Zinc oxide is a unique material that has been used as a pigment for thousands of years. In its pure form, zinc oxide is a fine white powder. But when used as a pigment, zinc oxide can produce a wide range of colors depending on factors like particle size, addition of other compounds, and more.
What is Zinc Oxide?
Zinc oxide is an inorganic compound with the chemical formula ZnO. It is primarily composed of the mineral zincite, though commercial zinc oxide also contains small amounts of related minerals. Some key facts about zinc oxide are:
- Zinc oxide occurs naturally as the mineral zincite, though most zinc oxide today is synthesized commercially.
- It is an amphoteric oxide, meaning it can react as both an acid and a base.
- Zinc oxide has a hexagonal crystalline structure.
- It is a semiconductor with a wide bandgap of ~3.3 eV.
- Zinc oxide is insoluble in water.
- It has a high melting point of 1975°C.
Some major uses of zinc oxide include applications as a pigment, in sunscreens, in cosmetics, as an activator in rubber production, and as a varistor in electronics. Its versatility arises from its unique physical and chemical properties that depend on the manufacturing process.
Zinc Oxide as a Pigment
The use of zinc oxide as a pigment dates back to ancient times. Prehistoric artists used zinc ores in paints, likely without realizing the chemical identity. Production of zinc oxide pigments on an industrial scale began in the 19th century after commercial processes were developed. Today, zinc oxide remains one of the most widely used white pigments in coatings and plastics.
As a pigment, zinc oxide offers many desirable properties:
- High hiding power and tinting strength
- Low moisture sensitivity
- Good chemical and heat stability
- Non-toxic and environmentally safe
- UV absorbance
The hiding power of zinc oxide arises from its high refractive index, which causes high light scattering. This allows it to conceal underlying colors with less pigment. Zinc oxide can produce white colors in paints, coatings, plastics, ceramics, foods, and other applications. It is also commonly used as a base for colored pigments.
Production Process for Pigmentary Zinc Oxide
Zinc oxide for use as pigment is produced through one of two main processes:
- French process – Zinc metal is melted and vaporized at high temperature (1000+ °C). The zinc vapor reacts with oxygen in the air to produce fine particles of ZnO, which are collected. This produces small and uniform particles desirable for pigments.
- American process – Zinc ores are milled and then roasted at around 600°C to produce ZnO powder directly. The particles are more coarse than the French process. This is cheaper but produces lower quality pigmentary zinc oxide.
The production process influences properties like particle size, purity, and color. Desired pigment properties can be achieved through careful control of the production conditions.
Factors Affecting Zinc Oxide Pigment Color
As discussed above, pure zinc oxide is white in color. So how can it produce other colors? There are several factors that affect the color produced by zinc oxide pigments:
Particle Size
The primary factor affecting color is particle size. As shown in the table below, smaller zinc oxide particles tend to scatter light, producing white colors. Larger particles result in light absorption and scattering, causing other colors:
| Particle Size | Color |
|---|---|
| 0.2-0.3 μm | White |
| 0.3-0.5 μm | Pale yellow |
| > 1 μm | Bright yellow/orange |
By controlling factors like temperature and vapor residence time, the production process can be tailored to produce zinc oxide pigments with desired particle sizes and colors.
Impurities
Impurities present can also impact the color. Transition metals like manganese, iron, cobalt, and copper can produce yellow/brown shades. Rare earth metals such as cerium may give pale pink colors. The table below shows some impurity effects:
| Impurity | Effect on Color |
|---|---|
| Iron | Yellowish |
| Cobalt | Greenish |
| Copper | Reddish brown |
| Manganese | Orange |
| Cerium | Pale pink |
While impurities are sometimes added intentionally to produce colors, minimizing impurities also allows whiter and brighter zinc oxide pigments.
Surface Treatments
Coating the surface of zinc oxide particles provides another route for modifying color. Surface treatments change light scattering properties and can improve pigment dispersion. Some examples include:
- Silica – Can reduce photocatalytic activity and give opacity.
- Alumina – Enhances durability and chemical resistance.
- Organic coatings – Improve dispersion and compatibility in organic media.
Doping
Finally, intentional doping with other metals provides a controlled approach to tuning zinc oxide color. Transition metals like manganese and iron can be added to produce yellow, orange, red, green, or brown hues. The concentration of dopant determines the final color. For example, low levels of manganese dopants result in pale yellow while higher levels give an orange color. Use of multiple dopant metals allows a wide gamut of colors.
Applications of Colored Zinc Oxide Pigments
Thanks to this range of colors, zinc oxide pigments see diverse uses. Some common applications of colored zinc oxide pigments include:
Paints & Coatings
Colored zinc oxide provides weather and fade resistant colors for interior and exterior paints and other protective coatings. Orange/yellow zinc oxide can provide corrosion inhibition as a primer pigment. The UV absorbance also improves coating durability.
Plastics
Zinc oxide dispersions are used as colorants and fillers in plastics such as PVC and polyethylene. Colored zinc oxide allows visually appealing consumer products while protecting the plastic from UV degradation.
Ceramics
The high heat stability suits zinc oxide for coloring ceramic glazes. Zinc oxide produces colors ranging from white, yellow, orange, green, red, to black in glazes. The color depends on furnace atmosphere, temperature, and other mineral content.
Foods & Cosmetics
Zinc oxide can add white or colored hues to foods like candy and icing. Its non-toxic properties also allow use in cosmetic products including makeup, lip balm, and sunscreen.
Rubber & Other Polymers
When compounded into polymers like rubber and adhesives, colored zinc oxide provides coloration and enhances mechanical properties through reinforcement and crosslinking.
Electronic Ceramics
Doped zinc oxide ceramics have semiconducting and piezoelectric properties useful for electronics applications. Colored zinc oxide ceramics are used in varistors, sensors, energy generators, and more.
Conclusion
While pure zinc oxide is white, it can produce a wide palette as a pigment. By controlling properties like particle size, impurities, and doping levels, yellows, oranges, reds, greens, and other colors can be achieved from this single material. The colorfulness and durability of zinc oxide has allowed it to be used as a pigment for thousands of years and its importance continues today across many modern applications.
