German chemical firm Evonik says it wants to help make rubber materials from scrap tyres easier to re-use in the manufacture of new automobile tyres. So far, the suitability of recycled rubber has been very limited because its chemical structure hampers interaction with new tyre materials.
A team of researchers at Evonik has now made a key step forward with a process that could make it possible to use up to four times as much recycled rubber in new tyres as in the past.
New tyre rubber is normally produced by vulcanisation of raw rubber, sulphur, and other components. When heat and pressure are applied, the sulfur forms bonds with the long carbon chains in the rubber, resulting in a robust, three-dimensional network. That is also the structure of ground tyre rubber from end-of-life-tyres.
However, since the material has already been vulcanised, its properties differ from those of non-vulcanised rubber. Currently, trade associations in the tyre recycling sector therefore set an admixture of about 5% ground tyre rubber from end-of-life-tyres as the upper limit.
Consequently, only small amounts of recycled rubber powder are re-used in the manufacture of new automobile tyres at present.
The majority of the recycled rubber is used, for example, in the production of protective elements for playgrounds and running tracks. In addition, many end-of-life-tyres still undergo thermal reprocessing as fuels for energy generation.
Evonik’s team of researchers have now succeeded in reversing the vulcanisation of rubber to a large extent. “By adding a special formulation containing vinyl silanes, the firm bonds in the recycled rubber can be split. We cleave the sulphur bridges in the rubber, yet leave as many of the long carbon chains as possible untouched,” explains the firm.
The research team has already used these vinyl silanes successfully for devulcanisation. In trials, the proportion of recyclate in the rubber blend could be increased to up to 20% —compared with the technical threshold of around 5% outlined above.
Evonik is now heading for the next milestones with test series and trials. The aim is to find a solution that customers could use in industrial production in the foreseeable future.
The sustainability benefits of a circular solution of this type would be tremendous: Globally, there are more than 1.3 billion passenger cars. More than 2,000 new tyres are produced every minute. There is no sign that demand will end—because even electric and hydrogen-powered automobiles will use rubber tyres in the future.
Annual global sales of tyres for passenger cars amount to over EUR100 billion. Evonik’s development project also supports the wide-ranging sustainability initiatives of tire manufacturers.