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Sulphur vulcanisation has for the longest time been the only way of making inflatable tyres since they were invented. But now researchers are about to reinvent the wheel, by offering a simple way of preparing commercial rubber without vulcanisation, but with self-healing properties. Thus, allowing cuts in tyres to be healed on their own, at room temperature.
Invented by Charles Goodyear, chemical crosslinking of rubbers by sulphur vulcanisation is the only method by which modern automobile tyres are manufactured. This involves adding sulphur or other compounds to the rubber, boosting the finished product’s durability by forming cross-links between the polymer chains that make up the material.
The formation of these cross-linked network structures leads to highly elastic properties, which substantially reduces the viscous properties of these materials. Unfortunately, though, once those links are broken, they can’t be repaired.
A team of scientists from the Leibniz Institute for Polymer Research (Germany), the Tampere University of Technology (Finland) and the Dresden University of Technology (Germany) are attempting to get around that limitation, by getting rid of the vulcanisation process altogether.
The scientists, who have published a report in the journal ACS Applied Materials & Interfaces, acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG), the largest independent research funding organisation in Germany. It promotes the advancement of science and the humanities by funding research projects, research centres and networks, and facilitating cooperation among researchers.
New method, no vulcanisation
This simple approach to converting commercially available and widely used bromobutyl rubber (BIIR) into a highly elastic material with extraordinary selfhealing properties does not use conventional cross-linking or vulcanising agents.
The researchers say they have discovered that adding a carbon/nitrogen compound has much the same effect, but additionally allows broken polymer links to reform over time.
Transformation of the bromine functionalities of BIIR into ionic imidazolium bromide groups results in the formation of reversible ionic associates that exhibit physical cross-linking ability.
Healing process – no more flat tyres?
Getting a flat tyre might never be an entirely hasslefree experience. A cut or torn tyre usually means that it has to be replaced with a new one.
But in time to come, that could change, since the tyregrade rubber without the vulcanisation, heals itself and could potentially withstand the long-term pressures of driving. Instead, damaged tyres could just be left a few hours to heal on their own.
The reversibility of the ionic association facilitates the healing processes by temperature or stress-induced rearrangements, thereby enabling a fully cut sample to retain its original properties after application of the self-healing process. In other words, using the new simple process that avoids vulcanisation altogether, the researchers chemically modified the commercial rubber into a durable, elastic material that can fix itself over time.
In laboratory tests, samples of rubber made using the process were able to heal cuts at room temperature. Heating the rubber to 100ºC for the first ten minutes accelerated the process. After a period of eight days, the healed pieces of rubber were able to withstand pressures of up to 52 bar – far higher than any tyre has to handle.
Other mechanical properties, such as the elastic modulus, tensile strength, ductility, and hysteresis loss, were found to be superior to those of conventionally sulphur-cured BIIR in test results.
According to the researchers, the addition of reinforcing agents such as silica or carbon black could boost the self-healing rubber’s strength even farther.
Thus, this simple and easy approach to preparing a commercial rubber with self-healing properties offers unique development opportunities in the field of highly engineered materials, such as tyres, for which safety, performance, and longer fatigue life are crucial factors.
The process is still at its trial stages and will have to go through further testing before it is launched commercially, say the researchers.