Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Pigment, Inorganic

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_180

Synonyms

Definition

Inorganic pigments, which have been widely used since prehistoric times, include naturally occurring substances prepared from minerals or their combustion products as well as synthetic compounds produced from appropriate raw materials and also hybrid pigment types derived from organic dyes and selected mineral supports. Inorganic pigments are insoluble in the surrounding medium and their optical effect arises from selective light absorption. They are classified according to their color, origin, method of production, and the character of the pigment material. Inorganic pigment technology is directly related to the development of new synthesis methods and the preparation of environmentally friendly colorants.

Introduction

This entry presents the main features of the classification and production of the most common inorganic pigments. The most important physicochemical and application properties of a wide range of pigments, including white pigments, are presented. Also discussed are issues relating to specialized pigments, including hybrid pigments.

The majority of inorganic pigments are fully suitable for use in a variety of consumer products such as paints and lacquers, plastics, inks, construction materials, paper, glass, and ceramics. The global consumption of pigments for 2012 shown in Fig. 1 clearly confirms the greater importance of inorganic pigments compared to organic pigments [1].
Pigment, Inorganic, Fig. 1

Structure of pigment consumption

Inorganic pigments are more inert and insoluble and are more resistant to both temperature and extreme pH than their organic counterparts. Characteristic features of inorganic pigments include excellent opacity, good color saturation, and stability. Inorganic pigments enable the production of durable and high-quality coatings.

General Information

Pigments are colored materials which, in a purified form, are also used for the dyeing of synthetic fabrics, rubber, and paper and which impart color as a result of wavelength-selective light reflection, absorption, or interference. They improve the aesthetic value of products and also possess practical functions such as protecting metal objects against corrosion by means of painting. Pigments are insoluble in oil, water, organic solvents, and resins. They are classified according to their color, origin (inorganic or organic), method of production (natural or synthetic), and the character of the pigment material (e.g., light and weather resistance, high opacity, color strength, ease of dispersion, etc.). Inorganic pigments usually display better resistance to light and atmospheric conditions, higher dispersion, and opacity [2, 3].

There are pigments which, apart from coloration, also display luminescence (luminescent pigments) or change color as a function of temperature (thermosensitive pigments); these are used for the production of luminescent or thermosensitive paints.

Pigments can include different components; according to the character of the material from which they are made, they are divided into:
  • Homogeneous pigments – comprise pigment particles of the same type, e.g., oxides of metals

  • Mixed or hybrid pigments – produced by mechanical mixing (vigorous mixing of the coloring substance with an extender) or by chemical synthesis (organic dyes or pigments grafted onto a mineral support)

Classification of Inorganic Pigments

Pigments can be classified on the basis of their chemical structure, optical properties, or technological properties.

A detailed classification of inorganic pigments according to their chemical structure and color subgroups is shown in Fig. 2.
Pigment, Inorganic, Fig. 2

Inorganic pigment classification

Properties

Pigments are mainly used for coloration and for endowing specific properties to paint coatings. They are important components of lacquer and paint coatings, influencing their final mechanical properties and resistance; they affect the paints’ rheology, stability, and some properties related to application. The physicochemical properties of pigments chiefly influence the application and technological conditions of their commercial use [3, 4, 5].

From an application point of view, the following functional properties of pigments are important for specific target demands:
  • Optical properties – color, tinctorial strength, opacity

  • Physical/ solid state properties – crystalline structure, grain morphology, character of surface, light refraction index, and defined structural properties

  • Resistance and protective properties

All of these properties are to some degree determined by chemical structure [3].

The pigment color, shade, opacity, resistance to chemicals, as well as viscosity depend mainly on the chemical and crystalline structure, light refraction index, size and shape of the pigment particles, as well as the character of the particle surface. Crystalline structure determines such properties as dispersion, rheology, solubility, color strength, and shade.

Exemplary morphological structures of selected inorganic pigments and extenders are shown in Fig. 3.
Pigment, Inorganic, Fig. 3

(a) Particle morphological structure. (b) TEM images of commonly known inorganic white pigments and extenders (Created based on Ref. [3] with permission from VincentzVerlag Publisher)

The fundamental properties of inorganic pigments include the following:
  • Color and intensity of coloration

    The color of a pigment depends on chemical composition and degree of purity, as these two aspects define the pigment’s ability to absorb certain wavelengths and to reflect the complementary color. The tinctorial strength or color strength of a pigment is defined as the pigment’s ability to change the original color of a given material. This is related to the pigment’s introduction to a given material and depends on the pigment’s degree of refinement, purity, and homogeneity of chemical composition.

  • Opacity

    Pigment opacity (hiding power) depends on both the size and structure of the pigment particles as well as the difference in the light refraction index of the pigment and the binding agent. The covering ability of a pigment is greater when the pigment comprises small particles and the difference in light refraction index is then significant.

  • Shape and size of pigment particles

    The size of pigment particles is controlled so as to obtain optimum visible light scattering. The size of pigment particles impacts on the stability, coating resistance, and covering power. Pigment particle size can vary from 1 to 10 μm, with particles of the smallest diameters ensuring higher density and protective properties.

  • Resistance to UV radiation

    Pigments should display the highest possible resistance to light. A change in color of the pigment, usually brightening or darkening as a result of exposure to visible light or other radiation, is mainly caused by changes in the dispersion of the pigment in the binding agent and changes in that agent, rather than to changes within the pigment.

  • Thermal resistance

    Thermal resistance is of particular importance in the cases of paint coatings for use on hot substrates or when dyed plastic products are subjected to hot processing. The pigments used for such purposes should be resistant to high temperatures.

  • Resistance to chemical agents

    Of particular importance is a pigment’s resistance to both acids and alkalies. A pigment’s resistance to the binding agent, substrate, and exploitation conditions should also be analyzed. When pigments are mixed, their interaction must also be established.

  • Optical properties

    The optical properties of pigments are reflected in their color and light absorption coefficient. The color of a pigment is determined by the adsorbed or reflected wavelengths of light.

  • Wettability by binding agent

    The extent of the wettability of a pigment by a binding agent depends on the degree of the pigment particles’ refinement, pigment structure, and the wetting properties of the binding agent. For oil products, it is described by the so-called oil absorption number.

  • Toxicity

    Although mineral pigments often contain heavy metals, their presence is not a threat to human health or the natural environment. In the production of such pigments, the heavy metals used are so strongly bonded in the pigment structure that they are not released into the environment. In many countries, the content of heavy metals in pigments for use in special products, such as those that are in contact with food, toys for children, and cosmetics, is controlled by regulations that specify admissible heavy metal levels.

Most of the aforementioned physicochemical properties of pigments determine their potential uses. Among the most common types displaying these physicochemical properties are white and black pigments and more rarely brown, yellow, red, blue, and green pigments.

White pigments – the optical effect of these is achieved by nonselective light scattering; technologically the most important examples are titanium dioxide, zinc white, zinc sulfide, and lithopone.

Titanium dioxide (C.I. Pigment White 6; TiO2) occurs in three crystalline varieties, namely, brookite (not used as a pigment), anatase, and rutile. Irrespective of the technology of titanium dioxide production, the pigments are characterized by high purity and defined crystalline and particle size. To improve the functional properties of the pigment in a coating, the titanium white surface is modified with different chemical compounds. The attractive features of titanium white pigment include high light refraction index, chemical and physical stability, possibility of regulation of particle size distribution, and possibility of surface modification. The crystallites and particle size are the main parameters determining the paint properties. The optimum size of TiO2 crystallites to ensure maximum light scattering, and hence high opacity of paint coatings, is ~0.23 μm. As the polymers used in paint formulations usually undergo degradation under the influence of ultraviolet radiation, the addition of substances to absorb UV radiation increases a paint coat’s lifetime. Moreover, titanium dioxide has the highest refractive index (2.55 for anatase and 2.70 for rutile) of all inorganic pigments and commonly known extenders. Because of the universal properties of this pigment, it is produced by industrial methods on a wide scale [6, 7].

Production of TiO2

Titanium white pigments are produced on an industrial scale using two methods, namely, sulfate and chloride:
  • Sulfate process – TiO2 is precipitated from a solution of ilmenite ore using concentrated sulfuric acid; both rutile and anatase forms can be obtained.

  • Chloride process – TiO2 is obtained as a result of the oxidation of titanium tetrachloride (TiCl4) synthesized by reduction and chlorination of ilmenite ore or rutile sources; only the rutile form is obtained.

The chloride process is more often used; indeed, it is estimated that over half of the titanium white produced worldwide is obtained using this method. Although this process requires higher-quality and preliminarily enriched ore and employs more complex technology, it produces less waste (thus being less burdensome on the environment) and is cheaper than the sulfate method.

Sulfate Method

The sulfate method of titanium white production is complex and divided into two main technological parts, referred to as the “black” part (alluding to the color of the ore) and the “white” part (alluding to the color of white pigment).

The process aims to extract TiO2 from raw titanium ore at elevated temperature by means of concentrated sulfuric acid (85–95 %). The most often used titanium ores are ilmenite (FeTiO3) containing 45–65 % TiO2, and titanium slag. The prepared ore is mixed with concentrated sulfuric acid, and compressed air is blown through the reactor, as this accelerates etching. The reaction starts above 140 °C and is exothermic, which causes a further increase in temperature to 200–220 °C. The reaction mixture is left for 1–12 h to mature and the ensuing mixture is cooled to obtain a porous sinter, which is dissolved either in water or diluted sulfuric acid at low temperature (<85 °C) to prevent premature hydrolysis. The solution obtained (so-called titanium liquor) is reduced using steel scrap to convert Fe3+ into Fe2+ ions and partly also Ti4+ into Ti3+ ions. The solution is subjected to sedimentation, and the unreacted substrate is filtered out from the ensuing suspension. The content of TiO2 in the product differs depending on the type of titanium ore: when ilmenite is used, it is 8–12 %, and when titanium slag is used, it is 13–18 %. When the temperature is decreased to 10 °C, the main by-product of the process [hydrated iron sulfate (FeSO47H2O)] undergoes crystallization. When titanium slag with a TiO2 content >75 % is used, the iron sulfate crystallization stage is not necessary. The purified solution is concentrated to 200–250 g TiO2dm−3. Hydrolysis of titanium sulfate is then performed (at 94–110 °C) by diluting the solution and maintaining the suspension at its boiling point. This process influences the size of the product particles and the degree of flocculation. The hydrated titanium dioxide formed is filtered off, washed with water, and subjected to whitening by hydrogen. The hydrated titanium dioxide obtained contains 20–28 % sulfuric acid and sulfates which are removed by filtration and washing with water, followed by reduction in the process of whitening, in which most of impurities are removed; at the end of this process, the content of colored impurities is in the order of ppm, while the content of sulfuric acid is still significant, at around 5–10 %.

In the “white” part of the process, the hydrated titanium dioxide is again filtered, washed, and prepared for calcination. When the aim is to produce the rutile form, nuclei of rutile particles and substances favoring transformation of anatase to rutile are introduced into the mixture, and calcination is performed at ~950 °C. When the aim is to obtain the anatase form of the pigment, no admixtures are introduced and calcination is performed at a lower temperature. The hydrate formed is calcined to remove water and to obtain TiO2. When rotary filters are employed, the content of titanium dioxide in the product is 30–40 %, but when pressure rotary filters or automated filtration press is used, the content reaches 50 %.

A schematic presentation of the sulfate method of producing titanium white pigment is given in Fig. 4, and the reactions at each step are listed in Table 1.
Pigment, Inorganic, Fig. 4

Production of TiO2 by the sulfate and chloride methods

Pigment, Inorganic, Table 1

Characteristics of commonly known inorganic pigments

Pigment type

Production

Advantages

Disadvantages

Color

Carbon black

Decomposition of carbonaceous precursors

Rich color, color strength, light and weather resistant

Thickens paint, color limits application

Black

\( \mathrm{C}{\mathrm{H}}_4\overset{\mathrm{temp}.}{\to}\mathrm{C}+2{\mathrm{H}}_2 \)

Titanium dioxide

Synthesized mainly by sulfate or chloride technology (alternatively using sol–gel, hydrothermal, or solvothermal methods)

High opacity and color strength, relatively cheap, very good UV resistance, very high refractive index, good resistance to alkaline solutions, mostly chemically inert pigment

Tendency for agglomeration of primary particles, forms radicals that degrade the binder/polymer matrix

White

\( \begin{array}{c}\hfill {\mathrm{FeTiO}}_3+2{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{TiO}\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{FeSO}}_4\hfill \\ {}\hfill {\mathrm{TiO}\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{T}\mathrm{i}\mathrm{O}{\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{H}}_2{\mathrm{SO}}_4\hfill \\ {}\hfill \mathrm{T}\mathrm{i}\mathrm{O}{\left(\mathrm{O}\mathrm{H}\right)}_2\overset{\mathrm{temp}.}{\to }{\mathrm{TiO}}_2+{\mathrm{H}}_2\mathrm{O}\hfill \end{array} \)

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Iron oxides

Synthesized, obtained as minerals and also prepared from chemical wastes

Light and weather resistant, mostly unreactive

Cannot produce clean shades

Yellow, red, brown, black

\( \begin{array}{c}\hfill {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{O}}_2+2\mathrm{F}\mathrm{e}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}}_6{\mathrm{H}}_5\mathrm{N}{\mathrm{H}}_2+\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\hfill \\ {}\hfill 3\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\to 2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}\hfill \\ {}\hfill 3\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{C}\mathrm{O}\to 2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+\mathrm{C}{\mathrm{O}}_2\hfill \\ {}\hfill 2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+1/{20}_2\overset{\mathrm{temp}.}{\to }3\left(\upgamma -\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\right)\hfill \\ {}\hfill 2\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+1/{20}_2\overset{\mathrm{temp}.}{\to }3\left(\upalpha -\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3\right)\hfill \end{array} \)

Zinc oxide

Chemical or metallurgical synthesis

Excellent heat stability, light and weather resistant, very good UV resistance, anticorrosive properties

Expensive chemical synthesis, tendency to form agglomerations

White

\( \begin{array}{cc}\hfill \begin{array}{l}\left.\begin{array}{c}\hfill \mathrm{Z}\mathrm{n}\mathrm{S}{\mathrm{O}}_4+2\mathrm{NaOH}\to \mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_2+\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4\hfill \\ {}\hfill \mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{l}}_2+2\mathrm{NaOH}\to \mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_2+2\mathrm{NaCl}\hfill \\ {}\hfill \mathrm{Z}\mathrm{n}{\left(\mathrm{C}{\mathrm{H}}_4\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2+2\mathrm{NaOH}\to \mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_2+2\mathrm{C}{\mathrm{H}}_2\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{Na}\hfill \end{array}\right\}\\ {}\dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \dots \end{array}\hfill & \hfill \mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_2\overset{\mathrm{temp}.}{\to}\mathrm{Z}\mathrm{n}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hfill \\ {}\hfill \mathrm{Z}\mathrm{n}\mathrm{C}{\mathrm{o}}_3\overset{\mathrm{temp}.}{\to}\mathrm{Z}\mathrm{n}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2\hfill & \hfill \hfill \end{array} \)

Zinc chromates

Synthesized

Anticorrosive properties

Thickens paints, dangerous synthesis with regard to reagent

Yellow

\( 4\mathrm{Z}\mathrm{n}\mathrm{O}+2{\mathrm{Cr}\mathrm{O}}_3+{\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7+{\mathrm{H}}_2\mathrm{O}\overset{\mathrm{temp}.}{\to }{\mathrm{K}}_2{\mathrm{Cr}\mathrm{O}}_4+3{\mathrm{ZnCrO}}_4\;\mathrm{Z}\mathrm{n}{\left(\mathrm{O}\mathrm{H}\right)}_2 \)

Ultramarine

Synthesized, seldom obtained as mineral

Rich color

Low stability in acidic environment

Blue

\( 6\left[{\mathrm{Al}}_2{\mathrm{Si}}_2{\mathrm{O}}_7\right]+7{\mathrm{Na}}_2{\mathrm{CO}}_3+\mathrm{n}\mathrm{S}\overset{\mathrm{temp}.}{\to }2\left[{\mathrm{Na}6\mathrm{A}\mathrm{l}6\mathrm{S}\mathrm{i}6\mathrm{O}24\mathrm{N}\mathrm{a}\mathrm{S}}_6\right]+7{\mathrm{CO}}_2 \)

Chromium oxides

Synthesized as native pigments or produced from wastes

Light, weather, acid and alkali resistant, thermal stability

Does not give clear colors

Green

\( \begin{array}{c}\hfill 4{\mathrm{Cr}\mathrm{O}}_3+3{\mathrm{N}}_2{\mathrm{H}}_4\to 4\mathrm{C}\mathrm{r}{\left(\mathrm{O}\mathrm{H}\right)}_3+3{\mathrm{N}}_2\hfill \\ {}\hfill 2\mathrm{C}\mathrm{r}{\left(\mathrm{O}\mathrm{H}\right)}_3\overset{\mathrm{temp}.}{\to}\;{\mathrm{Cr}}_2{\mathrm{O}}_3+3{\mathrm{H}}_20\hfill \\ {}\hfill {\left({\mathrm{N}\mathrm{H}}_4\right)}_2{\mathrm{Cr}}_2{\mathrm{O}}_7\overset{\mathrm{temp}.}{\to}\;{\mathrm{Cr}}_2{\mathrm{O}}_3+{\mathrm{N}}_2+4{\mathrm{H}}_2\mathrm{O}\hfill \\ {}\hfill {\mathrm{N}\mathrm{a}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7+2\mathrm{S}\overset{\mathrm{temp}.}{\to}\;{\mathrm{Cr}}_2{\mathrm{O}}_3+{\mathrm{N}\mathrm{a}}_2{\mathrm{SO}}_4\hfill \end{array} \)

Cadmium sulfides

Synthesized

High opacity and color strength

Poor weather resistance, very expensive

Greenish yellow, red, deep red

CdCl2 + (NH2)2CS + 2NH4OH → CdS + (NH2)2CO + 2NH4Cl + H2O

Chloride Process

In the chloride process, the substrates are either rutile or enriched titanium ore containing >90 % TiO2. This method is more modern, more technologically advanced, and less burdensome on the natural environment than the sulfate process; however, it enables the production only of rutile. The problems related to this method are the high temperature of the process; the aggressive medium; the used toxicity of chlorine, carbon monochloride, and titanium tetrachloride; as well as the risk of uncontrolled emission of chlorine gas. The advantages of this process are that the pigments obtained have higher resistance to light and are brighter than those obtained by the sulfate method.

The substrate, which is maintained in the fluid phase by a flux of air, is heated to 650 °C and then refined coke (250–300 kg per tonne of TiO2) is introduced. The substrate should be dry and should not contain small particles that could be carried by the stream of forming TiCl4 and contaminate the product. The burning coke increases the temperature to about 1,000 °C at which point chlorine gas and a continuous supply of titanium ore (substrate) and coke are provided.

The ensuing reaction leads to the formation of titanium tetrachloride gas, which is the main product as well as by-products that include chlorides of contaminating metals (e.g., iron, manganese, and chromium), which together with the unreacted substrate are separated from the reaction mixture by fractional condensation. The titanium tetrachloride formed and the accompanying chlorides are distilled off from the fluid bed, and these gases are then cooled with liquid TiCl4. As some metal chlorides (mainly iron chloride) can deposit on the surface of the TiCl4 upon crystallization, the chlorides are condensed off, thereby removing the impurities of the TiCl4. Cooling is carried out in stages; firstly, the gases are cooled to below 300 °C and then the chlorides co-occurring with titanium tetrachloride are separated by condensation or sublimation. After this stage, the gas mostly comprises TiCl4 and is cooled to <0 °C, at which point, condensation of TiCl4 occurs. The raw titanium tetrachloride is carefully purified, at first, by a chemical method to separate the substances that could not be removed upon distillation, such as VCl4 and VOCl3, and then by vacuum distillation, which gives highly pure TiCl4. The pure titanium tetrachloride is evaporated and combusted in a stream of oxygen or air to obtain TiO2 and chlorine; the reaction involved is weakly exothermic and requires a high temperature in the range 900–1,400 °C. The required pigment is obtained together with a mixture of gases (Cl2, O2, CO2) which are cooled and then separated using either an indirect or direct method. The stream of gases is recirculated to the cooling zone of the furnace and to the chlorination stage.

A schematic presentation of the chloride process of titanium white production is given in Fig. 4, and the reactions within each stage are listed in Table 1.

After cooling, titanium dioxide obtained via either the sulfate or chloride method is ground using a wet or dry method and dispersed in water. The pigment can be subjected to surface modification with hydrated oxides (e.g., silica, alumina, or zirconia) to obtain varieties of pigment for special applications. The main purpose of this surface treatment is to improve the pigment’s stability, reactivity, and dispersibility; treatment is carried out in the water phase, and solutions of the substances used to modify the particles of titanium dioxide are added in a defined sequence or simultaneously. By regulating the pH of the suspension, hydrated oxides are precipitated on the surface of the pigment particles; the form of the oxides strongly depends on the conditions of precipitation. Alumina, silica, zirconia, or other oxides do not occur in the pigment as an admixture, but as a molecular layer bonded to the pigment surface. After treatment, the pigment is filtered again, dried, and ground. The product is finally packed and stored.

Other alternative methods of TiO2 production, which allow the control of process parameters as well as the physicochemical properties of the products, include the sol–gel, hydrothermal, and solvothermal methods, as well as chemical vapor deposition.

Sol–Gel Method

In a typical sol–gel process, a colloidal suspension or sol is formed as a result of hydrolysis and polymerization of precursors, which are usually inorganic metal salts or organometallic compounds such as metal alkoxides. Polymerization and loss of solvent result in conversion of the liquid sol to a solid state gel phase.

The sol–gel method is based on hydrolysis and condensation of metal alkoxide or metal salt. In the reaction of the metal chloride with either metal alkoxide or organic ether, the latter two compounds act as oxygen donors, according to Eqs. 1 and 2:
$$ \mathrm{M}\mathrm{C}{\mathrm{l}}_{\mathrm{n}}+\mathrm{M}{\left(\mathrm{OR}\right)}_{\mathrm{n}}\to\ 2\mathrm{M}{\mathrm{O}}_{\mathrm{n}/2}+\mathrm{nRCl} $$
(1)
$$ \mathrm{M}\mathrm{C}{\mathrm{l}}_{\mathrm{n}}+\left(\mathrm{n}/2\right)\mathrm{R}\mathrm{O}\mathrm{R}\to \mathrm{M}{\mathrm{O}}_{\mathrm{n}/2}+\mathrm{nRCl} $$
(2)
In these reactions, M–O–M bond formation is promoted by condensation between M–Cl and M–OR, according to Eq. 3:
$$ \equiv \mathrm{M} \hbox{--} \mathrm{C}\mathrm{l} + \equiv \mathrm{M}\hbox{--} \mathrm{O}\hbox{--} \mathrm{R}\to \equiv \mathrm{M}\hbox{--} \mathrm{O}\hbox{--} \mathrm{M}\equiv + \mathrm{R}\hbox{--} \mathrm{C}\mathrm{l} $$
(3)
In the reaction with ether, Eq. 4, metal alkoxide is formed as a result of alcoholysis with M–Cl:
$$ \equiv \mathrm{M}\hbox{--} \mathrm{C}\mathrm{l}+\mathrm{R}\hbox{--} \mathrm{OR}\to \equiv \mathrm{M}\hbox{--} \mathrm{OR}+\mathrm{R}\hbox{--} \mathrm{C}\mathrm{l} $$
(4)
At room temperature, the reactions are slow, and oxide formation is usually favored at an elevated temperature in the range 80–150 °C. The main reaction (Eq. 5) between metal chloride and metal alkoxide takes place at room temperature:
$$ \equiv \mathrm{M}\hbox{--} \mathrm{C}\mathrm{l} + \equiv \mathrm{M}\hbox{--} \mathrm{OR}\to \equiv \mathrm{M}\hbox{--} \mathrm{OR}\equiv +\equiv \mathrm{M}\hbox{--} \mathrm{C}\mathrm{l} $$
(5)
An important advantage of the sol–gel method is the possibility of obtaining TiO2 particles of nanometric size and controlled shape.

Hydrothermal and Solvothermal Methods

This method is commonly used for obtaining titanium dioxide of small-diameter particles required for the ceramic industry. TiO2 nanoparticles are obtained by peptization of titanium precursors with water. The hydrothermal method is commonly used for the production of different morphological forms of TiO2 such as nanotubes, nanowires, and nanorods.

The solvothermal method is very similar to the hydrothermal method, the difference being that in the former process, an anhydrous solvent is used instead of water. The range of temperatures used can be much greater than in the hydrothermal method and depends on the boiling point of the organic solvent used. In the solvothermal process, it is easier to control the size, shape, and crystallinity of the TiO2 nanoparticles than in the hydrothermal process. The solvothermal method is regarded as a versatile method for the synthesis of nanoparticles with narrow particle size distribution.

Chemical Vapor Deposition

Chemical vapor deposition (CVD) is used for the thermochemical processing of materials. The aim of the method is to deposit thin films of TiO2 on a particular material so as to change its physical, chemical, or mechanical properties. In the reaction chamber, titanium precursors, mostly in the gaseous phase, deposit on the hot substrate surface. Usually CVD methods require temperatures of ≥900–1,100 °C or higher, necessary for film formation, and this condition significantly restricts the use of the method. In order to obtain the target products, different gas or liquid substrates (precursors) are used, e.g., halogens, chlorides, carbonyls, and volatile organic compounds of titanium.

The wide range of production methods, as well as the outstanding physicochemical and utilizable properties of TiO2, makes this pigment an extremely attractive product in the paint and lacquer industries. The greatest amount of titanium white is used for the production of paints. Thanks to very high light refraction index, paint coatings containing titanium dioxide are intensely white and have high opacity, which means that only small amounts of TiO2 are needed to produce an excellent white opaque coating. Titanium white also increases coating durability and ensures high covering power, so that the coating can be thinner, which improves its functionality. This pigment is used to obtain high-quality lacquers, both white and in pastel colors, in mixtures with other pigments and emulsion paints. Paint production uses a wide variety of titanium white pigments, namely, rutile, anatase, pigments with modified and unmodified surface, or pigments modified for special needs. The type of pigment used depends on the type of paint and its application. For external use (paints exposed to sunlight), rutile varieties are used, while for indoor applications, anatase is preferred. Anatase varieties with lower resistance are used in cheap dispersion paints, in self-cleaning paints, and in paints for road markings. Rutile varieties have much wider use and can be employed in all types of paints.

Although TiO2 is also used for the production of printing inks, less than 3 % of global production of titanium white is used for this purpose. TiO2 pigments in printing inks ensure opacity and high brightness of the coating. Because of the thinness of printing coatings, TiO2 pigments employed for this purpose should have a narrow particle size distribution and should not contain agglomerates. The choice of pigment is very important as it affects many properties of the ink, such as luster, color, covering power, susceptibility to sedimentation, wearing, and rheological properties. The varieties of titanium white pigment should be chosen depending on the final intended use of the ink.

The second best-known and widely used white pigment is zinc oxide (C.I. Pigment White 4; ZnO) which is produced by a metallurgical process based on zinc ore calcination [8].

Metallurgical Process

Zinc oxide products are divided into:
  • Type A – obtained by a direct process (American process)

  • Type B – obtained by an indirect process (French process)

In the direct (American) process, zinc ore is reduced by heating with coal (e.g., anthracite) followed by oxidation of the resulting zinc vapors in the same reactor. This single production cycle was proposed by Wetherill. In the indirect (French) process, molten, metallic zinc is evaporated at ~907 °C, and the immediate reaction of the zinc vapor with oxygen from air gives ZnO. Zinc oxide particles are transported through a cooling channel and collected by cyclones and sock filters. This process was popularized by LeClaire in 1844 and since that time is has been called the French process. The product is obtained in the form of agglomerates of mean size from 0.1 to a few μm. The particles of ZnO are of mainly spheroidal shape. Zinc oxide of type B is of higher purity than type A.

Of great importance in the technology of production of ZnO with defined physicochemical and structural properties are the chemical processes described below.

Controlled Precipitation

This is widely used as it enables products with reproducible properties to be obtained. The idea of the method is to perform the fast and spontaneous reduction of a zinc salt using a reducing agent that halts the growth of particles at a certain size, followed by precipitation of ZnO precursor from solution; in the next step, the precursor is subjected to thermal treatment and grinding. Breaking up of the agglomerates formed is very difficult, so the calcined powders show a high level of agglomeration. The precipitation stage is controlled by pH, temperature, and time. The most important zinc precursors are zinc acetate and zinc hydroxide. A necessary stage of this process is the calcination of hydrated ZnO above 120 °C (see Table 1).

Mechanochemical Process

This cheap and simple method for the synthesis of nanoparticles on a large scale is based on high-energy dry milling, which initiates a reaction upon ball–powder collisions in a ball mill at low temperature. The main difficulty in this method is the uniform grinding of the powder and reduction of the pigment grains to the target size. Although pigment size decreases with increasing time and energy of milling, longer milling times lead to the greater amounts of impurities. The benefits of this method are that the particles are of low cost, of small size, and of limited tendency to form agglomerations, as well as their highly uniform crystalline structure and morphology.

The initial materials used in this method are mainly anhydrous ZnCl2 and Na2CO3. The reactor is charged with NaCl, which is the reaction medium and which separates the forming nanoparticles. The zinc precursor formed (ZnCO3) is subjected to calcination at 400–800 °C.

Other significant methods for the synthesis of zinc oxide are the hydro- and solvothermal methods and synthesis of morphologically defined particles in emulsion and microemulsion systems, as described above for TiO2 [8].

Besides TiO2 and ZnO, commonly known white pigments are lead white (C.I. Pigment White 1; 2PbCO3Pb(OH)2) and zinc sulfide (C.I. Pigment White 7; ZnS).

Black pigments – the optical effect of these is achieved by nonselective light absorption; among this group of pigments, the most common are carbon black and black iron oxide.

The most common black pigment is carbon black (C.I. Pigment Black 6, 7, 8). The majority of carbon black pigments have highly developed surface area, which means that their oil absorption numbers are high. Their high surface activity is responsible for their poor wettability, the effect of flotation, and adsorption of paint components on the surface. Carbon black is an inorganic pigment, but it has many properties of organic compounds. The paint industry uses only modest amounts of black pigments compared to the rubber industry. In the production of rubber for tires, carbon black not only provides color but also increases mechanical strength. Black pigments based on carbon black have excellent resistance to light and solvents, although certain solvents can extract some colored contaminants from cheap types of pigments. Black pigments show excellent chemical and thermal stability. The intensity of the black color depends on the size of the particles: the smaller the size, the more intense the color.

A very attractive black pigment is also iron oxide (C.I. Pigment Black 11; Fe(II) and Fe(III) oxide; FeO, Fe2O3). In natural form, it is found as magnetite (also maghemite, hematite, goethite, lepidocrocite – Fig. 5), which can be used as a pigment, although most iron oxide black pigments are nowadays synthesized. Iron oxide is a low-cost, inert pigment of excellent chemical resistance, insoluble in organic solvents, and of excellent durability, though unfortunately it also has limited thermal stability and low tinctorial strength. This group of pigments also includes graphite (C.I. Pigment Black 10), formed by crystallization of carbon in hexagonal form.
Pigment, Inorganic, Fig. 5

Typical forms of iron oxide-based pigments

Color pigments – the optical effect of these is achieved by selective light absorption often in combination with selective scattering; examples include brown, yellow, red, blue, and green pigments.

Brown pigments, exemplified by brown iron oxide (C.I. Pigment Brown 6 synthetic and 7 natural; Fe2O3), which naturally occurs as burnt sienna or burnt umber. Different shades can be obtained depending on the presence of impurities, in particular, manganese oxide. The pigments are more often used for artists’ paints than for commercial paints. They have low tinctorial strength; synthetic pigments give an intense brown color of excellent resistance. They are not widely used for paint production, as similar shades can be obtained by mixing together cheaper pigments.

Lead chromates (C.I. Pigment Yellow 34, Orange 21; PbCrO4/PbSO4; orange PbCrO4.Pb(OH)2) are a good example of yellow pigments. The pigments can be composed of pure lead chromate or of a mixture of lead chromate and lead sulfate. They provide colors from green yellow to orange. Chrome yellow displays excellent opacity, low oil absorption number, high brightness, and high color saturation, which make it an ideal pigment for the production of paints in a wide gamut of yellow shades. Yellow pigments have very high resistance to solvents and relatively good thermal stability, which can be improved by chemical stabilization. They are sensitive to both alkaline compounds and acids, which cause them to fade. Their resistance to light is usually satisfactory, although they can become slightly darker under exposure to light. These drawbacks can be counteracted by surface modification with such materials as silica, antimony, aluminum oxide, or different metals. Other yellow pigments are cadmium yellow (C.I. Pigment Yellow 37, 35 lithopone; CdS; lithopone – CdS containing ZnS) and yellow iron oxide (C.I. Pigment Yellow 42 synthetic, 43 natural; Fe2O3∙H2O or more exactly FeO(OH)).

Excellent functional properties, such as thermal and chemical stability, and resistance to solvents are provided by red iron oxides (C.I. Pigment Red 101 synthetic, 102 natural; Fe2O3), which belong to the category of red pigments. They are also highly opaque and have quite low tinctorial strength. Despite the serious drawback of its dull brown-red shade, it has the great advantage of low cost.

Ultramarine (C.I. Pigment Blue 29, Violet 15; general formula Na7Al6Si6O24S3), which belongs to the group of blue pigments, has attractive features, including dazzling color and a pure shade that can vary from pink through violet to green. It is also characterized by excellent thermal stability, resistance to solvents, and good resistance to light and alkaline compounds. It is not resistant to acids, and thus it cannot be used in exterior paints. Other well-known blue pigments are Prussian blue (C.I. Pigment Blue 27; FeK4Fe(CN)6) and cobalt blue(C.I. Pigment Blue 36; Co(Al,Cr)2O4).

Last in sequence but not least in importance are the green pigments, such as green chromium oxide (C.I. Pigment Green 17; Cr2O3) and hydrated chromium oxide (C.I. Pigment Green 18; Cr2O(OH)4). These pigments have an opaque shade, good opacity but low tinctorial strength. They also offer excellent thermal stability and chemical resistance to solvents and light [2, 9, 10].

Because of their specific properties, inorganic pigments are used mainly in the paint and lacquer industries. However, in order for the final products (paints and coatings) to meet set requirements, they must be formulated with other additives that do not provide any staining or opacity. These additives are known as extenders (Table 2). They may be used to change typical pigment properties as well as specific functions [3, 11].
Pigment, Inorganic, Table 2

Example pigment extenders

Extender type

Formula

Uses

Calcium carbonate

CaCO3

Additive for paints containing solvent, printing inks

Silica

SiO2

Additive for rheology and thixotropy control, anti-settling, thickening, and anti-sagging agent

Talc

Mg3Si4O10(OH)2

Gives high hiding power and a stain finish, provides a matting effect and decreased moisture permeability

Barytes

BaSO4

Additive for waterborne paint systems

Kaolin

Al2Si2O5(OH)4

Decreases viscosity

Mica

AB2−3(Al,Si)Si3O10(OH)2; A = K,Na,Ca; B = Al, Fe, Mg, Li

UV and chemically resistant, improves water resistance, provides pearl effect

Inorganic pigment technology is directly related to the development of new synthesis methods and the preparation of environmentally friendly colorants. The manufacture of pigments containing metals such as Pb(II), Cr(VI), Cd(II), etc. in aqueous media involves the risk of releasing harmful substances to the environment and a need to dispose of materials which contain them; this restricts or prohibits their application. Such pigments cannot be used in paints, but are potentially useful and environmentally safe in other industries, such as catalysis and ceramics, but only as stable spinels. The manufacturers and users of such materials are still obliged to comply with environmental and regulatory restrictions in their synthesis and use. The production of the new pigment group targets first of all safe materials; waste materials, though, such as chromium(VI) compounds, can also be used for the direct synthesis of Cr2O3-based pigments. Those aspects by no means diminish interest in inorganic pigments and their use; on the contrary, due to environmental reasons, the focus on and the industrial role of TiO2-based pigments or hybrid pigments prepared using alternative methods continue to increase.

A special group of inorganic pigments consists of pigments with luster (mirror effect), luminescent pigments, and also hybrid pigments [12, 13, 14, 15, 16]. The first group of these special pigments includes the following:
  • Metallic pigments – (only inorganic) contain lamellar metal particles of high refraction index, e.g., flakes of copper, gold, aluminum.

  • Pearlescent pigments – contain transparent lamellar particles, which are responsible for a pearl shine which is the result of multiple reflections from particles oriented in parallel; examples include specially treated alkaline lead carbonates, bismuth oxychloride, and titanium dioxide supported on mica.

  • Interference pigments – here the optical effect is a result of the thickness of the oxide layer deposited on supports (similarly as in pearlescent pigments); examples are oxides of iron or chromium supported on mica; pigments in this group also display opalescence.

Also very important are luminescent pigments, including:
  • Fluorescence pigments – of which the optical effect is based on selective absorption of light and simultaneous luminescence and is initiated by high-energy radiation from the ultraviolet or shortwave visible range; examples are radioactive luminescent pigments.

  • Phosphorescence pigments (only inorganic) – here the optical effect is based on selective light absorption and scattering followed by delayed luminescence and is also initiated by high-energy radiation from the ultraviolet or shortwave visible range; examples are zinc sulfide and alkaline earth metal sulfides “dotted” with heavy metal ions.

Hybrid Pigments

Recently, much interest has been paid to the synthesis of hybrid pigments. These can occur in three main combinations, namely, organic/organic, inorganic/inorganic, and organic/inorganic. This group of pigments includes those obtained on the basis of organic dyes and oxide support such as SiO2. Substances of this type are produced mainly as a result of adsorption of organic molecules on to a synthetic or mineral oxide support. Adsorption of organic compounds such as dyes is determined first of all by the type of interactions involved (covalent bonds, electrostatic force, hydrogen bonds, etc.) and the character of the inorganic support (SiO2, TiO2, SiO2-TiO2, Al2O3, aluminosilicates, and carbonaceous materials), such as its surface properties and dispersion abilities [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27].

The most often used selective adsorbent of dyes is silica synthesized by the sol–gel method. Alternative supports include silicas synthesized in the precipitation reaction from water solutions of alkali metal silicates, mainly sodium silicate. A large number of selective adsorbents are provided by mineral substances such as kaolin and montmorillonite. The core of the pigment composites can also consist of biopolymers of natural origin (chitin, chitosan, lignin) or synthetic compounds.

Hybrid pigments of this type can be obtained in different colors depending on the color of the initial dye. They display many advantages over conventional pigments. The particle size of such pigments depends on those of the initial support. Moreover, from the same core, it is possible to secure pigments with the same particle size, density, and surface properties, but differing in color and hue.

Pigments Obtained by Grafting of an Organic Dye to a Modified Silica Surface

Such pigments are composed of a core that is a fine powder – SiO2, TiO2, Al2O3, etc. – and a dye chemically bonded with the silica surface by a coupling agent, e.g., aminosilane. They display high color strength, excellent durability, and resistance to solvents, water, temperature, and light. Their structure can be described by the formula:where:
  • S is the support.

  • CB is a coupling bridge – an alkyl group (3−6 carbon atoms) containing −NH− or −NHCO− in a chain like:
    $$ -\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2-,-\mathrm{N}\mathrm{H}-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2-\mathrm{N}{\mathrm{H}}_2,-\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_2-\mathrm{N}\mathrm{H}\mathrm{CO}-,\mathrm{R}-\mathrm{N}=\mathrm{C}\mathrm{H}-,\mathrm{R}-\mathrm{N}\mathrm{H}-\mathrm{C}{\mathrm{H}}_2-,\mathrm{or} \mathrm{R}-\mathrm{N}\mathrm{H}\mathrm{S}{\mathrm{O}}_2-. $$
  • D is the organic dye.

The most popular inorganic support for hybrid pigment formulation is silica. Hydrophilic silicas, which are usually colorless and have silanol groups on their surface, can react with many functional groups through covalent bonds. The process of dyeing these silicas is multistaged: silica is first reacted with a coupling agent, and then the dye (reactive, acid, or cationic) reacts with the coupling agent (e.g., silane) to give silica particles that contain covalently bound dye. The dye is permanently bonded to the silanol coupling agent and, therefore, cannot be leached by a solvent, which leads to reduced toxicity of the components within a paint or ink.

The sequence of reactions in this method is shown schematically in Fig. 6 and includes modification of the silica surface with 3-aminopropyltriethoxysilane in order to introduce amino groups, which subsequently react with the reactive dye (e.g., C.I. Reactive Blue 2) to give dyed silica particles [28].
Pigment, Inorganic, Fig. 6

Mechanism of hybrid pigment formation by adsorption of organic dye onto aminosilane-functionalized silica support (Created based on Ref. [28] with permission from Elsevier Publisher)

Through simple and economical processes, it is possible to obtain colored particles suitable for use as pigments for inks which display high resistance to water and temperature and low toxicity. The inks can be made in a wide range of colors and can be used for printing on foil, smooth, or coated paper.

This process can be also realized by the multistep synthesis of a chromophore on the surface of an inorganic support.

Pigment synthesis involves modification of the support surface – which becomes the core of the pigment – with an aminosilane coupling agent to introduce surface amino groups and then bonding the dye with amino groups. For this purpose, silicas of small diameters are used, as when silica particles are small, the area of contact is usually larger, which is beneficial for mixing with other materials.

Pigments Obtained in the Process of Silica Synthesis

Pigments of this type are obtained using silica synthesized in a process of hydrolysis and condensation of tetraethoxysilane (TEOS) in a mixture of ethanol/water/ammonia and in the presence of cationic dyes such as C.I. Basic Blue 9 (Fig. 7).
Pigment, Inorganic, Fig. 7

Modification of the pigment and binding with the silica surface

The initial dye solution is prepared in a water medium and is then filtered off through a suitable membrane. In order to synthesize inorganic particles containing a dye, the silica sol and dye solution are mixed to obtain a permanent product. Often, the whole amount of the dye used in the process is incorporated into the structure of the pigment, as indicated by a colorless solution remaining after the process. The color particles are filtered off and washed with distilled water until no dye is left in the filtrate.

Retention of dye in the support particles is tested by dispersion in ethanol or acetone, followed by ultrasound treatment and centrifugation. This procedure is repeated a few times to check the stability of the bonding of the dye to the support surface.

There is also interest in combining the functional features of inorganic pigments (TiO2) or their derivatives (TiO2-SiO2) with selected dyes used in the food and/or pharmaceutical industries (e.g., C.I. Food Yellow 4, C.I. Food Blue 5:2, C.I. Food Red 9) [16]. An example mechanism of synthesis of such a hybrid pigment is illustrated in Fig. 8.
Pigment, Inorganic, Fig. 8

Mechanism of C.I. Food Blue 5:2 interactions with TiO2-SiO2 surface (Created based on Ref. [16] with permission from Elsevier Publisher)

Such a combination makes it possible to obtain a homogeneous hybrid system, which is a great advantage from the application point of view, as separate introduction of dye and pigment at the stage of drug formulation (e.g., tablets) is no longer needed and problems with their steric and electrokinetic stability are eliminated.

The physicochemical and structural parameters of the hybrid pigments depend on many factors, among which the most important are the chemical structure of the dye and the support, concentration of reagents, reaction time, pH, etc.

Summary

Technologies for the production and application of a wide range of inorganic pigments are closely interrelated and are constantly being developed. In an era in which many branches of industry are undergoing dynamic development, there is a need to improve and obtain new compounds of various kinds, including inorganic pigments, with specific physicochemical and application properties. New trends in the development of pigments relate mainly to:
  • The introduction of new pigments or improvement of the properties of existing ones

  • Broadening of the assortment and range of uses of pigment composites

  • The introduction of nanoparticle pigment structures

The above is confirmed by the numerous R&D projects carried out at scientific centers and in industry, which to date have published more than 25,100 scientific papers relating to inorganic pigments.

The main reason for the high level of interest, particularly in the use of nanoparticles, is the possibility of improving the barrier properties of coatings and thereby also corrosion resistance and mechanical properties. Other drivers of innovation in the field of pigments are environmental awareness, economic pressures, and the constantly changing needs of the industries which consume these products. Hence the greatest level of development is observed in relation to pigments with special metallic and interference effects, including hybrid pigments. All of these aspects ensure unceasing demand and continued efforts to obtain a wide color range of inorganic pigments, whose importance can be expected to continue to increase.

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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Institute of Chemical Technology and EngineeringPoznan University of TechnologyPoznanPoland