Wonder material? Commercialising graphene
Although the possibilities of graphene are seemingly endless, commercialising this material has proved tricky. Gerry Boyce explains how one company is approaching the issue.
The world is continually striving for new materials that are lighter, stronger, cheaper and more environmentally friendly. Since the war, this has led to the development of numerous new polymers and, more recently, advanced polymer composites, which are now used to construct everything from the fuselage of the Boeing Dreamliner to large wind turbine blades.
Advanced polymer composites consist of long or continuous fibres embedded in a polymer matrix. These materials have a very high strength and stiffness to weight ratio, which makes them particularly attractive to the designers of aircraft, cars, trains and yachts. These fibres provide the strength and stiffness and the polymer provides the environmental resistance as well as transferring the loads into the fibres. Traditionally, the reinforcing fibres have been carbon, glass or aramid and the resin systems have typically been thermosetting liquid resins such as polyester, epoxy and vinyl ester. In general, the advanced composite material consists of 50% reinforcement and 50% polymer matrix.
Over the past 30 years, enormous strides have been made on the development of high strength and stiffness fibre reinforcements and now, with the development of graphene, materials scientists have the potential to develop a whole new range of enhanced polymer resins which, in turn, could be used to significantly enhance the properties of the composite materials.
Each gram of graphene has an estimated surface area of more than 2,600m2. It is essential that there is uniform distribution of the graphene in the epoxy resin so that there is continuity of mechanical, electrical and thermal performance throughout the material both in plane and through thickness to ensure that the material behaves in a uniform and isotropic manner. The best way to evenly distribute the graphene into various resins for use is, I believe, Haydale’s proprietary plasma treatment process, which activates the surface of the graphene as well as adding functional groups necessary for the graphene to bond to the host polymeric resin. Our research has shown that the best results are achieved by using carbon nanotubes (CNTs), which are sheets of graphene in a cylindrical form. The even dispersion that results from the functionalisation of the CNTs gives the epoxy resin uniform increases in mechanical, electrical, thermal and physical properties throughout the whole material.
It is possible to add a range of end groups to this process, such as oxygen (O2), carboxylic (COOH), amine (NH3), fluorine (F) and chlorine (Cl), although the choice of end group is very much dependent on the polymeric resin. For example, NH3 groups are more appropriate for epoxy resins and O2 are more suitable for polyester resins. The choice of end groups is also known to affect the resulting mechanical, electrical, thermal and physical properties of the polymeric resin. We are currently undertaking considerable research to determine the optimum surface treatment and quantity and type of functional groups for a given polymeric resin.
We have now successfully developed a range of highly loaded (up to 25% by weight) graphene-enhanced polyester and epoxy resins that can be used as a master-batch. This can then be diluted to a variety of alternate concentrations, depending on what property enhancement a company is looking to achieve. The development and supply of highly loaded master-batches enables resin and composite moulding companies to handle graphene in a safe and controlled manner, as it is embodied within a liquid matrix. The master-batch can also be readily diluted down by adding additional resin and using standard mixing techniques.
By passing a known light source through the test sample, we can then measure the dispersion of graphene within a resin using a sensor. This unit can take measurements on a continuous basis over time or at specified intervals and is useful for measuring the dispersion of graphene within a liquid and any settling out with time. It can also be used to measure the efficiency of mixing or stirring to re-disperse the graphene solution.
In order to develop optimised resin formulations with enhanced mechanical, electrical, thermal and physical performance, detailed development and testing programmes have to be undertaken. We have been working with two leading suppliers of polyester and epoxy resins on this area, which involved mixing and casting resin test plaques. These were then tested mechanically, electrically, thermally and physically.
As can be seen from the preliminary results, shown on the left, a 2% loading of graphene nano platelets (GNPs) leads to a 200% increase in ultimate tensile strength coupled with a 60% increase in tensile modulus. The mechanical properties seem to be optimised at 2% loading, after which the properties generally plateau.
In April 2014, the Aerospace Corporation published a peer review paper on the addition of our functionalised GNPs with O2 end groups into epoxy resin. It showed a 270% increase in tensile modulus and a 200% increase in tensile strength at 4% by weight loading of GNPs in the epoxy resin.
During 2014, we also supplied CNTs and few layered graphenes (FLG) functionalised with COOH end groups for the Cleansky project at Cardiff University. In this project, the functionalised CNTs and FLGs were added to an epoxy resin that was then resin-infused into carbon fibre fabrics to form carbon fibre test plaques. These were subsequently tested for a range of properties. This work also showed a doubling in modulus of the pure epoxy resin, which translated into some interesting properties in the carbon fibre reinforced epoxy composite laminates, as shown bottom left.
The addition of 0.5% COOH functionalised FLGs to the carbon fibre reinforced epoxy gave a 50% increase in compression after impact, 10% increase in compression performance, 35% increase in in-plane shear, 18% increase in double cantilever bend and a 25% increase in the tensile strength. This work is encouraging because it shows that enhancing the properties of the base resin can increase the properties of the carbon-fibre-reinforced epoxy composite structures.
Based on these initial exciting findings, we are now working to develop a range of graphene-enhanced thermosetting resins with improved mechanical, thermal, electrical and physical properties. This project involves assessing the optimum type of graphene, functionalisation time, chemical end groups, graphene concentration and mixing/dispersion method. Once we have identified any key resin dominated properties the plan is to manufacture and evaluate a series of fibre reinforced composite case study applications manufactured with graphene-enhanced resins. If we consider that 50% of the composite structure is fibre and 50% is resin, the ability to radically change the performance of the resin could lead to a whole new range of composite materials and structures.
Gerry Boyce is Managing Director of Haydale Composite Solutions Ltd, formerly known as EPL Composite Solutions Ltd. For more information about this process, please email email@example.com