Лако-красочные материалы — производство

Cadmium sulfides and sulfoselenides provide a range of moderately intense colours ranging from yellow through orange and red to maroon. They are of particular importance in the coloration of thermoplastics, especially in engineering polymers which are processed at high tempera­tures, because of their outstanding heat stability. Cadmium sulfide, CdS (C. I. Pigment Yellow 37) is dimorphic, existing in a — and ^-forms. The shade range of the commercial pigments is extended by the formation of solid solutions. When cadmium ions are partially replaced in the lattice by zinc, greenish-yellow products result, whilst replacement of sulfur by selenium gives rise progressively to the orange, red and maroon sul — foselenides (C. I. Pigment Orange 20) and (C. I. Pigment Red 108), depending on the degree of replacement.

Lead chromate pigments provide a range of colours, from greenish — yellow through orange to yellowish-red. They offer good fastness proper­ties, a remarkably high brightness of colour for inorganic pigments, and high opacity at relatively low cost. Historically, lead chromate pigments were found to exhibit a tendency to darken, either on exposure to light (due to lead chromite formation) or in areas of high industrial atmos­pheric pollution (due to lead sulfide formation). These early problems were overcome by the use of surface treatment with oxides, for example silica, and the modern range of pigments now offer excellent durability. The variation in shade of lead chromate pigments is achieved by the formation of solid solutions. In this respect they resemble the cadmium sulfides, although structurally the lead chromate pigments present a more complex situation by exhibiting polymorphism. The mid-shade yellow products are essentially pure PbCrO4 (C. I. Pigment Yellow 34) in its most stable monoclinic crystal form. Incorporation of sulfate ions into the lattice while retaining the monoclinic crystal form gives rise to the somewhat greener lemon chromes. The greenest shades (primrose chromes) consist similarly of solid solutions of PbCrO4 and PbSO4 but stabilised chemically in a metastable orthorhombic crystal form. Incor­poration of molybdate anions into the lattice gives rise to the orange and light red molybdate chromes (C. I. Pigment Red 104). Molybdate chromes usually also contain small amounts of sulfate ions, which are thought to play a role in promoting the formation of the appropriate crystal form.

Cadmium sulfides are prepared by aqueous precipitation processes using suitable water-soluble sources of cadmium and sulfide ions. The zinc-containing pigments are formed when appropriate quantities of soluble zinc salts are incorporated into the process, while the sulfoselen — ides are prepared by dissolving elemental selenium in the sulfide solution before the precipitation. Since the pigments usually precipitate from solution in the less stable ^-form, an essential final step in their manufac­ture is a controlled calcination at 600 °C which effects the conversion into the desired а-form. Lead chromate pigments are manufactured by mixing aqueous solutions of lead nitrate and sodium chromate or sodium dich­romate. The mixed phase pigments result when appropriate quantities of sodium sulfate or molybdate are incorporated into the preparation.

The use of cadmium sulfide and lead chromate pigments is limited to a considerable extent on the grounds of potential toxicity due to the presence of cadmium, lead and chromium(vi). Their use is restricted by voluntary codes of practice reinforced by legislation in certain cases, including toy finishes and other consumer paints, graphic instruments and food contact applications. In the European Community, for example, a Directive restricts the use of the cadmium sulfides in applications where they are not seen to be essential. However, at present, completely satisfac­tory substitutes for cadmium pigments are not available for use in certain high temperature plastics applications, especially in terms of thermal and chemical stability, while lead chromates remain by far the most cost — effective durable yellow and orange pigments. It is argued, particularly by the manufacturers of these products, that as a result of their extreme insolubility they do not present a major health hazard. Nevertheless, it seems likely that the trend towards their replacement by more acceptable inorganic and organic pigments will continue in an increasing range of applications.

Two types of inorganic pigments of relatively recent origin, bismuth vanadates and cerium sulfides, are potential replacements for the so — called ‘heavy metal’-containing products. Bismuth vanadates, which can contain variable amounts of bismuth molybdate, are brilliant yellow pigments with high opacity and good durability, and are used primarily to provide bright deep yellow shades in industrial and automotive paints. The pigments are manufactured by an aqueous precipitation reaction involving bismuth nitrate, sodium vanadate and sodium molybdate. The hydrated products which result are then calcined at 600 °C to both remove water and develop the appropriate crystalline form. Cerium sulfides provide yellow, orange and red shades with excellent durability. The colours, however, are tinctorially rather weak.