Zinc Oxide

OVERVIEW

Zinc oxide (ZINC OX-side) is a white to gray to yellowish powder with no odor but a bitter taste. It occurs in nature as the mineral zincite. Zinc oxide is perhaps best known as an ingredient in cosmetics, personal care products, medicines, and sunscreens. In terms of volume, however, a number of industrial applications are of greater importance.

HOW IT IS MADE

Three commercial methods of preparation are available for zinc oxide. Two of these methods are known as thermal procedures because they involve heating either zinc metal or an ore of zinc to obtain zinc vapor. The zinc vapor is then reacted with oxygen to produce zinc oxide. The formula for this reaction is 2Zn + O₂ → 2ZnO

KEY FACTS

OTHER NAMES:

Chinese white; philosopher's wool; zinc white; flowers of zinc

FORMULA:

ZnO

ELEMENTS:

Zinc, oxygen

COMPOUND TYPE:

Metallic oxide (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

81.39 g/mol

MELTING POINT:

1974°C (3585°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Insoluble in water; soluble in dilute acids

The third method is called a "wet" procedure because it takes place in aqueous solution. In this process, zinc compounds are leached out of ores and converted to basic zinc carbonate (ZnCO₃·2Zn(OH)₂). Leaching is a process by which soluble salts are dissolved out of soil, ore, or some other material. The basic zinc carbonate is decomposed by heat, yielding pure zinc oxide:

ZnCO₃·2Zn(OH)₂ → 3ZnO + CO₂ + 2H₂O

COMMON USES AND POTENTIAL HAZARDS

One of zinc oxide's most important uses historically is in paints and pigments. Common names by which it is now known, such as Chinese white and zinc white, are terms long used by artists for the compound. White paints and pigments made with zinc oxide retain their luster and purity longer than most other types of white paints, partly because zinc oxide does not react readily with hydrogen sulfide in air that causes most white paints to darken.

One of zinc oxide's most important properties is its ability to absorb ultraviolet (UV) light in sunlight. Because of this property, zinc oxide is often added to sunscreens and sunblocks to help protect a person from sunburn. The same property accounts for other important applications for zinc oxide, such as their use in rubber and plastic products. By absorbing ultraviolet light, zinc oxide protects the rubber or plastic from decomposing. The compound also has other applications in the rubber and plastic industries, including:

• In the curing of natural and synthetic rubber products, increasing the rate at which the product reaches its final chemical state;

• As a fungicidal additive to rubber and plastic, preventing fungi from attacking and destroying products made from those materials;
• To increase the temperature at which rubber and plastic products remain stable and the amount of exposure to light they can withstand;
• To maintain the proper acidic properties of a product, reducing the rate at which the are likely to decay;
• To provide additional strength to the rubber or plastic product.

Interesting Facts

• The term philosopher's wool dates to the period of alchemy when scholars often gave picturesque and descriptive names to the substances with which they worked. Philosopher's wool referred to the fluffy white material that was formed when alchemists (also known as natural philosophers) heated zinc in air. The substance was also referred to as nix alba, or "white snow" for the same reason.

Besides its major uses in the rubber and plastic industries, zinc oxide has a number of applications in other fields, such as:

• As an additive in glass and ceramic materials, to provide greater heat resistance, greater resistance to breakage by shock, and high luster;
• In the manufacture of specialized types of sealants and adhesives;
• As a light-gathering agent in photocopy machines;
• As an additive in lubricants for the purpose of reducing wear and helping the lubricant withstand high pressures;
• In smokestacks as an aid in removing sulfur dioxide and other pollutant gases produced in factory operations; and

• In the manufacture of specialized packaging materials, especially those used for food products, because of the compound's ability to kill certain microorganisms that cause food decay.

In moderate amounts, zinc oxide is a relatively harmless compound. Exposure to zinc oxide dust may cause respiratory problems, such as coughing, upper respiratory tract irritation, chills, fever, nausea, and vomiting.

Words to Know

ALCHEMY

An ancient field of study from which the modern science of chemistry evolved.

AQUEOUS SOLUTION

A solution that consists of some material dissolved in water.

FOR FURTHER INFORMATION

"Application." Nav Bahrat Metallic Oxides Industries. http://www.navbharat.co.in/clients.htm (accessed on November 19, 2005).

"Occupational Safety and Health Guideline for Zinc Oxide." Occupational Safety & Health Administration. http://www.osha.gov/SLTC/healthguidelines/zincoxide/recognition.html (accessed on November 19, 2005).

"Zinc Oxide." Center for Advanced Microstructures and Devices, Louisiana State University. http://www.camd.lsu.edu/msds/z/zinc_oxide.htm (accessed on November 19, 2005).

"Zinc Oxide Producers Association." http://www.cefic.be/Templates/shwAssocDetails.asp?NID=5&HID=25&ID=172 (accessed on November 19, 2005).

zinc ox·ide • n. an insoluble white solid, ZnO, used as a pigment and in medicinal ointments.

zinc oxide n. a mild astringent used in various skin conditions, usually mixed with other substances. It is applied as a cream, ointment, dusting powder, or as a paste, often in the form of an impregnated bandage.