1 Fundamentals, general aspects, color, application

1.1 Definitions and classification

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 also becomes clear that organic white pigments do not exist.

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

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, lead chromate, bismuth vanadate, cadmium sulfide) up to red (iron oxide, cadmium selenide, lead molybdate, 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, gray), including bright and dark color shades by the use of pure single pigments or pigment mixtures.

Fig. 1.2: Classification of inorganic pigments.

Special pigments are subdivided into the classes of 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 a part 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.

The optical behavior of effect pigments is based either on the directional reflection of visible light from predominantly two-dimensional and aligned metallic (metal effect pigments) or highly refractive transparent pigment particles (pearlescent pigments) or on the phenomenon of interference (interference pigments).

Transparent pigments are characterized by very small particles with sizes in the range below 100 nm and large specific surface areas. Most of the technically relevant pigments consist of particles which are even smaller than 30 nm. They are classified as nanomaterials. Pigmentation with these pigments leads to a transparent appearance of the application systems.

Luminescent pigments (luminescent materials, luminophores, phosphors) show optical effects based on the ability to absorb radiation and emit it as light of longer wavelength with a time delay (phosphorescence) or without a time delay (fluorescence). Light emission often occurs in the visible spectral range. External energy is necessary to enable luminescent materials to generate light.

Functional pigments are not about color but about different physical properties that cannot be derived from the original term “pigmentum”. Such materials, which exhibit magnetic, anticorrosive, electrically conductive, IR-reflective, UV-absorbing, and other physical properties can be appropriately described by the term “functional pigments”. Some of the transparent pigments and the luminescent pigments can also be classified as functional pigments. The reason why functional pigments are classified as pigments is because of their similar morphological and application technical properties.

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 the chemical composition. Table 1.1 contains relevant chemical compositions of inorganic pigments together with examples. Some of the pigments mentioned here are historically significant, but are no longer of any practical importance today. These include not only some of the sulfides (HgS, As2S3) but also stannate, phosphate, arsenate, and antimonate-based pigments. The main reason why these pigments are no longer used is their content of toxic components. The formerly important lead oxides PbO and Pb3O4 are becoming less important due to the undesired lead content. The carbonate hydroxides of lead and copper have also lost the great importance they once had. Some newer developments such as γ-Ce2S3 or the oxonitrides have not entered the pigment market because they do not meet important application criteria or are too costly to produce.