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ABSTRACT
Concern about the release of eco-toxic zinc species from rubbers into the environment leads to an increasing interest in potential substitutes. Although alternative metal oxides and zinc compounds as activators for sulfur vulcanization have been studied thoroughly, at present no viable alternative has been found to eliminate ZnO completely from rubber compounds, without significantly jeopardizing processing as well as performance characteristics.
In this paper, the application of a new activator for sulfur vulcanization will be discussed. This activator is developed based on the assumption that an increase in the availability of Zn^sup 2+^-ions could lead to a considerable reduction of ZnO in rubber compounds. Montmorillonite clay was used as carrier material and loaded with Zn^sup 2+^-ions via an ionexchange process.
Application in a wide range of natural and synthetic rubbers has been explored. Results clearly demonstrate that this Zn-Clay can substitute conventional ZnO, retaining the curing and physical properties of the rubber products but reducing the zinc concentration with a factor 10 to 20. Model Compound Vulcanization studies have been used to gain an insight into the mechanism of this activator.
It can be concluded that systems with Zn^sup 2+^-ions on a support represent a new and novel route to reduce the zinc level, and therefore to minimize its environmental impact significantly.
INTRODUCTION
In view of the increased attitude of protecting the environment and the upcoming legislations regarding the use and application of Zinc Oxide and Zinc containing compounds, the demand for alternative products that entail the least amount of environmental pollution impact has increased. To meet ecolabeling requirements for vehicle tires, for instance, it is desirable to keep the Zinc Oxide content as low as possible.1 This not only leads to a lower pollution impact but is also advantageous from an economical point of view.
Although zinc is considered one of the least harmful amongst heavy metals, soluble zinc compounds are classified as eco-toxic to aquatic organisms. Release of zinc into the environment from rubber occurs during production, during disposal and recycling of rubber products, for instance through leaching in land-fill sites. Also during service conditions, e.g. through wear of tires.
Another reason to search for possibilities to reduce the Zinc content in rubber vulcanizates is associated with the problem related...