Less Moisture in Natural Fibres
Various benefits of natural fibres make them attractive as an alternative to glass fibre-reinforced plastics (GFRP) and carbon fibre-reinforced plastics (CFRP): they are renewable, biodegradable and robust, as well as use less energy in being produced, are lighter and have better acoustic properties. Their drawback is that they absorb water very easily and hence become damaged and lose their otherwise positive mechanical properties. The Fraunhofer Institute for Structural Durability and System Reliability LBF, together with the Institut für Textiltechnik (ITA) of RWTH Aachen University in Darmstadt, Germany, has now solved this problem in the exploratory study "Bastfix", which is funded by the German Federal Ministry of Education and Research (BMBF).
Fibre treatment and yarn technology cleverly combined
"A surface treatment of natural fibres with water-repellent layers alone doesn’t provide any progress," explains Dr. Roland Klein, Group Manager Design of Interfaces in the Research Division Plastics of Fraunhofer LBF. "The water then continues to penetrate the natural fibre via cut edges or other damage." For this reason, the scientists have treated the fibres, so that they are moisture-repellent on the inside. They achieved this by producing polymers inside the natural fibres. "First, we let the monomers of the plastic penetrate into the cavities of the natural fibres. The polymerisation then took place directly inside the fibres," says Klein, describing the core of the new fibre treatment. Use of this process is particularly interesting for thermoplastic fibre composites since, during production, the molten polymer is very viscous, does not penetrate into the interior and only wets the fibres superficially.
In their experiments, the researchers used flax fibres in the form of a roving. In doing so, the fibres can be completely soaked with the monomer, since they are still present individually. Complete impregnation might not be guaranteed on fabric or twisted yarn. In the wrap spinning process, the Aachen scientists arrange the natural fibres in parallel and wrap the parallel core of the flax fibres with a filament. This means that the fibres are not twisted, which gives them extra strength. "By combining both processes, the physical properties of natural fibres can be fully exploited. These two developments greatly expand the area of application for NFRPs, with the result that they can also be used outdoors and in highly stressed components," says Klein, describing their added value. NFRPs have up until now been used mainly in cars, for components such as door trim panels.