Zinc oxide forms colorless hexagonal crystals or a white dusty powder. When heated, its color turns into lemon yellow changing into white again when cooled down. Zinc oxide has a density of 5.61 g/cm³. It evaporates at 1300°C and above and sublimates as it reaches a temperature of 1800°C. Hence, there is no (liquid) melt but a direct transition from the solid into the gaseous state of aggregation .
Occurence and Production
Zinc oxide (ZnO) occurs in nature as a coarse-grained mineral (zincite). Technically, it is obtained by oxidation of zinc or zinc vapor with atmospheric oxygen (zinc white) or by calcination of different components such as zinc hydroxide, zinc carbonate or zinc nitrate. Technical zinc oxides often contain lead. Such plumbiferous compounds must be marked with the corresponding hazard symbol (Xn) .
Zinc oxide has a very broad and versatile range of application including technical products, cosmetics, and pharmaceutical uses.
At present, zinc oxide is selling strongest in the rubber industry. Used as additive, it promotes the process of vulcanization in rubber that is used for tire manufacturing. In addition, its good conductivity improves the removal of heat that is generated during the churning motion of the tires. When added to cement, zinc oxide increases the water resistivity and prolongs the processing time. Zinc dust and “zinc white” are also used as pore fillers in surfacers and smoothing cements and as gray or white wall or artist’s paints.
Moreover, zinc oxide is used as catalyst in the chemical industry or for manufacturing cosmetic products. Zinc oxide together with ichors develops into antiseptic and astringent zinc salts, which is why it is used by the pharmaceutical industry for manufacturing zinc ointments, zinc pastes, adhesive tapes, and bandages for skin and wound treatment.
There are different electronic components, for example piezo-electric converters, transparent conducting oxides, sensors, luminous diodes, and optoelectronic or spintronic components, that at present are barely conceivable without zinc oxide. Also, zinc oxide-based semiconductors are used as transparent conductive layers in blue light-emitting diodes, liquid-crystal screens, varistors, and thin-film solar cells.
Nanoparticular zinc oxide is transparent in the visible range of the light spectrum and acts as a physical filter against the UV-B and particularly UV-A radiation of the sun. The fact that the ultraviolet rays are absorbed and are reflected like from small mirrors makes it interesting to use zinc oxide as a physical UV filter in sunscreens. Physical UV filters are mainly used in sunscreens with sun protection factors above 25. Unlike chemical UV filters that may trigger allergization, physical filters are suited for application to the sensitive skin of children and allergic persons. Chemical UV filters absorb UV radiation and convert it into heat.
The sizes of ZnO particles used for sunscreens are in the range of 20 to 60 nm. Before being added, these very small ZnO nanoparticles are coated with silicon or aluminum oxide. They then coalesce into aggregates sized 200 – 500 nm. Both, studies conducted by the industry and independent studies carried out within the EU project Nanoderm have shown that such particles contained in sunscreen products do not penetrate the body through the healthy skin, hence are not hazardous to the health of consumers [6,8].
Since zinc oxide nanoparticles are characterized by their UV-A and UV-B protection properties, their transparency, and antibacterial and fungistatic effects (see above), they are also used in textiles, clear varnishes for wood products and furniture, and in transparent plastics and plastic films (plastic glasses). The latter are characterized by a high transparency (> 90 % transmission) in the visible spectral range and UV permeability (< 10 % transmission) for wavelengths below 360 nm.