Cadmium pigments are stable inorganic colouring agents which can be produced in a range of brilliant shades of yellow, orange, red and maroon. Their greatest use is in plastics but they also have significant application in ceramics, glasses and specialist paints.
The pigments are based upon cadmium sulphide which produces a golden yellow pigment. Partial substitution of cadmium in the crystal lattice by zinc or mercury, and of sulphur by selenium, forms a series of intercrystalline compounds making up the intermediate colours in the lemon-yellow to maroon range of cadmium colours (Fig. 1). The pigments are fine, discrete particles of coloured powder, with diameters of around 1 gm, which are distributed and suspended in the material to produce a uniformly coloured product. Cadmium pigments have excellent heat stability which makes them essential in applications where elevated processing or service temperatures are encountered.
Most cadmium pigments are used in plastics. These pigments disperse well in most polymers to give good colouring and high opacity and tinting strength. The pigments are insoluble in organic solvents, have good resistance to alkalis and in most cases will remain colour fast for the life of the plastic. As a result, cadmium pigments have been used in a wide range of plastic products. Nowadays, their greatest application is in complex polymers which are processed at higher temperatures and require the unique durability and technical performance of a cadmium pigment. Their use is almost mandatory in many nylon, acrylonitrile butadiene styrene (ABS), polycarbonates, high density polyethylene, silicone resins and other modern thermoplastic polymers processed at high temperatures which preclude the use of organic pigments and also most alternative inorganic pigments in the range of hues provided by cadmium. Cadmium pigmented engineering polymers such as ABS are widely used in products which include telephones, gas pipes and fittings, electricity cables, beverage crates and motor vehicle radiator fans. Pigments are usually incorporated in plastics in proportions of 0.01 to 0.75 per cent by weight.
(b) Specialist and industrial paints
Bright cadmium yellows, oranges and reds are major pigments for artists' colours where their permanence and opacity are the accepted standards against which other pigments are judged.
Cadmium yellows and reds can have service temperatures well above 3OOO C and are used in coatings for process chemical and steam pipes. They can also be incorporated in latex and acrylic coatings.
Cadmium pigments are usually incorporated in these paints in proportions of 10 to 15 per cent by weight.
(c) Ceramics and glasses
The unique abilities of highly stable cadmium pigments to withstand high processing and service temperatures make them the only choice in much of their colour range for glasses, ceramic glazes and vitreous and porcelain enamels. In transparent glasses the cadmium pigment particles are colloidally dispersed to produce the colours by selective absorption and scattering. The addition of 0.5 per cent by weight of cadmium pigment produces bright transparent glasses with colour ranging from intense yellow through to ruby red depending upon the cadmium sulphide to selenium ratio. A ruby red produced using a cadmium sulphide to selenium ratio of 2:1 has excellent light transmission in the red part of the spectrum with a sharp cut-off of other colours. Selenium rubies of this type are used to colour glasses for standard signal lamps for railway, marine and other uses. Greater proportions of pigment (up to about 10 per cent) are employed to make darker decorative glasses.
The bright colours of cadmium pigments are ideally suited to ceramics, vitreous enamels for glass and porcelain enamels for iron and steel domestic products.
(d) Miscellaneous uses
Cadmium pigments have a number of other minor uses in rubber, paper and inks although these are small in terms of cadmium consumption.
Fig.1 Manufacture of cadmium pigments.