Addressing a pressing need to reduce ZnO level in rubber production and tire manufacturing

Rubber is a material used in many widespread applications as tires, tubes, footwear and gloves, glues thanks to its unique properties, as low hardness, high elasticity, and high elongation at break. However, despite the technological level of rubber materials and tires which represent the major application, the sustainability of their production, use, disposal, and reuse has become the main issue in the frame of a gradual but still far transformation of rubber materials lifecycle into a circular economy model.

For most applications, rubber undergoes a curing process to convert the raw sticky polymer into an elastic material by cross-linking the rubber chains. Among curing methods, vulcanisation is a consolidated process of the tyre industry to cure rubber with sulphur at high temperatures, improving its mechanical properties.

In the vulcanisation process, one environmental concern is related to the use of zinc oxide (ZnO), a fundamental and worldwide used curing activator, in conjunction with other additives as co-activator and accelerator, that increases the efficiency of the process by reducing reaction time, saving energy and costs of the process. Several studies demonstrated that zinc leaching occurs during the whole tire life, mainly in urban areas, due to tire tread consumption, leading to potential negative environmental effects associated with toxicity and cytotoxicity for the aquatic organisms. The amount of zinc released by tires in Europe is large, also considering that about 50% of the global ZnO annual production is used in the tire industries. For instance, at the end of the nineties, the amount of zinc that originated from tires was estimated about 150 tons per year only in Swedish and Great Britain. Thus, the reduction of ZnO level in rubber formulations has become an urgent issue.

EU project for more sustainable production of tires

The goal of SAFE-VULCA, funded by EIT RawMaterials, is to substitute or partially replace micro-crystalline ZnO, conventionally used in the industrial curing process, with a novel activator improving the efficiency of the curing process without compromising the rubber materials performance but reducing the total zinc content for more sustainable production of tires.

The novel activator ZnO-NP@SiO2-NP is constituted by ZnO nanoparticles anchored to silica particles, a common reinforcing filler utilised in rubber composite materials for tires. The novel activator may be described as a double function filler, behaving at the same time as a curing agent and reinforcing filler where the small sizes and the high dispersion of particles in the rubber matrix enhance their reactivity compare to conventional activator.

SAFE-VULCA deals with the upscaling of the production of the activator ZnO-NP@SiO2-NP, synthesised by a scalable and environmentally sustainable sol-gel procedure using water as a solvent, for a final application in the industrial compounding and vulcanisation processes to produce rubber material for tires with high performances, high mechanical behaviour and low energy dissipation.

Innovation project SAFE-VULCA is led by the University of Milano-Bicocca, Italy, and the consortium is made up of the partners Fraunhofer Institute, Germany; Radboud University, Netherlands; French Alternative Energies and Atomic Energy Commission – CEA, France; Monolithos Ltd, Greece; and the task partner Pirelli Tyre, Italy.