigments. Chemically, these pigments are synthetic crystalline metal oxides with structures identical to naturally occurring minerals. They are called complex pigments because they contain two or more different metal pigments. Their complex nature provides a range of metal combinations and, in practice, produces a wide variety of colors for this pigment class. Complex inorganic color pigments are used in situations requiring exceptional color stability. Because they are resistant to chemicals and solvents and insoluble, they do not cause migration during application. Their heat stability is hundreds of times higher than that of organic pigments, and their colors show no change or degradation in the presence of ultraviolet (UV) light. The ability of these pigments to absorb UV light without degradation makes them good UV absorbers and opacifying pigments in their own right. They are among the most stable and durable colorants on the market and are truly high-performance products. The elements found in complex inorganic color pigments are listed in the table below. Of these metal ions, only seven are color-producing metal ions among all these transition metals. The other metal ions in the pigments are colorless and are provided as modifiers to balance the charge of the crystal lattice or to slightly alter the color tone imparted by the coloring ions. Transition metal compounds are generally highly colored. Therefore, it is not surprising that complex inorganic color pigments are also colored. Colors in complex inorganic color pigments are produced by electronic d-d transitions on the coloring metal ions in the crystal lattice or by metal-conduction band transitions in the solids. These metal-based electronic transitions are spectroscopically unfavorable and thus cause relatively weak light absorption. This means that complex inorganic color pigments have weaker coloring power compared to other pigments. However, metal oxides are very stable structures and their colors are very durable.

Structure of Complex Inorganic Color Pigments

Complex inorganic color pigments are metal oxides that can exist in many possible structures. The most important determining factor in their structure is the oxygen/metal (O/M) ratio (Table 1). As long as the metal ions are of comparable size, the O/M ratio largely determines what the structure will be. Two crystal structures predominate in complex pigments within the class of complex inorganic color pigments. These are rutile and spinel. Hematite and corundum structures are also observed, though less frequently. Rutile is the name of one of the crystal phases of TiO2, the mineral titanium dioxide. This crystal phase is the densest phase of certain naturally occurring titanium dioxide with a stoichiometry of 2 oxygen to 1 metal or O/M = 2. Formulations containing metal ions of similar size to Ti (IV) with an O/M ratio of 2 are mostly accepted as having rutile structure. Complex inorganic color pigments in this structure contain a large portion of TiO2 as the base oxide. Not surprisingly, rutile complex inorganic color pigments share many physical properties with titanium dioxide. Metallic ions in rutile are arranged octahedrally with six oxygen ions, as shown in Figure 1. The spinel structure, named after the mineral MgAl2O4, is very common in most first-row transition metal oxides. It has an O/M ratio of 1.33, and many transition metal oxides with this M/O ratio adopt this structure. In fact, the structure is so stable that it can maintain the spinel configuration even with stoichiometry significantly different from M5O4. In such cases, the solids usually contain metal ion vacancies. A spinel structure contains metal ions in different oxidation states. Generally, the structure contains divalent (+2) and trivalent (+3) metal ions, but it can also accommodate other charged ions. There are two separate coordination environments for metal ions in spinel. The first region is coordinated octahedrally by oxygen ions while the second region is coordinated tetrahedrally. An example of a spinel lattice showing these regions is given in Figure 2. In normal spinels, only divalent metal ions are found in tetrahedral regions while only trivalent metal ions are found in octahedral ones. Inverse spinels form where some divalent metal ions are 6-coordinated and some trivalent ions are found in the 4-coordinated region. Another interesting structure is those with basic formula M2O3, O/M = 1.5. Two of these structures are found in complex inorganic color pigments. The first is named after the Al2O3 α-alumina phase: corundum; the other is named after the Fe2O3 mineral: hematite. There is a small difference in spatial geometry between the two structures, but both are quite similar. Metal ions in both are trivalent and arranged octahedrally.

Manufacture of Complex Inorganic Color Pigments

Complex inorganic color pigments are obtained by calcination of mixtures of metal oxides and/or oxide precursors such as metal salts, hydrates, and carbonates. Calcination temperatures typically range from 650 to 1300°C. At relatively low temperatures, all raw materials decompose to form metal oxides. At higher temperatures, this oxide mixture becomes reactive. Metal and oxide ions pass into a fluid state to mix with each other, forming a homogeneous solid. The ions in the solid subsequently rearrange into a stable crystal structure determined by the calcination temperature, O/M ratio, and metals present. This new structure is the color pigment. Following calcination, complex inorganic color pigments are generally washed, ground to a specific particle size, and finally mixed for uniform appearance. Commercially, complex inorganic color pigments are produced in three main groups. These are: 1. Titanium dioxide-based titanates 2. Aluminum oxide-based aluminates 3. Chromium and/or iron oxide-based chromites and ferrites.

Titanate Pigments

Titanate-based pigments contain titanium dioxide as the main component. Different types are listed in Table 2. Pigments using TiO2 as a modifier are not included in this list. The important crystal structures here are rutile and spinel. There are also a small number of other structures of commercial importance. Titanates currently make up the largest quantity of complex inorganic color pigments used, and the most commonly used types are rutile.

Rutile Titanates

Rutile titanates represent the largest commercial class of complex inorganic color pigments to date. Chromium antimony titanate yellows (C.I. PBr: 24) are the most widely used, followed by nickel antimony titanate yellows (C.I. PY: 53). Manganese antimony titanate browns (C.I. PY: 164) hold a much smaller market share, and other rutile types form an even smaller segment. Rutile complex inorganic color pigments contain significant amounts of titania as the main oxide. Typically, pigments contain TiO2 in the range of 70 to 90 percent by weight. Colorless ions Ti (IV), Sb (V), Nb (V), and W (VI) are present to maintain MO2 stoichiometry, whereas transition metal cations Ni (II), Cr (III), and Mn produce color. Colors range from light yellow to dark brown. The reflection curves for three antimony titanates are compared with rutile TiO2 in Figure 3. It is well known that pure rutile titanium dioxide has a photoactive surface. When exposed to UV radiation, highly reactive oxo radicals can form on the pigment surface that can photocatalytically decompose organic materials in contact with it. Commercial types of rutile TiO2 are passivated by coating with other metal oxides such as aluminum, silicon, or zirconium oxide to suppress this effect. Rutile complex inorganic color pigments do not exhibit photocatalytic activity as pure titanium dioxide does. The doping of the rutile structure with transition metals and other ions eliminates the mechanism of surface radical formation through UV irradiation. Commercial types of rutile complex inorganic color pigments do not require surface coatings to render themselves inert.

Spinel Titanates

Titanate spinels form a much smaller pigment class than rutiles. The spinel stoichiometry M3O4 is satisfied by reacting one unit of TiO2 with two units of +2 metal oxide, according to the equation below. Titanates are inverse spinels, with a number of +2 ions occupying octahedral arrangement sites in the crystal lattice. Commercially important types are cobalt titanate greens and iron titanate browns. 2MO + TiO2 —> M2TiO4 spinel M = Ni(II), Co(II), Zn(II), Fe(II) Cobalt titanate greens are generally modified with Zn (II) and Ni (II) oxides to give bright green colors. The greens can be the same color as chromium (III) oxide green and compete somewhat as they have similar durability properties. However, cobalt titanate greens are usually formulated to give bright and clean color tones, while some alternatively contain unusable bluish darker tones. Primarily as a result of their cobalt content, cobalt titanate greens are more expensive compared to other pigments. Iron titanates consist of a combination of iron (II) oxide and TiO2. These formulations are commonly modified by the addition of iron (III) oxide, zinc (II) oxide, and aluminum (III) oxide. Like cobalt titanates, these also have inverse spinel structures. Iron titanate pigments range in tone from light yellow-brown to dark reddish-brown. In most cases, these pigments exhibit higher heat stability compared to zinc ferrite or iron oxide browns and are generally used for this purpose.

Other Titanates

Another titanate pigment type of commercial importance is barium nickel titanium yellow priderite, other than those listed above. As with spinel and rutile above, the name priderite denotes the crystal structure of this compound. These pigments generally give a lighter and slightly greenish-toned yellow compared to the more common nickel titanate yellow. With this special case, other properties are similar to those of complex inorganic color pigments. The basic formula of yellow priderite is 2NiO x 3BaO x 17TiO2.

Aluminate Pigments

Complex inorganic color pigments containing aluminum oxide, Al2O3, or colorless base oxide alumina are called aluminates. They almost always use cobalt (II) oxide, CoO, as one of the coloring oxides. All of these pigments have spinel crystal structure and color tones in the range of blue to duck green.

Cobalt Aluminates

Combinations of cobalt (II) oxide and aluminum oxide in spinel stoichiometry yield cobalt aluminate blue spinel, CoAl2O4, C.I. PB: 28 pigment. Among the types of this pigment are modifications with zinc (C.I. PB: 72), magnesium, titanium, and lithium oxides. If lithium and titanium modifiers are provided, a turquoise blue pigment is obtained. Cobalt aluminate blues are among the most durable blue pigments on the market. They have excellent chemical and heat stability and can be used in chemically aggressive environments and in outdoor applications without color fading. A disadvantage of these blue pigments is that they are weaker UV absorbers compared to other complex inorganic color pigments.

Cobalt Chromium Aluminates

If part of the aluminum oxide in a cobalt aluminate is replaced with chromium (III) oxide green, a cobalt chromium aluminate blue-green spinel (CI PB:36) compound with simple formula Co(Alx,Cr1-x)2O4 is formed. Here x is a value between 0 and 1. These pigments can subsequently be modified with zinc, magnesium, and titanium oxides to give greenish blues and duck green tones. Like cobalt aluminates, cobalt chromium aluminates have excellent heat stability and outdoor durability. In most cases, they have much better UV resistance than PB; 28 types.

Chromites and Ferrites

Many complex inorganic color pigments contain transition metal oxides without significant amounts of a colorless base oxide. These pigments listed in Table 2 use green chromium (III) oxide, red iron (III) oxide, or a combination of both as a base. For this reason, they are called chromites or ferrites for chromium and iron bases, respectively. Most adopt spinel configuration and other important structures are corundum and hematite. Chromite and ferrite pigments have physical properties similar to titanates and aluminates. All such pigments consist of only a few greens and green-blues, along with brown and black tones. Some, in addition to durability and color stability, have unique infrared spectral properties that make these pigments of interest. M. Namık Kayaalp / Chemical Engineer / Ecelak Boya Kimya Ltd. Şti. References 1.High Performance- Pigments. Edited by Hugh M. Smith Copyright © 2002 Wiley 2.Pigment Handbook, Vol. 1, 2nd edn., Peter A. Lewis, 1988, , John Wiley. 3.Surface Coatings Science and Technology , 1985,Swaraj Paul, John Wiley