New approaches in the recycling of old tyres

Every year, more than 1.5 billion old car tyres are taken out of service around the world. If they were stacked on top of each other, the pile of tyres would reach from the earth to the moon – about 380,000 kilometres. Up to now, one third of old tyres in Germany have been incinerated to recover energy. Scientists from Chemnitz Technical University decided to look for a more environmentally sound way to recycle elastomers from old tyres.

What can be done with all the many old tyres? There are limits to the number of times they can be retreaded. At some point, they are so worn out that the solution in the past has often been to throw them into an incinerator. That at least is the fate of most old tyres. Although it would be desirable to put what were originally high-tech products to a more upscale use.

Talking about high-tech products: a conventional car tyre needs to withstand many different stresses. It is supposed to provide safe and reliable grip not only on loose gravel but also on hot or wet asphalt; it is supposed to do its job safely when temperatures are both extremely high and extremely low, without losing too much tread or producing noise, and, last but not least, it is expected to survive as undamaged as possible the tough treatment it has to take when it hits hard curbs or is subjected to reckless braking manoeuvres.

A modern car tyre really is a high-tech product – even if this is not obvious at first glance – and is, one has to add to present the full picture, not comparable by any means to early car tyres (LINK). A car tyre is a combination of vulcanised rubber, textiles and steel. Nowadays, natural rubber and such kinds of synthetic rubber as styrene-butadiene rubber (SBR) and polybutadiene (PB) are mainly used to make the rubber. They are finished and made roadworthy by the addition of up to 200 different fillers and additives (LINK). While the main components of tyre rubber are generally well-known, tyre manufacturers keep their tyre formulation and manufacturing process very much to themselves. After all, no-one wants to give the secret of his success away to the competition.

Since practical use of a tyre on the road requires the use of a combination of so many different materials and substances, manufacturers have come up with numerous formulations for specific models. This leads to major problems in material recycling, because it is not possible to separate and sort them properly. Elastomers are produced from a carefully chosen blend of ingredients and they react sensitively. Dissimilar constituents of a blend can easily lead to unintentional interaction. This explains why only very small amounts of recycled material – if any – are used to manufacture new tyres, while such materials are only used for uncritical components like the tread.

It makes little sense to dispose of old tyres on landfill sites, however, and this has been banned in the EU since 2006 anyway – how big a mountain of tyres do we want there to be? The option that remains is therefore incineration with energy recovery, which does make sense although it is not exactly ideal. Quite apart from the fact that the elimination of the provision about energetic value in German recycling legislation (§ 8, Para. 3) as per 01.06.2017 means that incineration is no longer a straightforward option either. So let us take another intensive look at the potential of material recycling. Which is exactly what scientists from the Institute for Lightweight Structures at Chemnitz Technical University have done – and have come up with some very interesting ideas for possible solutions. The experts from the extrusion technologies and recycling department say that their approach is a solution that is both smart and efficient.

Up to now, if old tyres have been recycled to produce new materials – which is been the case with about every third old tyre – they have generally been chopped up into relatively coarse granulates and have been compressed into floor and protection mats or rubber layers for automotive manufacturing applications following the incorporation of binding agents. These granulates can be used just as effectively for the production of artificial turf for sports pitches and playgrounds. But that is all. The scientists at Chemnitz Technical University thought that this was not enough and are focussing on high-quality and highly efficient new materials. More sophisticated materials, if you like, and ones that require different treatment.

The size and fineness of the granulate are the key. What the scientists headed by Dr Stefan Hoyer, a member of the scientific staff at the Institute for Lightweight Structures at Chemnitz Technical University, aimed to achieve was therefore to grind old tyre rubber up into extremely fine powder and then to process it blended with thermoplastics. It has become apparent that the thermoplastic-elastomer compounds manufactured by this process can be melted down and turned into complex components by the injection moulding process. Chemnitz Technical University writes in a press release that It is not obvious from the products that are created in this way that they were once tyres.

Stefan Hoyer and his colleagues developed a completely new processing technology for their recycling solution: single-stage direct extrusion. The special thing about this is the combination of the compounding and profile extrusion, i.e. the blending of the original materials and the shaping of them. “We eliminate one stage in the process, reduce energy consumption and protect the material from thermal damage”, says Stefan Hoyer. It is said that endlessly long, high-quality mats for wear protection and soundproofing can be produced in this way. Hoyer and his colleagues are so confident about the quality and promise of their research that they are already thinking about starting series production: “At the moment we are still in the market launch phase, but we are already co-operating with a company from the region.”

In line with the motto that one solution is not a solution, the scientists from Chemnitz adopted a second approach too. In this context, Stefan Hoyer and his colleagues focussed on improvement of the technology, which is now in operation on an international scale: the goal was to optimise the recycling of technical elastomers, says Stefan Hoyer. Let us take a look at the details:

Technical elastomers are rubber materials that are used for many different applications, e.g. in the form of sealing rings. In most cases, production of them leads to a large amount of waste, which is expensive to dispose of. Their method aims to process these residual materials in such a way that they can be fed back into the manufacturing process very easily, Hoyer explains. The recycled material, which consists of a single substance in most cases, is simply mixed in with the virgin materials again and is processed with them.

The recycled material is the fine powder that is produced from the residue left over from the manufacturing process and normally has dimensions of between 600 and 200 µ. It is ground in what is known as a “reactruder” involving the application of high mechanical stress. Advantage is taken in this context of what is known in Formula 1 car racing as “graining”:

Graining is a consequence of excessive racing car tyre stress. The tyre rubber on slicks in particular is unable to maintain its structure due to drive and/or cornering forces and comes off. The shredded rubber sticks together and to the tyre tread. Graining reduces the tyre contact area and thus changes the surface temperature of the tyre. Tyre temperature is the crucial factor in determining grip levels.

Although hot grinding extrusion technology is not new, Hoyer and his colleagues have developed and optimised it. They report that one of the objectives has been to simplify cleaning, to make structures more compact and to reduce wear and tear. Stefan Hoyer is convinced that their process makes it possible for the first time to recycle even the smallest of batches economically in a pure, sorted form. While succeeding in cutting the energy consumption of the process by about 60 per cent into the bargain.

Hoyer and his colleagues were thinking of potentially interested users when they were doing their work: their technology, it is claimed, gives small and medium-sized companies in particular the chance to recycle residual materials incurred in-house on an independent basis. Stefan Hoyer says that their method represents an efficient tool for the recycling of elastomers, with which original raw materials can be used and recycled sustainably. Savings in energy requirements, disposal costs, resources and CO₂ emissions are further positive effects.

Stefan Hoyer points out that this is not yet the end of the road, however: “We have a lot more planned.” One of the hopes of the scientists at Chemnitz Technical University is that they will be able to obtain funding from the German Research Foundation (DFG), in order to continue improving the formulations for thermoplastic-elastomer compounds in the context of the German cluster of excellence MERGE. The scientists are also looking at the work on thermosetting plastics and other elastomer composites. In order to implement sustainable principles in practice in the rubber processing field as well, the Chemnitz research scientists have also established the corporate network  „ElastoTech“ (sustainable technologies for highly elastic polymer composites. [http://www.elastotech-netwerk.de]. To Stefan Hoyer, this confirms the objective that the scientists at the Institute for Lightweight Structures at Chemnitz Technical University are trying to achieve: “to develop and characterise new materials and to develop efficient processing technologies for practical applications.” k-online wishes them great success. Guido Deußing

[1] Hoyer S., Kroll L., Nendel, W. et al.: Werkstoffliches Recycling von Elastomeren in Theorie und Praxis. In: GAK Gummi Fasern Kunststoff, 67. Jahrgang (2014), Nr. 12, S. 752–761

[2] Hoyer, S.: Neuartige Warmmahltechnologie zum Recycling von Elastomeren und Analyse prozessbedingter Eigenschaften. Chemnitz, Technische Universität Chemnitz, Institut für Strukturleichtbau, Dissertation, 2014. Dr. Hut Verlag, München, 2015; ISBN 9783843921275