In the tire industry, cobalt salts and resorcinol resins are commonly part of mixtures for steel cord adhesion. However, regarding sustainability, work safety and environmental issues, these compounds have been increasingly considered as critical in recent years. Within the scope of a joint research cooperation, Brüggemann and Allnex developed a product that allows the elimination of both cobalt and resorcinol from steel cord adhesion mixtures.
S. Mark, Ü. Özkütükcü, R. Blanco-Trillo, R. Schäfer
A significant part of modern tires today consists of special mixtures that ensure the best possible adhesion to steel or textile cord – and thus give a tire all necessary properties required for its application on the street. The performance profile of a tire strongly depends on the strength and durability of such bonds between cord and rubber matrix. Due to the naturally low tendency of steel to adhere to rubber, adhesion promotors are used. State-of-the art adhesion promotors are resorcinol or resorcinol condensates that are combined with a methylene donor as crosslinker – such as hexamethoxy methylmelamine (HMMM). This combination provides a resin network that contributes significantly to both mechanical and adhesion properties of the rubber. Cobalt and cobalt salts, on the other hand, accelerate the vulcanization and contribute to the adhesion mechanism, which results in a positive impact on aging characteristics. Thus, resorcinol and cobalt play an important role in the production and use of a tire.
However, resorcinol was placed on the ECHA (European Chemicals Agency) CoRAP list („community rolling action plan“), first by France in 2016, and again in 2019 by Finland. In spring 2020, resorcinol was proposed for inclusion on the so-called SVHC list („substance of very high concern“). The reason was, and is, its potential mode of action as an endocrine disruptor. Substances identified as SVHC substances must be classified accordingly – and the pressure to find suitable replacement products increases significantly. In addition, the physical inclination of resorcinol to sublimate represents a high potential for exposure during application.
Cobalt and its salts are considered similarly critical today. Several factors come together that make cobalt compounds increasingly unattractive for applications in tires in the future. Metallic cobalt and many cobalt salts are considered to be toxic to reproduction and are suspected of having carcinogenic effects. The largest mining areas of cobalt are located in developing countries and areas of conflict, that are monitored very critically regarding prevailing working conditions and health-, work- and environmental standards. Besides, the demand for cobalt is also increasing due to the dramatically increasing demand for batteries for electric cars – and thus the availability of this raw material for the tire industry decreases, while at the same time market prices are rising.
These are reasons for the efforts to reduce or even eliminate both components from tire mixtures. To actually achieve this, it is crucial to provide a replacement material that is able to meet the high requirements. Vulcanization speed, mechanical properties, adhesion as well as aging characteristics have to be taken into account. Furthermore, dosage form, processability, toxicological and environmental issues and sustainability must be considered to provide a suitable alternative to state-of-the art systems. This article is the result of a joint research collaboration of Brüggemann and Allnex to introduce a potential approach to solve this challenge.
The first marketable product has now been a ready-to-use masterbatch in granulate form. This masterbatch is based on a novolak-containing, resorcinol- and cobaltfree resin system, which was doped with activated zinc ions. Current theory is that this special combination of a resin with highly active zinc achieves high mechanical strength on one hand, while maintaining the adhesion to steel. On the other hand, aging processes can be slowed down due to the high availability of activated zinc.
Thus, all crucial properties of a tire adhesion mixture, which until now have been determined by resorcinol and cobalt, are considered. A positive side effect of the newly developed product is that the use of ZnO is no longer necessary – the zinc contained in the masterbatch also acts as a vulcanization activator. In addition to the elimination of cobalt and resorcinol, it is also possible to significantly reduce the amount of heavy metal zinc needed in a tire mixture.
S. Mark, Ü. Özkütükcü, R. Blanco-Trillo, R. Schäfer, RFP Rubber Fibres Plastics, 03 2020, 132-135.