Researchers use carbon nanotube derivatives to strengthen recyclable plastics

Reducing the environmental impact caused by plastics can be addressed through different strategies, such as making more sustainable 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 melted multiple 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 greater resistance to shock and mechanical stress, although they are also more fragile. Epoxy resin, silicone or melamine are examples of thermoset plastics, commonly used in construction.

To make these plastics stronger, engineers add reinforcing materials such as carbon fibers. They are already used to make objects like motorcycle helmets or sports equipment, which are very durable although they are not easily recyclable.

At IMDEA Nanociencia, the Low Dimensional Materials Chemistry group, led by Emilio Pérez, is studying a strategy to strengthen recyclable plastics in collaboration with the company Nanocore. The plastic studied is an “adaptable covalent network”, whose molecular structure is similar to that of a thermoset plastic but with the particularity of incorporating covalent bonds – strong – but at the same time reversible between polymer chains.

The work is published in the journal Advanced functional materials.

More precisely, they work with imines, whose bonds are dynamic: they can be broken by water or temperature and reorganized. The novelty of the study lies in the use of a derivative of carbon nanotubes surrounded by a ring molecule – the mechanically interlocked carbon nanotubes MINT. The ring molecules are attached to the carbon nanotube mechanically and not chemically, so the bond between the two is very strong, but at the same time allows some movement of the molecule along the nanotube.

The researchers equipped the ring with two anchor points (two amines) so that they covalently bond to the polymers. In this way, the nanotube becomes a structural element of the polymer network.

Carbon nanotubes are essentially a sheet of graphene rolled up on itself. To join a nanotube with other molecules, it is possible to do it directly by covalent bonds which break the tube a little, add defects and weaken it.

The strategy pursued by the researchers uses mechanical bonding – a ring molecule around the nanotube – to integrate the nanotubes into the polymer network, preserving all their properties and maximizing charge transfer from the matrix to the reinforcement. In other words, we can’t do better.

The concept is simple: by surrounding the nanotube with a ring, we avoid the agglomeration of these fibers which makes the reinforcement less effective. Additionally, polymer interaction sites are provided in the ring, which improves stress transfer. Adding just 1% by weight of nanotubes to the polymer blend achieves a 77% improvement in Young’s modulus and a 100% improvement in tensile strength. Remarkably, the mechanical properties of this reinforced plastic remain intact after being melted down and recycled up to four times.

In engineering, the law of mixtures states that the properties of a compound are the mixture of the properties of the original materials, in proportion. The study carried out by the Madrid researchers confirms that this is only the case when there is an effective transfer of mechanical stress between the two compounds, at the nanoscopic level.

In their work, the researchers achieved maximum efficiency in the transfer of mechanical stresses from the polymer to nanotubes, the strongest material. Nanotubes have a Young’s modulus of 1TPa, five times harder than steel, being a much lighter material.

Adding more nanotubes to plastic does not make it stronger, because the nanotubes begin to clump together and lose their effectiveness. The key to success lies in the covalent bond between the nanotubes and the polymer.

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 result in considerable fuel savings.” Manufacturing with fewer materials and guaranteeing recyclability opens up a promising horizon.