Graphene Gaining Strength in Composites Markets

In 2004, working at the University of Manchester in the United Kingdom, researchers Andre Geim and Kostya Novoselov produced graphene, the world’s first two-dimensional, man-made material. In the following years, graphene was hailed as a wonder material because of its many remarkable properties. Despite being extremely lightweight and a million times thinner than a human hair, graphene is the world’s strongest material, with 200 times the tensile strength of steel. It has high electrical conductivity and thermal conductivity, is practically transparent, is impermeable and extremely flexible and stretchable. What makes graphene truly unique, however, is its ability to provide all these properties at once. “With most materials, if you want to get one beneficial property you have to introduce some negative aspects, but with graphene you can impart multiple properties simultaneously and without the typical tradeoffs in many, many cases,” says Terrance Barkan, executive director of The Graphene Council. Like many other high-tech discoveries, graphene didn’t initially live up to the hype. Early attempts to capitalize on its properties were unsuccessful, and many companies became dubious about its ability to deliver what was promised. There were several reasons for these failures. “In many cases, the companies that thought they were working with graphene were actually working with material that may have had carbon in it, but wasn’t really graphene,” says Barkan. Manufacturers also found it very difficult to disperse graphene throughout a matrix to get the desired properties. Early adopters were also confused by graphene’s unusual behavior. Composite material manufacturers, for example, are accustomed to adding more of a substance to a composite when they want to increase the properties it imparts to a material. But graphene has the reverse effect; reducing the quantity used generally improves the graphene-enhanced composites’ performance. Companies that initially added 1% graphene by weight to their composite materials achieved better results when they decreased the amount to 0.5% by weight. “The magic of this material is that an incredibly small amount of it can dramatically improve the performance of other materials,” Barkan says. With continued research and experimentation, companies have gained a better understanding of graphene’s behavior and are finally realizing its benefits. The nanomaterial is proving to be a valuable asset for the composites industry since it can be used for thermoset and thermoplastic composites, incorporated into glass, carbon and basalt fibers, and used with a variety of resins. “The Graphene Council has identified more than 45 different vertical markets for graphene; composites are clearly the leading application market, and coatings is not far behind,” Barkan says. Producing Graphene In its purest form, graphene is a one-atom-thick sheet of carbon atoms arranged in a dense, hexagonal lattice pattern. Although scientists theoretically knew about graphene for several years, it was difficult to produce. The University of Manchester researchers used adhesive tape to lift flakes from a piece of graphite, then continued to split those flakes with more tape until they reached a single-atom sheet. Although manufacturing methods have progressed far beyond the adhesive tape level, producing monolayer graphene remains difficult and expensive. Researchers have found, however, that graphene doesn’t have to be in this pure form to be used effectively. “Few-layer graphene (FLG), typically less than 10 layers, still displays many of the unique properties of graphene. It is a more cost-effective solution for many applications, including composites,” says Lisa Scullion, application manager at the University of Manchester’s Graphene Engineering Innovation Centre (GEIC). “The common assumption is that graphene material that has fewer layers is superior quality, but it really does depend on what the particular application is,” says Barkan. “You could have a 20-carbon-layer-count material that still imparts the mechanical or thermal conductivity benefits you might want, but it also might be quite a bit less expensive than trying to find a single or two-layer material.” Composites manufacturers often use multi-layer graphene (MLG), which consists of 11 to 20 carbon atom layers. Graphene today is produced in many ways. Chemical vapor deposition (CVD) yields the purest form, which consists of one to two layers of carbon atoms. A 25-micron, thin foil copper sheet in a vacuum vessel, heated up to 1000 C, is bathed with a mixture of argon, helium and methane. Graphene sheets collect on the copper’s surface, and the copper is then digested by chemicals like hydrochloric acid, leaving the graphene. CVD is a very expensive approach, producing graphene costing up to $500,000 a gram, and even continuous CVD lines can’t produce the necessary output for large-scale manufacturing. So manufacturers have developed bulk production methods to obtain FLG and MLG, using physical, mechanical, chemical and thermal forces to exfoliate the graphite feedstock. In one technique, rock crushers and a ball mill break the graphite down into a fine powder, which is then put into a solvent like alcohol or methyl pyrrolidone and hit with ultrasonic vibrations to further break down the graphene layers. The longer the processing time, the fewer the layers of graphene. In powder or solution form, this graphene can sell from $100 to $500 a gram for pure graphene or FLG and for as little as $50 a gram for MLG. Some researchers are using filters to remove the graphene from biochar, a residue of biofuel production. Others are experimenting with a detonation method, introducing a hydrocarbon gas and oxygen into a chamber and detonating it using a spark plug. The gases burn off, leaving behind a graphene-containing carbon residue. Barkan says there are many other methods of graphene manufacture that are currently being explored and/or moved into commercial-scale production. Graphene comes in a variety of forms, including sheets, nanoplatelets, flake powder and dissolved in solutions. Each form has different properties and characteristics, including lateral size, that make it appropriate for specific applications. Graphene can also be functionalized with other materials to impart certain properties. For example, graphene functionalized with boron nitride makes a better insulator than a molecular conductor. “The main one that people use is graphene oxide; you can make that in a batch process, putting graphene in with specific chemicals and providing some agitation. You end up with few-layer graphene oxide flakes,” says Mark Dickie, application manager for composites at GEIC. Because graphene oxide disperses more easily than some other forms of graphene and is compatible with many organic systems and polymers, it is being used in many applications. The goal is to tune the properties of the graphene to the matrix of the materials to achieve the desired results, Dickie adds. Opportunities and Obstacles One of the biggest hurdles manufacturers face when adding graphene to composites is overcoming its tendency to agglomerate. “Where people fail is in not using the correct equipment to disperse it properly,” says Dickie. “They think they can just whisk it in there, but since it sticks to itself you need to use something like a high shear mixer so that it separates the agglomerates.” The rate of addition is also important; putting the graphene in slowly and mixing it well works best. Another approach is using graphene incorporated into liquid resins. “In many cases it makes a lot of sense to disperse your graphene material in a monomer and then introduce that into a polymer,” says Barkan. “You can do high shear mixing. You can do melt mixing. You just want to make sure the process is controlled enough that you get adequate dispersion.” Functionalizing graphene to alter its surface chemistry and surface energy may assist in dispersion, but there are drawbacks. “The problem is that if you don’t think it through very carefully, in the process of altering [graphene] to get good dispersion you may actually degrade or destroy the very properties that you are putting it in for,” says Steven Rodgers, technology consultant and principal, EmergenTek, and a board member of the National Graphene Association. If you’re incorporating graphene for its electrical properties, for example, introducing certain chemicals could tie up a graphene sheet’s receptors, reducing the ability of electrons to transfer between sheets. In addition, there are concerns about how the use of graphene powder could impact workers’ health. The National Institute for Occupational Safety and Health is currently conducting exposure studies; in the meantime, they advise companies to use the same precautions they would with any potentially hazardous dust. Ensuring the quality and consistency of the graphene supply is another problem. Even a small discovery or commercial development has the potential of exploding the world’s need for graphene, says Rodgers. “There are a wealth of suppliers out there but not all of the people who claim to be selling graphene have a quality product,” says Dickie. “We need to generate confidence in graphene material. The adoption of graphene by several big industries is helping to build on that, and companies are starting to test the materials extensively with a view to producing graphene-based products in the near future.” This quality assurance process will be made a little easier because some of the necessary equipment and procedures developed for carbon nanotubes could be used for graphene as well, Rodgers notes. Moving Mainstream Ford Motor Company has been a leader in the adoption of graphene-enhanced composite materials. In October 2018, it announced that it would include foam made with graphene in more than 10 under-hood components, including pump covers and fuel line covers, on the Ford F-150 and Mustang. Debbie Mielewski, Ford’s senior technical leader in materials sustainability, says the company had little success when it introduced graphene into hard plastics in 2011. A few years later, however, summer interns tried incorporating graphene into urethane foam. Although the graphene dispersed well, the 1% to 5% graphene loads they first tried produced good, but not outstanding results for temperature and sound absorption. To save money, the team began reducing the amount of graphene in the foam. “That’s where things got really interesting. Every time we cut back on the amount of graphene, the properties went up,” says Mielewski. At these lower amounts, the graphene was less likely to interact with other graphene. “You want it singularly dispersed and doing its job separately,” she adds. Working with XG Sciences and Eagle Industries, the team eventually settled on a foam that included 0.2% by weight graphene. “We got a 25% improvement in high-temperature properties and an almost 30% improvement in noise absorption properties,” Mielewski says. “All of a sudden we were able to take advantage of this really interesting molecule, since 0.2% by weight was certainly affordable. Since it also nucleated the foam more uniformly, we were able to use less urethane. So we balanced the cost, and we got all these improvements.” She says the graphene helps absorb the sound much better, gives a quieter ride in the cabin and can withstand the heat – all important qualities under the hood. There’s also room for improvements. Last summer, Ford interns reduced the graphene load down to 0.05% and the parts performed even better. Ford might someday use graphene urethane foams for engine covers and for headliners and door panels in vehicle cabins. “We’re also thinking about going back to hard plastics, but you have to be a little more creative because I think [graphene] is not going to be easy to distribute there,” Mielewski says. Meanwhile, in Europe, Briggs Automotive Company has reduced the weight of its single-seat racer by more than 15% using graphene-enhanced prepreg for the body panels. The tooling for the parts, also made from graphene-enhanced prepreg, enabled the automaker to reduce the process time. “They could heat the tooling up quicker and cool it down quicker,” says Dickie. Barkan suggests that Ford’s use of graphene may be a catalyst for its inclusion in more composite parts. “Many companies are somewhat conservative. They don’t want to be the first. They want to see that somebody else has proven the market,” he says. “So I think when we see a large-scale manufacturer using graphene in a way that takes them a step change up in competitiveness – when they get a 25% to 30% improvement for a class of materials – the rest of the competitive field has to decide how they’re going to act.” Expanding Possibilities Composite materials containing graphene are now being used in the manufacture of everything from golf balls, sports racquets and training shoes to fire retardance coatings and construction materials. “One company, Haydale, has just come out with a prepreg material specifically designed to be used for lighting strike protection,” says Barkan. “That would eliminate having to use a copper mesh or a silver nanowire mesh to protect aircraft.” That same technology could be used for unmanned aerial vehicles and offshore wind turbine blades. GEIC is working with industry customers on construction materials, incorporating graphene into concrete and asphalt mixes. Adding very small amounts of graphene powder to concrete mixes dramatically increases concrete’s compressive strength and reduces the amount that builders need by 30%. Since concrete production releases a lot of CO₂ into the atmosphere, the use of graphene provides an environmental benefit. Putting graphene into polymers, foams and textiles can improve their fire retardancy. “Several industries looking at fire prevention in aerospace have had some very positive results in thermoplastics and textile materials,” says Scullion. This flame retardancy is usually just one of the many beneficial properties that graphene delivers for these applications. Graphene imparts anti-biofouling and anti-corrosion properties to coatings, and that could have a global economic impact. “Biofouling on the hulls of commercial ships costs the shipping industry $36 billion a year in extra diesel fuel because it creates a drag in the water,” says Rodgers. “Corrosion on bridges, rebar, automobiles globally costs us about $2.4 trillion dollars.” Because graphene is a very flexible material, it can be used as a sensor in composite products. Crash helmets and sports helmets could be designed so that they measure the impact of a ball or other object; if someone gets hit in the head it would be easier to tell if they had a concussion and needed further medical attention. Incorporated into vehicle parts, graphene could provide a variety of more sensitive and less power-consuming sensors. “With graphene, composites manufacturers can take advanced composites, which are already amazing, and make them even better,” says Barkan. Composites with graphene may now be able to compete directly with metals because of the strength improvements the nanomaterial imparts. Adding graphene to thermoplastics raises the thermal deformation temperature, so they can now be used in applications in temperature ranges where they couldn’t be used before. While graphene may not be a wonder material, its multiple properties should open up a wide range of new opportunities for the composites industry. “As prices for graphene and graphene-related materials come down and the methods for making graphene at scale are refined, we should start to see more and more graphene composites in the market,” says Scullion. “The incredible properties that graphene materials can bring mean that there is a huge drive to make this happen.”