Inorganic Pigments

Gerhard Pfaff

• Publisher: De Gruyter
• Year: 2017
• ISBN:
  9783110484557
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Chimica inorganica

1Fundamentals, general aspects, color, and application

1.1Definitions and classification

Pigments are defined in a modern way as substances consisting of small particles that are practically insoluble in an application system and that are used as a colorant or because of their anticorrosive or magnetic properties. They differ from dyes, which also belong to the colorants, primarily by the fact that dyes are practically completely soluble in the medium of application. The most important application media for pigments are automotive and industrial coatings, paints, plastics, printing inks, cosmetic formulations, and construction materials. Other uses of pigments are in paper, rubber, glass, porcelain, glazes, and artists’ colors.

The term “pigment” descends from the Latin word “pigmentum”. It was originally used in the sense of a coloring matter. The use of the word was later extended to indicate colored decoration. The term “pigment” was also used in the late Middle Ages for plant and vegetable extracts, particularly for those with coloring properties. In biological terminology, it is still applied to indicate vegetable and animal colorants that are present in “solved” form as extremely small particles in cells or cell membranes, as deposits in tissues, or suspended in bodily fluids. In all these cases, the term “pigment” is misleading and would be better replaced by the more suitable term “dye” or “dyestuff”.

The term “colorant” covers all colored compounds regardless of their origin and utilization for coloration or other purposes. Colorants are not only divided into pigments and dyes, but also into natural and synthetic compounds. Some pigments and some dyes exist as natural and synthetic variants. Pigment particles, when applied, have to be attached to surfaces (substrates) by additional materials, such as binder systems (paints, coatings, printing inks, cosmetics) or plastics. Dyes are applied to various substrates, such as textiles, leather, paper, or hair, using a liquid in which they are solved. In contrast to pigments, dyes must have an affinity to the substrates on which they are fixed.

Pigments are differentiated according to their chemical composition and with respect to their optical and technical properties. A fundamental distinction is that between inorganic and organic pigments.

Figure 1.1 shows a rough classification of pigments and dyes within the category of colorants (coloring materials). Fillers as a substance class closely related to pigments are involved here. It can be clearly seen that dyes are based only on organic compounds. It becomes also clear that organic white pigments do not exist.

Figure 1.2 contains a detailed classification of inorganic pigments. White, colored, black, and special pigments exist. The most important representatives for the different pigment categories are shown. White pigments are represented by titanium dioxide (rutile and anatase), zinc sulfide including lithopone, and zinc oxide. Colored pigments show the broadest variation, ranging from blue (mixed metal oxides, ultramarine, iron blue) via green (chromium oxide, mixed metal oxides) and yellow (iron oxide hydroxide, mixed metal oxides, chromates, bismuth vanadate, cadmium sulfide) up to red (iron oxide, cadmium selenide, molybdates, cerium sulfide, oxonitrides). The main representative for black pigments is carbon black. Based on this variety of colors, whites and blacks, with inorganic pigments it is possible to design nearly all thinkable colors and noncolors (white, black, and gray), including bright and dark color shades, by the use of pure single pigments or pigment mixtures.

Fig. 1.1: Classification of colorants (* effect pigments, transparent pigments, luminescent pigments, functional pigments, ** effect pigments, luminescent pigments).

Fig. 1.2: Classification of inorganic pigments.

Special pigments are subdivided in the classes effect pigments (luster pigments), with the two subclasses special effect pigments (pearlescent pigments, interference pigments) and metal effect pigments, transparent pigments, luminescent pigments, with the two subclasses fluorescent pigments and phosphorescent pigments, magnetic pigments, and anticorrosive pigments. The last two classes, as well as some of the transparent pigments, belong to the category functional pigments. Other materials that count as functional pigments are electrically conductive, IR reflective, UV absorbing, and laser marking pigments.

Fillers (extenders) are powdery substances that, like pigments, are practically insoluble in the application system. They are typically white and are used because of their chemical and physical properties. The distinction between pigments and fillers is made based on the specific application. Another criterion is the refractive index, which for fillers is usually below 1.7 and above this value for pigments. There is, however, no fixed definition of the value for the refractive index to distinguish both product classes. A filler is not a colorant in the proper sense, but a substance that modifies the application medium in order to improve its technical characteristics, to influence optical and coloristic properties, or to increase the volume. Fillers are also used to lower the consumption of more expensive binder components. Those fillers, which are used mainly for cost reduction reasons are also called extenders. Fillers are often applied together with pigments to improve the properties of the medium in which they are incorporated.

Another classification for inorganic pigments is based on their chemical composition, for example:

–oxides and oxide hydroxides: α-Fe2O3, α-FeOOH, γ-Fe2O3, Fe3O4, Cr2O3, CrOOH, CoAl2O4, (Ti, Ni, Sb)O2;

–sulfides and selenides: CdS, Cd(S, Se), CdSe, Ce2S3;

–chromates, molybdates, vanadates: PbCrO4, Pb(Cr, S)O4, Pb(Cr, S, Mo)O4, BiVO4, 4 BiVO4 ⋅ 3 Bi2MoO6;

–ultramarine: NaAl6Si6O24(NaSn6);

–hexacyanoferrate: K[FeIIIFeII(CN)6];

–oxonitrides: CaTaO2N, LaTaON2.

Generally, inorganic pigments are more stable against light, weather, temperature, and chemicals than organic pigments. Another advantage of inorganic pigments are their lower manufacturing costs. Organic pigments need mostly multistage syntheses and more expensive raw materials for their production, which finally leads to higher prices for them in the market. They are, however, in various cases more color intensive and, therefore, more attractive than inorganic colored pigments. A disadvantage of organic pigments is their lower stability against the factors mentioned above. Degradation of organic colorants – pigments as well as dyes – by exposure to UV light and under atmospheric influences is an issue. The use of UV absorbers together with organic colorants is a very helpful possibility to ensure a long shelf life also for these materials in their application.

1.2History and economic aspects

The history of inorganic pigments goes back to prehistoric times. Human beings in primitive society were already able to color textiles, furs, and other things by using natural substances, predominantly from vegetables, but also from animals. Cave drawings like those in Altamira or Lascaux were made in the Ice Age (Figure 1.3)...