World-wide use of zinc oxide is in excess of 1.2 million tonnes annually.
Rubber is the most important application of zinc oxide. It has been used in this capacity for over a century. Zinc oxide along with Stearic acid activates sulfur crosslinking of rubber. In addition it provides pigmentary properties which enhance the ability to absorb frictional heat an important property in tire performance. The level of zinc oxide use is expressed in terms of ‘parts per hundred of rubber, phr’. The usual level is 3-5 phr. Essentially all rubber goods contain zinc oxide.
The ease of incorporation of the zinc oxide into rubber compound can be improved by coating the particles with propionic acid. The acid binds to the zinc oxide surface and the resulting hydrocarbon chain is more compatible with the rubber compound.
The important properties of zinc oxide that are relevant to rubber are:
- Low oversize – to prevent point defects in the compound.
- Particle size – to provide the required level of reactivity. Note particle size is usually expressed as the inverse measurement, surface area.
- Purity – Some elements, for example manganese, are detrimental to rubber curing at very low levels.
- Soluble salts – Soluble salts would be expected to reduce the resistance of rubber to degradation.