Fully impregnated fiber-reinforced thermoplastic sheets, or the so-called organic sheets, allow the thermoforming of parts within very short cycle times. This article describes the development of the next generation of organic sheet materials based on recycled carbon fibers and polyamide 6 staple fiber yarns. Regardless of the recycled nature of the fibers and an average fiber length of 25 mm, the organic sheets still reach a comparable level of the tensile strength and modulus of continuous fiber-reinforced organic sheets made of virgin CF with the same reinforcement structure. Due to the staple fiber yarn architecture, the organic sheets feature a deep-drawing ability of a total plastic deformation up to 50% in the fiber direction. The effect is enabled via an interfiber sliding when the organic sheet is processed in the molten condition. The creation of a finite element model for the thermoforming process simulation of the material is also presented. Predictions of the plastic strain distribution and its magnitude are shown to agree well with forming experiments where a curved geometry is formed to different depths.
The current growth in use of fiber reinforced polymer composites causes a strongly increasing amount of waste. Current approaches for fiber reinforced polymer composites recycling usually not exploit the potential of endless fibers as they are shortened during recycling and will not be properly aligned in the final product. Considering this, the present work aimed at the development of a recycling process for long recycled carbon fibers, where fiber length is preserved and load-related fiber orientation is possible. The starting point for the presented work was so-called slivers, which are long bundles of fibers resulting from a carding process that has been applied to fiber scrap. The main focus of this work was on the development of a binder mesh application rig that processes the sliver to a binder tape, processable in an automated tape laying process, which in turn required modifications to adapt to the novel tape. The functionality of the binder tape manufacturing process was validated with long recycled carbon fibers slivers with linear density of 4 g/m and fiber lengths between 70 and 120 mm. With the binder tape preform manufactured this way, two alternative routes for composite manufacturing were tested. First, the amount of binder was set so high that direct thermoplastic pressing of the preforms was possible. Second, the amount of binder was minimized, and the preforms were infiltrated with a thermoset resin system via resin transfer molding. While the thermoplastic route showed very deficient fiber–matrix adhesion, with the thermoset route, ≈68% of stiffness and ≈31% of strength of virgin fiber-based composites could be achieved in fiber direction in a unidirectional lay-up.
In order to sustainably establish carbon fiber reinforced polymer composites (CFRPC) in the market on an industry scale, solutions on how to recycle these new materials have to be developed. Quasi-continuously aligned carbon staple fiber structures in organic sheets made of recycled carbon are one approach which will be dealt with in this article. The process chain as well as the mechanical properties will be presented. Moreover, the specific feature of staple fiber yarns to be able to plastically deform under process temperature, enabling new degrees of deep-drawing of CFRPC organic sheets in the thermoforming process, will be highlighted.
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