The amount of carbon fibers (CF), which are used in carbon fiber reinforced plastics (CFRP) for lightweight applications in the aerospace and automotive industry, is increasing. They are put into plastics in different shapes with the aim of improving the mechanical properties, e. g. stiffness, and by that to reduce the weight of a component.
Approximately 90% of CF are produced out of polyacrylonitrile (PAN) as endless fibers. The central production steps, pyrolizing and carbonizing, take place at temperatures up to 2000°C. This fact causes the high amount of energy that is needed and the costs that come with it. From a sustainable and economic point of view the recovery of the CF and their continued use after the end of life of a component is very reasonable and desirable.
The recycling of CF bears many challenges. During the pyrolysis not only the polymeric matrix is disintegrated, but also depending on the surrounding gas and temperature the organic CF are affected. To remove only the polymeric matrix while conserving the fiber the conditions of the pyrolysis need to be maintained in the most optimum and homogeneous way. Damages that might occur due to oxidation of the fiber surface strongly decrease the mechanical properties and thus the possibilities for re-use. An additional challenge of the recycled carbon fibers (rCF) is their comprehensive characterization, which allows a better identification of damages and classification of the fibers’ quality. Examples are the condition and length of the rCF after the recycling process as well as their mechanical properties.
The Institute for Composite Materials (IVW) works on the solution of those problems in several research projects. Especially for fiber analysis, optical characterization using scanning electron microscopy was performed. The rCF samples being investigated were obtained by pyrolysis. The matrix polymer of the CFRP was removed at temperatures up to 700°C and a defined gas atmosphere and exposure time. If those conditions are matched ideally with the CFRP, the CF remain undamaged. It is the challenge to identify that CFRP specific process window. For this purpose, the rCF generated was optically characterized using scanning electron microscope images and thus the effect strength of the different process parameters was specified. It was determined that the process conditions have a strong influence on the complete exposure and the diameter of the rCF. Especially oxygen (respectively air) as atmosphere during the pyrolysis can lead to significant fiber damages by oxidation (see the pictures). The obtained results are the basis for further work about the characterization of CF and the recycling of CFRP by pyrolysis. The mechanical properties and also the geometry of the fibers will be investigated. The goal is to transfer rCF in different components while considering measurable specific values to better identify application possibilities.
M.Sc. Kerstin Steidle
Tailored & Smart Composites
Institut für Verbundwerkstoffe GmbH
Phone: +49 (0) 631/2017 242