NEW PROCESSES TO TREAT ELTS
By Sharad Matade
The growing mountains of end-of-life tyres (ELT) are a major issue that is negatively impacting the environment globally. Worldwide, the rubber industry is seeking possible economical and eco-friendly recycling options to get rid of waste tyres. Some new processes and improvements in existing recycling processes are being brought in to expand the range of recycled rubber applications. According Dr Wilma Dierkes, Associate Professor of Elastomer Technology and Engineering (ETE) group at the University of Twente, the devulcanization process is the most competitive and eco-friendly way to recycle used tyres for use in various high-end applications.
Management of tyres at the end of their useful lives (end-of-life tyres, or ELTs) is a global problem that threatens to damage the environment. Every year world-wide an estimated one billion tyres reach the end of their useful lives. Disposing of ELTs in an environmentally friendly way is considered one of the greatest challenges faced by governments. Recycling is the major option to handle the mounting number of ELTs. It is here that the groundbreaking research by Dr Wilma Dierkes assumes relevance. She is Associate Professor of the Elastomer Technology and Engineering (ETE) group at the University of Twente.
“Our research shows that the devulcanization of waste tyres, in the perfect case, provides no harm to the polymer, so the materials after devulcanization would have the same properties as the input material,” she told Tyre Asia in an interview.
“In other words, devulcanization gives a significantly higher quality recycled rubber compared to just granulate particles. After devulcanization, we really have a compound, which can be homogeneously blended with a virgin compound. It can be cured for a second time, resulting in a homogeneous product with the best possible properties,” she said. However, it cannot be better than the input material: the material we devulcanize is aged during the first life cycle, adds Dr Dierkes.
Devulcanization of used tyres can be done multiple times without losing much of the desired properties. “In my past work on conventional reclaim, I did a study on multiple reclaiming: tyre tread material was reclaimed and added in a concentration of 20 per cent to a new compound of the same type. This material was then again reclaimed, and so on, until the original material had undergone four recycling loops. The results were that there were no significant differences between 1, 2, 3, or 4 times recycled material in this compound. So I don’t see a risk of multiple recycling of rubber by re-using it in tyres,” adds Dr Dierkes.
With this high-quality devulcanizate, and if processing and compounding of the virgin rubber/devulcanizate blend are adjusted, 40 per cent of devulcanized rubber can be used in making new tyres, depending on the part of the tyre it will be used in.
Grinding of waste tyres for use in various applications offers limited benefits. Today one of the main usages of granulates are in road construction and artificial turf used in sports fields. However, according to Dr Dierkes, in both, road construction and artificial sports fields, there are issues that are needed to be addressed.
Explaining the issues for using granulates in road construction, Dr Dierkes says, “The problem of the usage of rubber granulate in road construction is that the blend of granulate with bitumen is not stable, resulting in processing problems. We need to address this; in my opinion, devulcanized rubber is a good approach for more stable rubber-bitumen blends.”
Today, such rubber granulates are also extensively used in making artificial turf for soccer fields. However, in the European Union, there have been heavy discussions on the environmental and health impact of granulates in the soccer fields. “
The Dutch ELT body RecyBEM has being doing major efforts to study polycyclic aromatics in tyre granulate. The result of this study is just published and shows that the PCA’s are indeed within the specification” adds Dr Dierkes.
Currently Dr Dierkes is working on a devulcanization project in two directions: scaling up the devulcanization technology to industrial scale, and an application study.
The up-scaling study is coming forth from a project financed by RecyBEM and assigned by a tyre recycling company. The product application study is publicly financed from the Netherlands.
Dr Dierkes says that within the EU, money is explicitly marked for rubber and tyres to motivate recycling and re-use. “Such programmes are to explore all new ideas to improve any rubber products for higher quantity and quality,” she adds.
However, the challenges for devulcanization of used tyres mainly lie in the availability of high quality raw materials. Granulates or powders generally are not clean enough for devulcanization.
“Especially in passenger car tyre rubber, a considerable amount of fibres is present, and that will reduce the desired properties of the devulcanizate. So, we really need cleaner materials. The finer the powder, the cleaner it is, but it is more expensive,” says Dr Dierkes.
However, the devulcanization process is depending on the input material, and one of the parameters is the particle size: smaller size particles require a shorter residence time for a complete devulcanization, but larger particles are more efficiently sheared in the extruder. “We will have to find a solution for balancing the process,” adds Dr Dierkes.
As tyres are changing from conventional to ‘green’ compounding, new ingredients for making tyres are being intensively sought. Today, in passenger car tyres, carbon black is replaced by silica owing better performance and safety.
However, devulcanization of silica containing tyres does not achieve the results as carbon black filled compounds do. “We have done a study on silica compounds. In an ideal situation, the degree of devulcanization is up to 80 per cent for passenger car tyre rubber, carbon black filled, while silica compounds reach 30- 40 per cent of devulcanization only under comparable process conditions,” says Dr Dierkes.
Recycling of used tyres through pyrolysis technology is another widely practised process globally. Pyrolysis projects in Europe so far have not been successful due to the fact that conventional technologies are used in pyrolysis plants.
“The problem is that there is not much variation possible in product quality obtained from the conventional pyrolysis technology. Carbon black obtained from the process is a N600 to N700 quality, so there is little scope for carbon black obtained from pyrolysis process for using in new tyres. There is limited scope of improvement of the conventional pyrolysis process technology. We need new, innovative technologies to produce a good quality carbon black by pyrolysis,” explains Dr Dierkes.
Currently Dr Dierkes and her team are doing a project on application of passenger car tyre rubber back into passenger car tyres. “We started the study on a small scale looking for what compounds and polymers we have in passenger car tyres, and we found that the most difficult to devulcanize would be SBR. Initially we elaborated the devulcanization process for this polymer: time, temperature, devulcanization aids and mechanical energy input. We then applied the technology to the other polymers used in tyre compounding.”
The outcome was that NR does not make any problem for devulcanization, BR devulcanizes less efficiently under these conditions but is still acceptable. Butyl rubber reacts differently, so her research team is now looking into an adjustment of the devulcanization process for butyl rubber.
The next step was to apply the thus developed devulcanization technology to complete compounds containing fillers as well. “We found that the presence and type of filler had a significant influence on the devulcanization behaviour.”
Other than granulates , pyrolysis and devulcanization, one more option for recycling is biological devulcanization. “Biological devulcanization means bacteria that eat sulfur in rubber. For this technology large reactors are needed and it takes a very long residence time to devulcanize only a superficial layer of the rubber particles. Besides, the treated rubber is wet and you need to dry it afterwards. I don’t see this as a too practical solution; I doubt if it is feasible on a large scale,” adds Dr Dierkes.