The small world of graphene

Paul Boughton

European researchers are investigating how graphene could boost the properties of thermoplastic components. Lou Reade reports.

Graphene is the new wonder material that will give us space elevators, nano-scaled microcircuits and impossibly light aircraft. At least, that's the theory. For now, the material simply holds lots of promise, and is still very much a laboratory curiosity. But this potential has led many research teams to investigate how graphene might be moved out of the laboratory and into the world of engineering.

A new pan-European research project called Nanomaster aims to develop new methods to make graphene (and similar nano-scaled carbon compounds) and incorporate it into thermoplastics, in an attempt to boost physical properties.

"It's the combination of electronic, thermal and mechanical properties that is of interest," says Ben Hargreaves, senior project manager at Nanomaster's coordinator NetComposites. "If we wanted to improve any one of these properties, there are other ways of doing it. But graphene can do all three, allowing multi-functional property enhancement."

Some of the physical properties boosted by graphene (and nano-particles in general) are flame redundancy, barrier properties (which is useful for plastic packaging) and mechanical strength. Just a small percentage of graphene can improve the stiffness, strength and electrical properties of thermoplastics. The aim of the project is to strengthen plastic components, allowing their weight to be cut by as much as 50 per cent. At the same time, the parts could be imbued with electrical and thermal functionality.

Graphene is one of a number of nano-structured forms of carbon being studied in the project. The others include expanded graphite and nano-graphite. In each case, the very small particle size has a far greater effect than conventionally sized additives - such as standard carbon black, which is commonly used as an anti-static additive in plastics.

Roll call

Nanomaster is a wide-ranging project, encompassing materials specialists, research organisations, manufacturing companies and health and safety experts.

As well as being project co-ordinator, UK-based NetComposites will take charge of moulding graphene nanocomposite test panels. Roechling, which makes plastic car parts such as air intake manifolds, is also an expert in plastic moulding. Among other tasks, it will provide moulds to verify the processability of the new materials. At the same time, Netherlands-based Promolding will handle pilot-scale compounding, as well as analysing the electrical and mechanical properties of the materials.

Materials specialists in the project include: Lati, which makes electrically and thermally conductive compounds; Timcal, a leading producer of graphite and carbon black; Aimplas, a Spanish research organisation with experience in preparing nanocomposite compounds; and MB Proto, a small French company involved in rapid prototyping technologies.

Manufacturers in the consortium include: Dutch electronics giant Philips; and Aero Engine Controls, a joint venture between Rolls-Royce and Goodrich that makes aerospace components.

An extra element of the project is the safety aspect, and involves evaluating the potential hazards of working with these materials, and devising suitable risk management strategies. It is led by the UK's Institute of Occupational Medicine.

Production boost

The first step in the project is to take production of the materials up to something approaching industrialisation - but, being such specialised materials, these volumes are not exactly astronomical. At the outset, the team was capable of making 10g batches, but later raised this to 250g - with remarkable success. This was a crucial breakthrough: as with many nano-particles, the aim is to ensure as little 'agglomeration' as possible, because particle size is the key property. In the case of graphene, the particles are take the form of carbon 'sheets' just one molecule thick. The better these sheets remain separate, the greater the effect. (The process of separating nano-particles is called exfoliation.)

Production is now hovering at the 1kg scale, and will soon be upgraded to 5kg. The eventual target is to raise this to 25kg.

"Right now we can achieve 90 per cent yields in around 24 hours," says Hargreaves.

Nano particles are notoriously difficult to mix with plastics. For this reason, the project will attempt to develop a range of pre-mixed graphene compounds (known as masterbatches). This ensures that a traditionally difficult step of the process - incorporating the graphene into the plastic matrix - has already been done.

The graphene-enhanced plastics are made into pellets and processed in the conventional way, to produce plastic components. Nanomaster will look at two commonly used processes - injection moulding and compression moulding.

The project will focus on lab-scale compounding of graphite-polypropylene (PP) compounds. The specifics of how the compounds are made will be key to their eventual success, says Hargreaves. One important factor is shear - they physical force that acts on the materials as they are compounded together.

"Because shear is important, and affects how the graphene is ultimately distributed in the plastic, we're doing some compounding techniques that are effectively 'zero shear'," he says.

There will also be computer simulation work, to allow effective scale-up of the process from lab scale to pilot scale production.

In addition to taking the pre-made graphene and dispersing it within a masterbatch, another aim of the project is to determine whether an expanded or partially exfoliated graphite can be further exfoliated during the compounding process.

Additive manufacturing

As well as targeting conventional plastics processes, Nanomaster will attempt to develop materials for use in additive manufacturing. Additive manufacturing - also known as 3D printing - relies on building up components in a 'layer by layer' fashion, to create products that would be impossible to make in conventional ways. These kinds of techniques - such as selective laser sintering (SLS) and fused deposition modelling (FDM) - were originally used to make 'rapid prototyping' models. More recently, they have been used to make limited-run finished products.

The mechanical properties of these components are not generally very high. So more robust materials - such as those being developed by Nanomaster - could expand the scope of additive manufacturing.

Materials must be specially adapted for processing in this way. This is something the project intends to study.

"One method is to make standard SLS grades and blend them with graphene," says Hargreaves. "But, you would have free graphene flying around and that's not ideal."

Alternatively, the graphene could be embedded into the surface of the material, and then be fully encapsulated (and safe for use). This could be achieved using techniques such as spray drying or cryo-milling, he says. The team has already had some success compounding nylon 12 with expanded graphite, and creating an ABS grade for FDM processing.

"One thing we need to do is refine the manufacturing process for these additive manufacturing grades," says Hargreaves. "Graphene could help us move this technique further towards rapid manufacturing."

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