Carbon fibre reinforced plastics are lightweight and stable materials. They are often used in shipbuilding, aircraft construction and the car industry, since they ensure safety and minimise transport weight. BAM’s researchers want to further improve fibre-reinforced plastic (FRP) composites by adding nanoparticles. The aim is to develop a novel plastic and to make it suitable for the market.
Precise measuring methods for fibre-reinforced plastics
“We are developing a modular system that allows us to predictably change the properties of the plastic composites using specially selected nanoparticles. This way we can develop a new class of plastics,” explains Dr. Dorothee Silbernagl, who leads a German Research Foundation (DFG) project on fibre reinforced plastics (FRP) materials of the future. BAM has developed several new measuring methods with a resolution of up to ten nanometres, which provides a significantly higher data density, to help study the complex inner workings of FRPs. This allows a very precise view into the material.
“Not only do we study the structure of plastic composites, but we also measure how this tiny section of the structure reacts to external forces,” says Prof. Dr. Heinz Sturm who is also involved in the project. This information is important for understanding the properties of a material in order to make a targeted change.
Carbon fibre reinforced plastics are complex systems. They consist of two components: carbon fibres and the matrix in which the fibres are embedded, such as an epoxy resin. Both ingredients are important: carbon fibres ensure high tensile strength, but it is the epoxy resin that enables the individual fibres to form a network and react to an external force. During plastic production, the fibres are usually formed into the shape before adding the liquid epoxy resin. The resin cures and glues the fibres together in a matrix. “For decades, there have been very strong and successful efforts to optimise the adhesion between carbon fibres and the matrix, or the epoxy resin,” says Sturm. “It was recognised early on that it was the weak point of the material. Now we are working on optimising the matrix.” If the resin hardens too much, it becomes brittle. If it reacts too weakly, it does not reach the rigidity for which the component is designed. It is becoming increasingly complex to control this process because the fibre’s binding to the matrix must also be considered.
Nanoparticles alter mechanical properties
The DFG project investigates the way in which nanoparticles affect the mechanical properties of the matrix. It used to be a common belief that increasing the stiffness of FRPs requires nanoparticles that are stiffer than the matrix into which they are stirred. The researchers were able to refute this with boehmite particles. Boehmite is a special form of aluminium hydroxide. The mineral occurs in nature, but also occurs as a byproduct in aluminium production. “We have chosen boehmite because not only can we use the pure mineral, but we can also optimise the surface of the particles using chemical methods,” explains Sturm.
The research teams have used two effects. On one hand, the presence of nanoparticles alters the curing process of the epoxy resin. On the other hand, they modify the surface of the boehmite. This creates additional anchors for bonds within the matrix or to the carbon fibres. “If we use particles whose surface has been changed by acetic acid, the product becomes more brittle. If we use taurine for the modification, the opposite happens,” says Silbernagl about the initial results of the research. “Thus, we can change the properties of the FRP by adjusting the matrix while the contribution of the carbon fibres remains unchanged.”
This knowledge enables targeted improvement of the material’s rigidity or damage tolerance. “We can now improve a material much more decisively, because we can look closely at what happens at the nano level,” says Silbernagl. BAM can measure quantitative changes in the plastic composite’s properties. “We can calculate the elastic part of the surface and the portion that absorbs external energy like a shock absorber,” explains Sturm. Being able to change this ratio in a targeted way provides an important tool for designing new materials. “This opens up new applications in the lightweight construction of aircraft and automobiles,” explains Sturm. “And after the transfer to production processes, it will also enable the construction of more efficient and cost-effective wind turbine blades.”
Material of the future should react individually
However, the plans of the DFG research group go even further. They want to use different types of nanoparticles with different surfaces. “Our vision is to add a lot more additives that react individually depending on the thermal, chemical or mechanical stress on the material,” says Sturm. Thanks to the modular principle, the material subsequently gets the previously specified properties.