Description
Lithopone is a bright white pigment used in paints, inks, leather, paper, linoleum and flour. It was developed in the 1870s as a replacement or supplement for lead carbonate (white lead), in order to overcome its disadvantages of toxicity, poor weathering properties and darkening in atmospheres containing sulphur compounds. It is an insoluble mixture of barium sulphate and zinc sulphide, which precipitates when barium sulphate and zinc sulphate solutions are mixed. The precipitate is recovered by filtration and then calcined (roasted) at temperatures above 600° C (1,112° F). It is now replaced by titanium dioxide in most applications, but it is still widely used in many products such as water paints.
Chemical Properties
Lithopone is a white powder that is insoluble in water, but soluble in acids. It has high resistance to hydrochloric acid and silver ions.
Uses
Lithopone is a mixed zinc sulfide-barium sulfate pigment available in
two types; one containing 30 percent zinc sulfide and one containing
60 percent zinc sulfide. Coprecipitation is achieved by reacting an aqueous
solution of zinc sulfate with barium sulfide. The barium sulfide
solution is prepared by reducing barite ore (BaSO4) with carbon.
Uses
Lithopone is mainly used in coating materials with relatively high pigment concentrations. Examples are primers, plastic masses, putties and fillers, artists’ colors, and emulsion paints. An important property of lithopone is its low binder requirement, giving paint products with good flow and application properties. It is suitable for almost all binder media, and has good wetting and dispersing properties. With optimum feed composition, good dispersion can be achieved simply by the action of a dissolver. It can be economically advantageous to use lithopone in combination with TiO2 pigments; the good hiding power of the TiO2 pigments is combined with the economic and technical advantages of lithopone. Due to the strong shift of the absorption band towards the blue, lithopone is especially useful as a white pigment for UV-cured paint systems. Zinc compounds have fungicidal and algicidal effects, and inclusion of lithopone or Sachtolith in paint formulations for exterior use therefore helps prevent the growth of algae or fungi.
The material advantages of lithopone are used in plastics (e.g., good lightfastness and clear bluish-white shade). The product also imparts very good extruding properties to the plastic resulting in high throughput rates and economic extruder operation. In fire-resistant systems, ca. 50% of the flame retardant antimony trioxide can be replaced by nontoxic lithopone without any adverse effect.
Uses
In water and oil paints to provide thixotropy, improve gloss and flow.
Application
Lithopone is used as a pigment in paints, plastics, and printing inks. The particle size of it can be reduced through the use of polyvinyl alcohol (PVA) or sodium carbonate. PVA reduces particle size by absorbing the liquid and forming a gel-like substance around the particles. Sodium carbonate reduces particle size by forming a protective layer on the surface of the particles, which prevents them from clumping together. Lithopone also has analytical chemistry applications because it absorbs radiation well and can be used for identification purposes.
Synthesis
(1) Roasting-Leaching Method: Barite is mixed with coal and roasted to produce barium sulfide (BaS). The BaS is leached and then reacted with a zinc sulfate solution. This forms a precipitate of ZnSBaSO. The precipitate is filter-pressed, roasted, washed with sulfuric acid, washed with water, treated with a color-fixing agent, dried, and milled.
(2) Barite Calcination Method: A barium sulfide solution and a zinc sulfate solution are prepared separately. They are mixed to carry out a double decomposition reaction, precipitating ZnSBaSO. The precipitate is filter-pressed, calcined, wet-ground, dried, and crushed.
Toxics Screening Level
The initial threshold screening level (ITSL) for each of the above listed chemicals is 0.1 μg/m3 based on an annual averaging time.
