Zinc oxide reduction becoming a necessity.

For many of us who have developed rubber compounds over the years, it has been 'automatic' to use 5 phr of zinc oxide and 2 phr of stearic acid as the activator for accelerated sulfur cure systems. Among the many variables in compounding, this activator system was known to work well in most compounds. Now, we are being faced with an initiative to re-design those compounds to reduce or eliminate zinc content because of environmental concerns. Elevated concentrations of zinc in water are known to be toxic to aquatic organisms, in particular many species of algae, crustaceans and salmonids. Although zinc is a common natural element that is abundant in the Earth's crust, the human influence is causing alarmingly increased concentrations in waterways, ponds and lakes, and this is altering the balance of the ecosystem.

Many of the processing aids used in rubber compounds are zinc salts of fatty acids. Some zinc-free alternatives for these have been introduced (ref. 1). Finding a suitable replacement for zinc oxide in sulfur cure systems will be more difficult. Researchers at the University of Twente in the Netherlands have published a number of papers on their investigation of zinc oxide alternatives and zinc reduction. This work is discussed in detail in the thesis of Dr. Geert Heideman that is available online. In the summary at the end of chapter two of his thesis, Dr. Heideman states: "The chances of finding a completely zinc-free curing system must be regarded as rather small, and would involve a fundamental change in the practice, as well as the chemistry of sulphur vulcanization." However, some potential options for significantly reducing the zinc concentration are presented that appear to be feasible as shorter-term developments (ref. 2). Their studies also indicate that the zinc oxide level can be reduced by about 50% in many compounds without adverse effects.

The largest usage of zinc is in anti-corrosion protective coatings for metals. Galvanized metal sheets have been a popular building and roofing material for industrial buildings and storage sheds throughout the world for many years. Galvanized water pipes, storage tanks and galvanized nails are also exposed to water erosion on a continuing basis. These galvanized metal products are undoubtedly primary contributors to increasing concentrations of zinc in the aquatic environment. Other common uses of zinc include batteries, dietary supplements, animal feed supplements, brass and other metal alloys. Zinc oxide is used as a pigment in paints and coatings, in ointments and lotions for sunburn protection and other cosmetics, as a cement additive, in batteries and photocells, and a number of other uses.

With this abundance of zinc-containing products in common use, it is hard to believe that eliminating zinc oxide from rubber compounds would have a measurable impact on the ecosystem. The typical 5 phr level of zinc oxide would be about 2 to 3% by weight of the compound, and only about 0.5 to 0.6% by volume. Only a small fraction of the zinc near the surface of a rubber part would be susceptible to leaching by water. Rubber parts that remain intact throughout their life cycle and disposal would not contribute any significant amount of zinc contamination. However, there is one rubber application that is a known contributor to increasing zinc levels. The rubber dust from worn tires that becomes airborne or collects along the side of roads and highways is a concern due to the high surface area of the finely divided rubber powder and its ready access in drainage and runoff streams. Zinc reduction should be a priority for tire tread compounds. For most other rubber compounds--probably not.

References

(1.) K. Menting, J. Bertrand, M. Hensel and H. Umland, Rubber World, Vol. 229, No. 2, November, 2003.

(2.) G. Heideman, J. W.M. Noordermeer, R.N. Datta and B. van Baarle, Rubber Chem. & Tech., 77, p. 336 (2004).

John Byers

Technical Editor