Crack length versus annealing temperature of samples. Inset shows optical images of the silicone rubber/graphene nanoplatelets composite before and after thermal annealing demonstrating healing of the initial crack length.Silicone rubbers are the workhorse of the industrial and automotive sectors – maintaining strength and flexibility at both high and low temperatures. This type of composite is commonly reinforced with carbon black or – more recently – carbon nanotubes. But now researchers report that using nano-sized flakes of graphene instead can bring novel and useful properties.
When Nicola M. Pugno from the University of Trento, Fondazione Bruno Kessler, and Queen Mary University of London, along with colleagues from the University of Perugia, added graphene nanoplatelets (GNPs) to silicone rubber (SR), the resulting composites not only showed a rapid decrease in electrical resistance with increasing temperature but also self-repaired damage via a simple thermal annealing process [Valentini et al., Composites Science & Technology 134 (2016) 125].
“The development of elastomers with self-healing properties, i.e. the realization of structures able to repair mechanical damage, is an important challenge from an industrial point of view,” says Luca Valentini of the University of Perugia.
The SR/GNP composite maintains its properties up to temperatures of 250°C, which could meet many of the demanding requirements placed on materials used in applications like hoses, seals, and automotive components. These parts are susceptible to scratches, cracks, and punctures, which can result in dangerous spillages, safety hazards, or simply loss of performance. Self-healing materials like the SR/GNP composite could be extremely helpful in these applications, extending the lifespan of essential components and avoiding the need for frequent checks and repairs.
“Our approach does not take into account supramolecular chemistry or coordination complexes,” explains Valentini, “but is based on the simplest self-healing mechanism proposed in publications in the 1950s.”
The researchers believe that the self-healing behavior is thanks to ‘living’ reactive species in the composite that not used up during the original curing process at room temperature because they are protected by aggregates of graphene platelets. The reactive species remain dormant until thermally activated in the self-healing process when they serve to catalyze new crosslinks in the damaged part of the composite network.
“It is generally acknowledged that the aggregation of graphene platelets is detrimental for the final mechanical properties of polymer composites,” says Pugno. “In our composite, the aggregation of graphene promotes the decrease in electrical resistance with temperature and the high healing yield.”
The composite is produced using a rapid mixing method, which could be easily scaled up to industrial levels. The researchers believe it could be suitable for injection molded or extruded automotive parts or even temperature sensors.