Making Recycled Plastics Stronger With Carbon Nanotubes

Reducing the environmental impact caused by plastics can be addressed through different strategies, such as the manufacture of more durable plastics or recycling. In general, there are two main types of plastics. The first is thermoplastics, which can be melted and molded to form other objects, although their mechanical properties weaken if they are melted several times. And the second, thermosets, do not melt at high temperatures, since the chains of the polymers that form them are intertwined by chemical bonds.

Thermoset plastics have advantageous properties compared to thermoplastics. They tend to have a higher resistance to impact and mechanical stress, although they are also more brittle. Epoxy resin, silicone or melamine are examples of thermoset plastics, commonly used in construction. To make these plastics stronger, engineers add reinforcement materials such as carbon fibers. They are already used to manufacture objects such as motorcycle helmets or sports equipment, which are very durable although they cannot be easily recycled.

Putting rings on nanotubes: a simple and very effective strategy

Carbon nanotubes are essentially a sheet of graphene rolled up on itself. To join a nanotube with other molecules, it is possible to do so directly by covalent bonds which break the tube a little, add defects and weaken it. The strategy pursued by the researchers uses the mechanical bond – a ring molecule around the nanotube – to integrate the nanotubes into the polymer lattice, preserving all their properties, and maximizing the load transfer from the matrix to the reinforcement. In other words, it cannot be done better.

The mechanical properties of this reinforced plastic remain intact after being melted down and recycled up to 4 times.

In engineering, the Law of Mixtures indicates that the properties of a compound are the mixture of the properties of the original materials, according to their proportion. The study led by the Madrid researchers confirms that this is only the case when there is an efficient transfer of mechanical stress between both compounds, at the nanoscopic level. In their work, the researchers have achieved maximum efficiency in transferring mechanical stress from the polymer to nanotubes, the strongest material. Nanotubes have a Young's modulus of 1TPa, 5 times harder than steel, being a much lighter material. Adding more nanotubes to the plastic does not make it stronger, as the nanotubes begin to agglomerate and lose efficiency. The key to success lies in the covalent bond between the nanotubes and the polymer.

It all started with an ERC grant

The story of this scientific result begins in 2012, when researcher Emilio Pérez received a prestigious 'Starting' grant from the European Research Council (ERC) to develop an innovative idea: putting molecular rings on carbon nanotubes. With this grant of 1.5 million euros, Pérez consolidated his research group at IMDEA Nanociencia institute. For 5 years, they created mechanical bonds between ring molecules and carbon nanotubes and studied their properties, although without yet focusing on their possible applications. In 2017, Pérez received a call from Nanocore ApS, a Danish company pioneering the transfer of results to the materials production market. They also intended to modify carbon nanotubes, from a biochemical approach. They began collaborating together on a year-long project, and then obtained an ERC 'Proof of Concept' grant that promoted the experiments. In 2020 they would sign a contract of more than 3 million euros to work together on the reinforcement of plastics with carbon nanotubes.

A myriad of applications

The collaboration with Nanocore continues to explore all the most commercially relevant polymers that can be reinforced. Producing plastics almost as strong as carbon fibers, which can be melted down and recycled, is a dream. A before and after, which can firmly contribute to a new, greener and more sustainable scenario. Pérez explains: “Producing lighter structures, such as cars, planes, etc., would mean considerable fuel savings.” Manufacturing with less material and ensuring recyclability draws a promising horizon.

Reference: Isasti I, Miranda S, Jiménez DM, et al. Reinforcement of polyimine covalent adaptable networks with mechanically interlocked derivatives of SWNTs. Adv Funct Materials. 2024:2408592. doi: 10.1002/adfm.202408592

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