The rubber industry in general, and tyres manufacturers in particular, are the largest users of zinc oxide, owing to its important chemical, physical and optical properties.
Since heat build-up is critical at the higher operating speeds of heavy-duty pneumatic tyres (as compared with solid rubber tyres) they carry high loadings of zinc oxide for heat conductivity, as well as for reinforcement.
The important role played by zinc oxide in the rubber industry is expressed in various processes:
- Activation In the curing process of natural rubber and most types of synthetic rubbers, the chemical reactivity of zinc oxide is utilized to activate the organic accelerator. The unreacted share of the zinc oxide remains available as a basic reserve to neutralize the Sulphur bearing acidic decomposition products formed during vulcanization. Adequate levels of zinc oxide distinctly contribute to chemical reinforcement, scorch control and resistance to heat aging and compression fatigue.
- Acceleration Zinc oxide serves as the accelerator with some types of elastomer. The cross-linking that it induces takes several forms. With some systems, zinc oxide serves as an effective co-accelerator in the vulcanization process.
- Biochemical activity Zinc oxide is useful in the preservation of plantation latex, as it reacts with the enzyme responsible for decomposition. The oxide also inhibits the growth of fungi, such as mold and mildew.
- Dielectric strength In high voltage wire and cable insulation, zinc oxide improves the resistance to corona effects owing to its dielectric strength. At elevated operating temperatures it contributes to maintaining the physical properties of the rubber compound by neutralizing the acidic decomposition product.
- Heat stabilization Zinc oxide retards the devulcanization of numerous types of rubber compounds operating at elevated temperatures.
- Latex gelation Zinc oxide is particularly effective in the gelation of the foam, with sufficient stability, as part of the production process of latex foam rubber products.
- Light stabilization Zinc oxide's absorption of ultraviolet rays is exceptional among white pigments and extenders. It therefore serves as an effective stabilizer of white and tinted rubber compounds under prolonged exposure to the sun's destructive rays.
- Pigmentation Zinc oxide provides a high degree of whiteness and tinting strength for rubber products such as tyre sidewalls, sheeting and surgical gloves, owing to its high brightness, refractive index and optimum particle size.
- Reinforcement Zinc oxide provides reinforcement in natural rubber, as well as in some synthetic elastomers, such as polysulfides and chloroprene's. The degree of reinforcement appears to depend upon a combination of the oxide's particle size, with the finest size being the most effective; and the oxide's reactivity with the rubber. Under conditions characterized by rapid flexing or compression, zinc oxide also provides heat conduction to enable more rapid heat dissipation, thereby providing lower operating temperatures. Additionally, it imparts heat stabilization by reacting with acidic decomposition products.
- Rubber - metal bonding In the bonding of rubber to brass, zinc oxide reacts with copper oxide on the brass surface, forming a tightly adhering zinc-copper salt.
- Tack retention One of the unique properties of zinc oxide is its ability to retain over many months of shelf -storage the tack of uncured rubber compounds for adhesive tapes. French process zinc oxides impart heat-aging resistance superior to that of American-process zinc oxides. The former type, being sulfur-free, has a higher pH and, thus, can neutralize more effectively the acidic decomposition products formed during aging. Moreover, the finer French-process zinc oxides prove superior to coarser grades in heat-aging resistance