Cadmium, pigments

Cadmium orange Pigment Orange 20 formula CdS-CdSe 

Cadmium pigments have good hiding power, soft texture, and good light fastness and thermal stability. They are resistant to alkali, but are decomposed by acids. Despite these merits, their use is declining because they are relatively expensive, and in most applications, they are prohibited due to environmental problems associated with cadmium. 

In yellow CdS, which is the basis of the cadmium pigments, both the cadmium and the anion can, within certain limits, be replaced by ions with similar radii. The incorporation of zinc is industrially important in the manufacture of greenish yellow pigments, (Cd,Zn)S, as is that of mercury in (Hg,Cd)S and selenium in Cd(S,Se) for the production of a color range from orange to red to bordeaux red. 

Cadmium yellow pigments (CdS) are manufactured by adding sodium sulfide to dissolved metallic cadmium or a cadmium salt solution to precipitate the fine particle raw colorant, which does not yet have pigment properties. Calcination at temperatures between 600 and 700°C induces particle growth to ca. 0.2 pm at which optimum tinting strength and hiding power are attained. 

An alternative frequently used process is the calcination of a mixture of CdC03 (from metallic cadmium), zinc salt and sulfur (for the manufacture of yellow pigments), or with selenium in the place of zinc salt in the case of red pigments. 

In the manufacture of orange to deep red cadmium sulfoselenides using the precipitation process, selenium is dissolved in the sodium sulfide solution in an amount consistent with the color required. Particularly brilliant hues are obtained, if the precipitated raw colorant undergoes brief heat treatment in molten salt at ca. 800°C whereby an optimum particle size of ca. 0.2 to 0.4 pm can be obtained. 

Variation of the hue of yellow cadmium sulfide pigments hy mixed-crystal formation  

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A third form of cadmium pigments includes the mercury cadmiums. Mercuric sulfide (HgS) forms soHd solutions up to about 20 mol % with the oranges, reds, and maroons. The heat stabiUty is improved up to 370°C, and the costs are somewhat lower than the CP grades. The mercury cadmiums are slightly more reactive, but have excellent bleed resistance. 

Cadmium Pigments. J. R. J. van Asperen de Boer, Molecular Aspects of Ageing in Painted Works of Art, Progress Report 1995-1997, FOM Institute for Atomic and Molecular Physics, Amsterdam, NL, http //www.amolf.nl/research/biomacromolecular mass spectrometry/molart/progress report98.pdf... 

Table 2.6 Colours obtainable with cadmium pigments...

It is announced in this short article that the European Commission has voted for no further restrictions on the marketing and utilisation of cadmium pigments to colour plastics, thereby halting the trend of recent years in reducing the number of applications in which these pigments are permitted. Brief details are provided. EUROPEAN COMMISSION... 

Journal of Coatings Technology 71, No.896, Sept. 1999, p.14 EUROPEAN COMMISSION FOREGOES ADDITIONAL RESTRICTIONS ON CADMIUM PIGMENTS USED IN PLASTICS... 

An EC study has found that cadmium pigments present no significant threat to human health or the environment, says the International Cadmium Association. In view of the positive life cycle assessment, EC Member States voted earlier this year that there should be no further restrictions on the marketing and use of cadmium pigments in plastics. INTERNATIONAL CADMIUM ASSOCIATION... 

Because of their brilliant pure red and yellow shades cadmium pigments, e.g. Cl Pigment Red 108, CdS-xCdSe, have found widespread use in the coloration of plastics, but toxicological problems have led to restrictions on their use and to the introduction of safer replacement products. 

Preparation of Cadmium Pigments. Place two thin layers (from 0.5 to 1 g) of cadmium carbonate spaced 6 cm apart into the middle part of a refractory glass tube 40 cm long and 5 cm in diameter and pass a stream of hydrogen sulphide through the tube. When the colour of the first layer changes, heat the place of the reac-... 

In the 19th century, ultramarine, Guignet s green, cobalt pigments, iron oxide pigments, and cadmium pigments were developed in quick succession. 

Figure 27. Crystal lattice of cadmium pigments (wurtzite structure) a) Sulfur (selenium) b) Cadmium (zinc, mercury)...

Figure 28. Reflectance curves of cadmium pigments (pigment volume concentration 10%)...

The use of zinc yields greenish yellow pigments due to the lower lattice constants mercury and selenium lead to expansion of the lattice. With an increasing content of selenium, or especially mercury, the shades of the pigments change to orange, red, and ultimately to deep red (bordeaux). The brilliant colors of cadmium pigments are primarily due to their almost ideal reflectance curves with a steep ascent (Fig. 28). 

When hydrogen sulfide is passed into cadmium salt solutions, cadmium sulfide is formed as a yellow precipitate with a zinc blende structure (cubic, (i-form). The P-form can be converted into the a-form (e.g., by heating). a-Cadmium sulfide shows photoconductivity due to defects in the crystal lattice (usage in photovoltaic cells) [3.106]. The solubility in water at 25°C is 1.46 x 10 10 mol/L [3.107], Cadmium sulfide forms the basis for all cadmium pigments. 

Cadmium pigments are mainly used to color plastics. They have brilliant, pure shades (yellow, orange, red, and bordeaux), good hiding power, and moderate tinting strength. 

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