Compounds used in making tires - and rubber products in general - are made from many different elements, all of which are necessary to improve the qualities of the tires. Among them, an increasingly important role is being played by silica, which is now produced in sustainable fashion.
The intrinsic mechanical properties of elastomers in rubbers, whether natural or synthetic, are not sufficient to create products with adequate mechanical properties. This is why many other additives are being added to rubbers and, among these, silica - more precisely silicon dioxide (SiO2) the production of which is being made increasingly sustainable - is gaining greater importance. Silica has many applications in a variety of industries, for example, it keeps skin, hair and nails healthy, greatly improves the performance of cement materials and, most interestingly, makes an important contribution to improving the mechanical properties of elastomers, making them suitable for the production of many goods. Several additives are used in tire manufacturing - curing accelerators, antioxidants, antiozonants, homogenisers - but silica plays a very important role because it is able to better 'fill' the ‘magic triangle’.
Reconciling what is incompatible
In fact, grip, mechanical resistance, and low rolling resistance conflict with each other. A tire that grips well to the road owes this to the fact that the compound sticks to the road surface, but this will increase attrition and therefore rolling resistance. The same goes for wear resistance: the tougher the compound the greater the wear resistance but this will inevitably conflict with grip. Silica allows these characteristics to be improved simultaneously, which is a solution that all manufacturers welcome. The qualities of silica also depend on its 'appearance': being in smooth rather than corrugated granules, for example, changes its ability to be dispersed - that is, its ability to distribute itself evenly throughout the compound - and its interactions with the polymer chains of the elastomer.
Sustainability in silica production
The effectiveness of adding silica to compounds is now beyond question, and the development of this filler is increasingly focusing on sustainability in order to further reduce the environmental impact of tire production. Think of silanes: these components are used to improve the mechanical properties of rubber compounds. Their use, however, introduces other chemicals into the tire, increasing its environmental impact, which is why PPG has developed the Highly Dispersible Silica Agilon range. This silica promises not to require the use of silanes, which saves a production step and reduces emissions of VOCs - Volatile Organic Compounds associated with silanes, which contain sulphur. However, further experiments are being made in an effort to increase the sustainability of silica itself, for which a chlorine-containing agent, silicon tetrachloride, which is both toxic and corrosive, is used.
Companies investing in sustainable silica
Although silica is very common in nature – easily found in several sedimentary rocks (such as sand, radiolarites, quartz arenites, flint etc.) - obtaining it in its pure state implies, as seen above, materials that are not exactly friendly. The need to mitigate the environmental impact of silica production had already been made explicit, for example, by Pirelli in 2015. the company had in fact developed, in its Brazilian plant in Meleiro, a process to extract silica from rice husks. This is the outer husk of the grain, worth 20% of the weight of raw rice and is the main waste product of this crop. The husk, which contains 18% silica by weight, is also used to produce the energy needed for the process at low temperature due to its high heat value. This production process not only uses fewer polluting elements but is energy self-sufficient and allows for a significant reduction in carbon dioxide emissions because husk is a biomass and its combustion releases atmospheric CO2 fixed in the plant without extracting additional carbon from the subsoil.
Sustainable silica for Michelin and Kumho
Returning to the present day, Michelin presented two road tires, bus and car, that contain 45% and 58% respectively of sustainable materials such as natural rubber, vegetable oils, butadiene from biomass, recycled steel and silica from rice husks. Carbon black - supplied by Scandinavian Enviro Systems, a subsidiary of the French Group - comes from the recycling of end-of-life tire casings. Michelin aims to reach an average of 40 per cent sustainable materials in all its tire lines by 2030 and reach 100 per cent in 2050. Khumo Petrochemical took the same path with the use of ecologically sustainable bio-silica, extracted from rice husks, to be used in the company's high-performance synthetic tires. This method will reduce carbon dioxide emissions by up to 70% compared to previous silica production methods, which was extracted from quartz.
Goodyear's proposal and Evonik's agreements
Earlier this year, Goodyear unveiled a prototype with 70 per cent of the material used being low environmental impact as they are made from renewable raw materials, recycled or produced by capturing atmospheric CO2. The prototype is another step on the path that will lead the US group to introduce a tire made entirely of sustainable materials by 2030.
Thirteen 'green' ingredients are used in nine different tire components with three different types of carbon black obtained from methane, carbon dioxide and vegetable oil instead of burning oil. The flexibility of the compound is the result of using soybean oil, while the silica is produced from rice husk ash; the plies are made of polyester from recycling bottles and other PET-based waste. Equally noteworthy is the strategic agreement between silica producer Evonik, the Pörner Group and Phichit Bio Power to supply sustainable Ultrasil silica to various tire manufacturers. Ultrasil is produced, using green energy, from sodium silicate made from rice husk ash. Phichit Bio Power, which owns rice mills and biomass power plants, is a licensee of Pörner's technology for the production of bio-silica from sodium silicates derived from rice husk ash. The push for sustainability often comes from customers, and therefore, as Emmanuel Auer, head of the Silica business line at Evonik, puts it, “for the first time we can offer our customers an Ultrasil silica with an improved sustainability profile, thus supporting their goals of circularity and reduced carbon footprint”.
