Lithopone  B-301 (CAS: 1345-05-7)

Lithopone, C.I. Pigment White 5, is an mixture of inorganic compounds, is a widely used as a white pigment powder. It is composed of a mixture ofbarium sulfate and zinc sulfide. These insoluble compounds blend well with organic compounds and confer opacity. It was made popular by the cheap production costs, greater coverage. Related white pigments include titanium dioxide, zinc oxide (“zinc white”), and zinc sulfide.

History

Lithipone was discovered in 1870s by DuPont. It was manufactured by Krebs Pigments and Chemical Company and other companies. The material came in different “seals”, which varied in the content of zinc sulfide. Gold seal and Bronze seals contain 40-50% zinc sulfide, offering more hiding power and strength. Although its popularity peaked around 1920, approximately 220,000 tons were produced in 1990. It is mainly used in paints, putty, and in plastics.

Stability and darkening

Although barium sulfate is almost completely inert, zinc sulfide degrades upon exposure to UV light, leading to darkening of the pigment. The severity of this UV reaction is dependent on a combination of two factors; how much zinc sulfide makes up the pigments formulation, and its total accumulated UV exposure. Depending on these factors the pigment itself can vary in shade over time, ranging from pure white all the way to grey or even black. To suppress this effect, a dopant may be used, such as a small amount of cobalt salts, which would be added to the formulation. This process creates cobalt-doped zinc sulfide. The cobalt salts help to stabilize zinc sulfide so it will not have as severe a reaction to UV exposure.

Production

Lithopone is produced by coprecipitation of barium sulfate and zinc sulfide. Most commonly coprecipitation is effected by combining equimolar amounts of zinc sulfate and barium sulfide:

BaS + ZnSO4→ ZnS • BaSO4

This route affords a product that is 29.4 wt % ZnS and 70.6 wt % BaSO4. Variations exist, for example, more ZnS-rich materials are produced when zinc chloride is added to the mixture of zinc sulfate and barium sulfide.

Barium sulfide is produced by carbothermic reduction of barium sulfate. Zinc sulfate is obtained from a variety of zinc products, often waste, by treatment with sulfuric acid.

Safety

Lithopone is not highly poisonous, reflecting the insolubility of the components. Barium sulfate is used widely in medicine as a radiocontrast agent. Lithopne is allowed to be in contact with foodstuffs in theUSand Europe.