Scrap tyres: the mother lode
Every time the price of raw materials shoots up, so does the price of virgin, natural and synthetic rubber. Scrap tyres could satisfy 45 % of European demand for virgin rubber, according to an estimate by the CRIOSINTER project. The project is opening up new prospects for the manufacture of products made from recycled tyres. Here is an overview.
When waste gains in value
Almost 3.5 million tonnes of old tyres are added each year to the already substantial European stockpile, of which disposal is difficult.
A third of these tyres were recycled last year, largely to make aggregate for use in road construction, quarry rehabilitation and other projects. Shredded and relieved of their textile and metallic components, the tyres are used to make synthetic grass for football and rugby pitches. They are also crushed and mixed to make flooring for playgrounds or moulded to form shopping trolley wheels and other items.
But all too often tyres end up in the landfill, despite the fact that European Directive 99/31/ EC outlawed this practice since 2006. "We can increase the proportion that is recycled," assures Valérie Shulman, Secretary General and co-founder of the European Tyre Recycling Association - ETRA (1). "In view of the technical requirements, tyres could be used as a substitute for virgin rubber in many products," she says. Naturally there is no question of replacing the incomparable strengths of natural latex in a product as sensitive as a tyre, but if old tyres can penetrate a segment of the rubber market (such as shoe soles, urban furniture or automobile accessories), they will gain in value.
An underrated material
Rubber is not actually recycled. To make tyres it is often mixed with some 200 ingredients before undergoing vulcanisation, an irreversible process that changes the rubber's molecular structure and endows it with the properties needed by the manufacturer: elasticity, shock absorption and sound-proofing.
Shredded used tyre material nevertheless retains a wide range of exploitable properties.
To convince a reluctant market to use this underrated material, the CRIOSINTER project has taken up the challenge of using 100 % recycled tyres to manufacture prototypes for three products representative of the sector: shoe soles (essentially an exercise in style for the would-be rubber substitute); road surfacing material with no added binding agent; and truck bumpers, a large-scale automobile part.
Three project partners are manufacturing the prototypes in their factories. With co-financing of more than half a million euros from the 6th Framework Programme over two years, CRIOSINTER ended in early 2008. "At first sight it did not seem possible to make these prototypes," recalls Tomás Zagora, a biomechanic and the project's technical coordinator at the Valencia Biomechanics Institute (Instituto de Biomecánica de Valencia, ES). "The properties of the materials from recycled tyres depend largely on a range of factors outside our control." The size of the tyres and their provenance govern the physical and chemical properties of the finished product. The way they are crushed (using traditional or innovative methods, mechanical techniques at ambient temperature, or cryogenic techniques at vitreous transition temperature) greatly influences the result.
Finally, the size of the particles is critical: tyres recycle differently when they are reduced to powder particles of around one-hundredth of a micrometre in size than when they are reduced to granules of one centimetre or more.
Which of these materials should be used to meet the demands of each particular manufacturer?
It is impossible to say without an indepth analysis.
The sole at the journey's end...
At this stage, the powders and granulates are sent to the laboratory for assessment. The project partners include three suppliers whose methods represent the diversity of industrial practices. "The morphology of the particles, granulometry, surface state, thermo-mechanical analyses and many other tests are used to build a complete characterisation of the materials," explains Tomás Zagora. Each supplier's products are used to create a prototype: "Using this 100 % recycled rubber and a few necessary additives, we moulded the soles of a sports shoe model targeted at the younger market," says José Ramón Sempere, the technical manager of one of the manufacturerpartners in the project, Analco Auxiliar Calzado S.A. (ES). "We used the normal moulding process, applying the right temperature and pressure to suit the material's specific properties. For mass production we would need to adapt the production process based on the data output from the project." The partners' optimisation work on the entire manufacturing process, including moulding the soles, is now bearing fruit. In the final stage, an expert system inputs the manufacturer's three specified mechanical properties for its product, weighted by the respective importance of these properties. The system then outputs the optimum choice of material, the right composition of mix to inject, and the temperature and duration of the moulding process for manufacturing the item. "To our knowledge, no prototype has ever been manufactured by this process using 100% rubber from recycled tyres, although there have been some preliminary studies," says Tomás Zagora. Of the three prototypes created, only the sole has failed to live up to expectations. "Although we could market a simply designed shoe that does not have overly demanding physical requirements, we have not yet succeeded in making a 100 % reliable product in terms of conditions of use, and still have to resolve the problem of this rubber's odour," explains José Ramón Sempere.
Coming soon to a store near you?
Will shoes made from recycled old tyres become a new fashion trend? As the image of recycled products is not always entirely positive, it may not be enough for the shoes to have all the required qualities and a fashionable design. How can we overcome the barriers to their introduction? Specialists in productconsumer interaction looking for an answer to this question have created ‘affective engineering' methods aimed at enhancing consumer perceptions of recycled products. The study conducted for the project reveals that "if consumers are properly informed, their purchase intention increases by almost 50 %. The words ‘green', ‘quality', ‘innovative', ‘beautifully-finished' and ‘top-grade' help to change consumer perceptions." Inspired by these results, the researchers have devised a label that sums up the environmental benefits of buying an item made from recycled tyres. "Substituting this material for virgin rubber," they explain, "will reduce the greenhouse gases caused by extracting new natural resources by around 72 % and will save almost 88 % of the energy." Will tyres worn out from eating up road miles soon be beating the pavements? Only time will tell...
- Etra - European Tyre Recycling Association, www.etra-eu.org