Lithopone

Mixture of inorganic compounds, widely used as a white pigment powder

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Chemical compound

Lithopone, C.I. Pigment White 5, is a mixture of inorganic compounds, widely used as a white pigment powder. It is composed of a mixture of barium sulfate and zinc sulfide. These insoluble compounds blend well with organic compounds and confer opacity. Recently, there have been advancements in production techniques which have further reduced costs and increased availability, allowing for broader use in various industries. Related white pigments include titanium dioxide, zinc oxide ("zinc white"), zinc sulfide, and white lead.

History

Lithopone was discovered in the early 20th century by DuPont. It was first 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 the mid-20th century, approximately 223,352 tons were produced in the most recent year recorded in various industry reports. 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, increasing its stability and resistance to UV light.

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 typically yields 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 the carbothermic reduction of barium sulfate, while zinc sulfate is obtained from various zinc products, often waste, by treatment with sulfuric acid.

Safety

Lithopone is considered non-toxic due to the insolubility of its components. It has been used in medicine as a radiocontrast agent and is allowed to be in contact with foodstuffs in the US and Europe.

See also

References

Historical references

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Lithopone White Pigment Powder