DFG-SFOB – Process analysis of pseudo-plastic behavior of unidirectional reinforced staple fiber organic sheets

In the project “Process analysis of pseudo-plastic behavior of unidirectional reinforced staple fiber organic sheets”, the application of staple fiber yarns made from recycled carbon fibers (rCF-SF) for the production of organic sheets and their behavior in the thermoforming process is investigated.

Organic sheets are established in the field of fiber-reinforced thermoplastics due to their possibility of being processed into components quickly during the thermoforming process. They also have a good shelf life as well as an uncomplicated handling and offer excellent mechanical properties based on continuous reinforcement with fabrics/non-crimp fabrics. However, a major disadvantage of organic sheets is that they are not plastically formable due to the continuous fiber reinforcement. The loading of the continuous fibers almost immediatelly leads to material failure due to low elongation at break. Organic sheets are only drapable and thus only allow limited geometric component complexity.

With the increasing application of carbon fiber reinforced plastics (CFRP), there will also be an increasing number of CFRP components in the future that will require recycling at the end of their life cycle. Currently, the focus is on recycling production waste generated during component manufacture, such as scrap parts or CF semi-finished product waste. These materials need to be classified into pre-impregnated CFRP semi-finished products and dry CFRP waste. The dry wastes, which accumulates in significant quantities, includes coil remnants or faulty production during CF manufacture, as well as offcuts from manufacturing textile intermediates.

One way of converting this dry CF waste into a semi-finished product suitable for further processing with the longest possible fiber length and maximum orientation is to produce so‑called staple fiber yarns. The process begins by pre-dissolving and cleaning the waste. In the carding process, the fiber flakes are broken down to single fibers, parallelized and further cleaned, removing short fibers and forming a sliver. In subsequent process steps, the parallelized fibers are stretched and further parallelized. Finally, in the yarn formation process, the sliver is stretched to the desired yarn count and compacted for consolidation. Compacting can be achieved by twisting, wrapping the sliver with winding filament or wrapping it with sheath fibers, depending on the yarn formation process.

In the project " Process analysis of pseudo-plastic behavior of unidirectional reinforced staple fiber organic sheets", staple fiber yarns made of rCF and PA6 fibers, produced by Wagenfelder Spinnerei GmbH, are further processed into tape semi-finished products in a modified impregnation and calandering process. Yarns are pulled off the spools and compacted by the first pair of rolls. They are then heated above melting temperature of the polymer by hot gas nozzles and stretched by a speed difference to the second pair of rolls to improve the fiber alignment of the recycled carbon fibers. In the second pair of rolls, the stretched yarns are impregnated and consolidated into a staple fiber tape (SF tape). The influence of the crucial process parameters (temperature, process speed and stretching) on the degree of fiber orientation and the resulting width and thickness of the tape are determined using a polarization camera and a laser profile sensor. The degree of consolidation, fiber distribution and orientation of the tapes are determined by microscopic examinations. By digitizing the SF tape characteristics, possible problems and questions along the process chain up to the thermoforming result can be traced back directly to the SF tape.

The produced rCF SF tapes are further processed into staple fiber organic sheets (SF OB) in the tape laying process with additional stretching. The resulting thinning of the SF tapes due to the additional stretching is compensated by additional tape layers in sheet width and thickness. To ensure a consistent laminate quality, all laminates are post-consolidated in an autoclave prior to characterization. The mechanical properties of the SF OB determined in tensile and flexural tests are compared with those of laminates made from virgin tape material to define an evaluation measure for fiber orientation. In addition, the SF OB are analyzed in detail for their pseudo-plastic material behavior in temperature-controlled tensile tests. For this purpose, a novel test module was (developed that uniformly heats the tensile test specimens in the loading zone and then reconsolidates them after the tensile test. The results of the temperature-controlled tensile tests are verified in the thermoforming process and, based on these results, an existing description model for process behavior in the thermoforming process  is calibrated.

The overall objective of the project is to enable new fields of application through an innovative process chain that allows for the recycling of CF primary waste and offers new application possibilities through a novel pseudo-plastic deformation behavior in thermoforming.

The project "Process analysis of pseudo-plastic behavior of unidirectional reinforced staple fiber organic sheets" is funded by the German Research Foundation (DFG) – funding reference 471480678.

Further Information:

Martin Detzel M.Sc.
Leibniz-Institut für Verbundwerkstoffe GmbH
Press & Joining Technologies
Erwin-Schrödinger-Straße 58
67663 Kaiserslautern
Phone: +49 631 2017-164
Email: martin.detzel@ivw.uni-kl.de