By Liam Critchley (chemistry and nanotechnology writer)
Graphene is often referred to as the ‘miracle material’ due to its vast range of fantastic properties. These properties have elevated graphene above other additive materials, and it has since become a material of interest in many applications and industrial sectors, one of which is composites.
However, when we talk about graphene, we’re not actually talking about a single material. Graphene comes in many forms based on its size, number of layers and defects. In recent years, the collective family of graphene materials has been dubbed ‘graphenes’.
This is important because each type of graphene has different properties that make it suitable for different applications. If you try using the ‘wrong’ type of graphene for an application, then you likely are not going to achieve the desired results. Therefore, choosing the right graphene material is the key to success, and failure with other types of graphene materials should not put you off. You just need to find the right fit.
There’s a lot of interest in graphene for different composites, from wearable technologies to battery electrodes, medical implants, aerospace and automotive lightweighting composites, concrete, polymer composites and many more in between. In this article, we home in on one of the graphene materials known as large, thin and nearly defect-free (LTDF) graphene flakes and why it is a beneficial choice for polymer composites compared to other graphene materials.
What is a composite?
A composite is made of two or more materials that, when combined, create a final material or product. By selecting and combining materials you can create a product designed for a specific application. Graphene is increasingly recognized as a game changing additive to make composites stronger, lighter and more flexible while also providing improved electrical and thermal conductivity (depending upon the need).
What is LTDF Graphene and Why is it the Best Choice for Composites?
As the name suggests, LTDF graphene flakes have very few defects (there are always going to be a few at the atomic level), and the lateral size of the flakes are much larger than the average graphene particle you find in graphene powders and nanoplatelets. LTDF flakes are also very thin and have an average thickness below 1 nanometre.
The lateral diameter of your average graphene powder typically ranges from 15-200 nm. Other forms of graphene (namely graphene oxide) used in composites can have nanometre-sized flakes ranging up to single digit microns. By comparison, the LTDF graphene flakes being produced by Avadain are on average 55 microns in lateral diameter.
So, why does all this matter? With composites, you get out what you put in. If you put in highly defected, small sized or many-layered graphene materials (especially as some ‘graphene’ materials are technically graphite), then the end product will be disappointing. However, if you put in a higher quality additive material with better mechanical properties, then you will get out a more durable, robust and structurally sound composite.
When it comes to composite materials, LTDF graphene significantly outperforms smaller flake size graphene on many fronts. There are several reasons for this, including the larger flake size having a much higher surface to volume ratio that improves the mechanical properties of the composite by providing a larger platform for the host to interact with at the molecular level.
The larger flake size of LTDF graphene also reduces the amount of graphene that needs to be added into the composite to see transformative effects. Depending upon the materials, the amount of LTDF graphene added can range from 1/10th of 1% up to 3%. This low dosing, combined with planar bonding, reduces the agglomeration risk posed by small flake or powder graphene dispersions.
Graphene has excited a lot of interest and engendered enormous hype due to its amazing mechanical properties, such as excellent flexibility and high tensile strength. However, the best tensile strength properties are found in graphene materials that have a lateral flake size larger than 30 microns. Above this diameter, you also start to see a much-improved filler-to-matrix contact area, good stress transfer efficiency at the filler-matrix interface and better crack bridging properties in the composite.
When it comes to utilizing the optimal properties of graphene as an additive, the ideal layer range is between 1 and 5 layers, so you don’t want a graphene material that extends beyond this for high value or demanding applications.
The overall benefit of using LTDF graphene is that the interactions between the larger flakes and the host enables less material to be used while achieving a superior mechanical and material performance. The low number of layers in LTDF graphene enables harnessing graphene’s extraordinary properties.
LTDF Graphene Flakes for Polymer Composites
The polymer composite market is huge and growing. According to Vantage Market Research, the global market value of composites was $93.46 billion in 2022 and was projected to increase to $151.24 billion in 2030. Analyst firm Markets and Markets estimated the global composites market at $113.7 billion in 2022, growing to $168.6 billion in 2030.
Polymer composites rely on additive materials to give them the desired physical properties. Graphene is increasingly being integrated into polymer composites to make them more durable and structurally sound. Many polymer composites, including epoxies, PVA, PC, PVDF and PS have all exhibited better properties when integrated with graphene. Polymer-graphene composites show significantly superior mechanical (flexural toughness and tensile strength), thermal, gas barrier, electrical and flame-retardant properties compared to ‘pure’ polymer materials and have garnered a lot of interest in both structural and conductive composite applications.
The quality of graphene polymer composites is influenced by the intrinsic properties of graphene, including its level of defects, size and interfacial reactions with the polymer host. The key to a strong composite is the interaction between the filler and the host. The larger flake size provides a large surface area for the graphene filler to interact with the polymer at the molecular level and provide a tightly bonded network between each graphene flake and the polymer host.
However, agglomeration is an issue in many graphene-polymer composites. Agglomeratin is caused by van der Waals forces, causing graphene particles to be drawn to other graphene particles. Agglomeration causes clumps of graphene and prevents even dispersion. Agglomeration of the filler material in polymer composites often leads to a reduced interfacial interaction with the host. A lower degree of interfacial interactions in the composite ultimately reduces the effectiveness that the graphene has on improving the properties of the composite. This often leads to below-expected mechanical properties, and agglomeration is one of the biggest challenges in the nanocomposite field today. From a mechanical property perspective, agglomerations also cause weak points between the clumps of agglomerated graphene. Thus, the ’wrong’ type of graphene actually makes the composite susceptible to breaking or deforming.
Given the challenges that agglomeration presents, there have been several methods developed that can remove this agglomeration. However, these methods—such as inducing mechanical stress to the composite—can disrupt the internal structure of the graphene-polymer composite, leading to poorer mechanical properties. Among other concerns, these methods also tend to break the graphene, resulting in smaller size which lacks graphene’s fantastic strength.
By using larger flake sizes, you avoid this agglomeration issue (as they don’t naturally agglomerate like smaller flakes) and there’s little or no need for post-processing treatments that could affect the overall strength and flexibility of the material. This is why LTDF graphene can be used to overcome some of the key challenges when manufacturing graphene-polymer composites.
There are also polymer composites that have a small amount of conductivity (small compared to other materials). The addition of LTDF graphene into these electronically-focused composites means that you can achieve a more robust electronic performance while still utilizing the mechanical properties of the polymer for the intended application—for example, using conductive polymer composites in wearable technologies, flexible sensors or EMI shielding applications.
Conclusion
The beneficial properties afforded by LTDF graphene flakes over smaller graphene materials are important and numerous. Many of the current approaches towards using smaller graphene flakes or graphene powders have been financially driven, but this doesn’t necessarily yield the best results. In fact, it may introduce weak zones and deformation. Achieving the best performance results for your composites (regardless of whether it’s a structural, thermal or conductive composite) comes from using a more pristine and larger than 30 microns graphene flake.
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