Materials increasingly self repair

Self repairing materials are showing evermore impressive results. Here we look at two recent impressive examples that could soon be set for real world application.

Materials that repair themselves are becoming increasingly possible. Though, many of the most promising materials remain in development and in the lab, commercial applications are becoming tantalisingly close.

The first example comes from researchers at the Adhesion and Adhesives Laboratory at the University of Alicante in Spain. There, researchers have developed a flexible polymeric material capable of self-repairing.

Its behaviour seems pure sci-fi, but when the transparent resin is cut in half with scissors and the two parts put back in contact, it re-joins itself within 10 to 15s, without any external stimulus. This cutting and joining process can also be performed in water or indeed any kind of fluid.

“This property gives way to the development of different materials for applications in sectors such as medicine, cosmetics, the space industry, automotive, and many others,” says Jose Antonio Jofre, a chemical and industrial engineer at the University.

In addition, the material is claimed to have shape memory, which means that even if it is crushed and kneaded it recovers its original shape in just a few seconds.

When the transparent resin is cut in half with scissors, if the two ends are the placed back in contact with one another, they rejoin within 10-15s.

Another example comes from scientists from the Leibniz Institute for Polymer Research, in Germany, where non-vulcanised tyre-grade rubber has been produced. Though the initial research was just to avoid the vulcanisation process, the resulting material showed it was able to ‘heal’ itself.

Vulcanisation involves adding curatives or accelerators, such as sulphur, that modify the polymer by forming cross-links (bridges) between individual polymer chains. This has the result of making the rubber more durable, while maintaining its elasticity. However, once it has been punctured it cannot be patched for long-term use.

Invented by Charles Goodyear, chemical cross-linking of rubbers by sulfur vulcanisation is the only method by which modern automobile tyres are manufactured. The formation of these cross-linked network structures leads to highly elastic properties, which substantially reduces the viscous properties of these materials.

However, the researchers have modified commercial bromobutyl rubber into a durable, elastic material that can fix itself over time. Testing showed that a cut in the material healed at room temperature, a property that could allow a tyre to mend itself while parked to potentially withstand the long-term internal tyre pressures of driving.

The research claims that chemical cross-linking of rubbers by sulfur vulcanisation is the only method by which modern automobile tyres are manufactured. Vulcanisation substantially reduces rubber’s viscous properties, however the researchers have been able to take commercially available bromobutyl and transform its bromine functionalities into ionic imidazolium bromide groups creates cross-linking abilities without vulcanisation.

The researchers claimed: “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.”

The rubber’s mechanical properties, including elastic modulus and tensile strength, actually improved with ionic modification compared with vulcanisation.

Tests showed that a cut in the ionised bromobutyl healed in eight hours at room temperature. After eight days, the rubber could withstand stress of 51 atm. Heating the rubber to 100°C for the first 10 minutes accelerated the repair process, and reinforcing agents such as carbon black or silica would further strengthen the rubber.

However, it noted that the rubber prototypes may be unstable over time, and the researchers are addressing that problem. The research is still in its infancy, so pilot production of self-healing rubber is still some time in the future.

The research describes the process of converting commercially available and widely used bromobutyl rubber (BIIR) into a highly elastic material with extraordinary self-healing properties without using conventional cross-linking or vulcanising agents.

Transformation of the bromine functionalities results in the formation of reversible ionic associates that exhibit physical cross-linking ability. 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 Bromobutyl rubber.

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.

Justin Cunningham

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