Organic Sheets Made of Recycled Carbon Staple Fiber Yarns
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In order to establish carbon fiber reinforced polymer composites (CFRPC) in the branch, solutions on how to close the material loop must be found. Quasi endless, aligned staple fiber structures are one approach for a true recycling of carbon fibers (CF) while maintaining their mechanical properties the best possible.
In the year 2020, around 20.000 tons of CFRPC waste is expected . That corresponds to an estimated amount of 12.500 (Estimation based on a FVC of 50% and an average resin mass density of 1.2 g/cm3) tons of CF, which is approximately 10 % of the today’s global production capacity . This significant amount of waste is divided into two thirds of in-house (post-industrial) and one third of end-of-life (post-industrial) of waste . These dimensions demonstrate impressively that an answer on how to recycle CFRPC must be found.
Less Downcycling, More Recycling
Today, different processes are available for transferring CFRPC waste into a second life cycle. All state of the art processes have in common though, that during the processing of the material a significant degradation of material properties must be accepted. This is due to the shortening of fiber length as well as a poor orientation in the final parts. Thus, state of the art recycling processes shall be classified as downcycling rather than recycling. Examples are rCF non-wovens, rCF compounds for injection molding or even rCF powder as a filler material for related processes.
A “true” recycling of CFRPC can only be achieved by fulfilling two major requirements. First, the length of fibers should be maintained since the length determines the mechanical properties of the final part. Literature recommends fiber lengths of at least 50 mm for being able to presume a continuous reinforcement . Second, reinforcing fibers have to be aligned in the final part in order to unfold their whole mechanical potential. Thus, it should be ensured that the fiber orientations in final parts are known and controllable. One possibility to fulfill these two requirements is provided by using staple fiber yarns made of rCF.
rCF in Structural Components by Applying Staple Fiber Yarns
One path for producing structural components made of rCF is therefore to use staple fiber yarns, which can be processed into biaxial non-crimp fabrics (NCF). By adding thermoplastic resin, the NCF can be used for producing organic sheets which are then serving as semi-finished products for the production of final parts via thermoforming processes. Also other applications as for example the combined process of thermoforming and injection molding can be imagined. Along the process chain it is a beneficial circumstance that industry-scale and efficient machinery can be used, which only have to be partially modified for the use of rCF.
Within the research project InTeKS (Innovative textile structures made of carbon staple fibers; development of a novel, plastically deformable organic sheet) the research consortium contributes to such development and is representing the whole process chain from regaining carbon fibers up to a final demonstrator part. In a first step, the company Altex gains the rCF from spool residuals from carbon fiber textile production and cuts them on a cutting machine from the company “Pierret” to a defined length of 80 mm. The prepared rCF are homogeneously mixed with polyamide 6 (PA6) fibers on a pin roll. The resulting advantage is that the material directly brings the necessary matrix with it and the fiber volume content (FVC) in the organic sheet can be adjusted by the yarn.
The development of novel organic sheets is accompanied by a tailored simulation model, which will be developed by the Institute for Composite Materials (IVW) and the company DYNAmore GmbH. Its main purpose is the simulation of draping behavior during the thermoforming process. Typical failures such as roving splaying, wrinkles, and material thinning can be represented and anticipated.
Mechanical Properties Comparable with Virgin Carbon Fibers
For the determination of tensile strength and modulus, unidirectional (UD) reinforced specimens were produced at the ITV by winding the yarns on a steel core and a following impregnation and consolidation in the autoclave. The plates were cut into multiple tensile bars. The tensile properties of these bars were tested at room temperature.
The average tensile strength at a FVC of 53 % was 1250 MPa, the tensile modulus was identified to be 94 GPa. Furthermore, the 0°/90° non-crimp fabrics were used to produce organic sheets which were impregnated and consolidated in a static press. At a FVC of 39 %, the average tensile strength of the balanced 0°/90° reinforced organic sheet was 290 MPa and a tensile modulus 26 GPa.
Out of the view of process, the flowability distinguishes the metals from organic sheets.
Apart from the CFRP Bond Laminates organic sheet, all materials contain a PA6 matrix. The comparison reveals that the not yet optimized press process of the 0°/90° rCF organic sheets already provides slightly better tensile strengths and roughly doubled tensile modulus compared to the GFRP alternative. Considering the theoretically achievable values (InTeKS 0/90 Autoclave Theoretical), it is shown that rCF organic sheets can approach those of vCF in their tensile properties after a process optimization. All in all, the development of rCF organic sheets is closing the out of a design perspective interesting niche between GFRP and CFRP organic sheets.
Drapability Leads to New Applications of Organic Sheets
Organic sheets are effective semi-finished products for the production of fiber reinforced components in various ways. They are easy to handle, to store and offer a wide range of application due to their out-of-autoclave technology. Moreover, many manufacturers already have the necessary production technologies available in order to process organic sheets. The main advantage though is the extremely short cycle time, which is the key for industrial applications of CFRPC.
Due to the quasi-isotropic properties of biaxial or even multiaxial reinforced organic sheets with at the same time high mechanical characteristics, they are generally suitable for substituting metal components. However, today there are still restrictions for the substitution of metallic parts by CFRPC solutions. This is also caused by the limited geometrical part complexities which can be realized with organic sheets since their deep drawing abilities are restricted. Having said that, metals show an excellent deep drawing behavior. For example, the forming of corrugations is feasible by local thinning of the material.
Out of the view of process, the flowability distinguishes the metals from organic sheets. In the case of the latter the lack of flowability is to be considered as a disadvantage. At this point, the concept of organic sheets made of rCF staple fiber yarns offers a solution: based on the staple fiber structure of the material a plastic deformation of the organic sheets at thermoforming process temperature is possible and can be compared to the deep drawability of metal materials. In consequence, the novel staple fiber organic sheets do not only improve the drapability but also introduce a deep drawability to this class of semi-finished products.
The organic sheets do not only improve the drapability but also introduce a deep drawability to this class of products.
The ability to plastically deform has been validated at the IVW on UD tensile bars in a test rig specifically designed for this purpose. The tensile bars were 250 mm long, 15 mm wide and 1,75 mm thick. The specimens were clamped horizontally with solid steel clamps. In order to avoid the fixation of single fibers at both ends, a clamping length of 200 mm was defined. Afterwards, the bars were heated up with infrared (IR) heaters from below and above, at a distance of around 150 mm between heater and sample. Calibrated infrared cameras were used to control the specimens’ temperature. At 245 °C the specimens were drawn with 50 mm/min speed until a defined stretching length was reached. The chosen lengths for the series were 15 mm, 25 mm and >50 mm, until the specimen was torn apart.
Economically and Ecologically Attractive?
Through the use of favorably priced rCF it can be assumed that rCF organic sheets are less expensive than conventional ones. Indeed, the costs for rCF are 25–50% lower than for vCF, thus there is potential for cost reduction in the raw materials [5, 6]. But these cost reductions are opposed to additional process steps in the manufacturing of organic sheets, namely the preparation of the fibers, the carding and the yarn spinning. Since these are industrially common processes designed for big volumes, it is possible that the cost savings in the raw material will exceed the additional costs in the processes. Further investigations are necessary though in order to quantify this approach.
It can be stated that a plastic deformation of the staple fiber organic sheets up to 25 % is possible.
Beside the expected economic advantage the ecologic benefits are obvious, if vCF are replaced by rCF. Since the production of the CF is consuming most of the necessary energy for manufacturing CFRPC components, the use of rCF is representing significant energy savings. Having that put into numbers, the production of vCF consumes 55 – 165 kWh/kg while the pyrolysis process for rCF only consumes 3 – 10 kWh/kg . The energy savings are even higher in the case of in-house wastes.
By the holistic consideration of the process chain and the inclusion of every single process step the production of organic sheets made of rCF staple fiber yarns has been succeeded. Thereby, the use of organic sheets in complex structural parts has come closer. Alongside the newly gained degrees of freedom regarding part complexity, the use of rCF will reduce material costs and improve the ecological impact of such semi-finished products. These factors combined with the short cycle times of the thermoforming process will raise the attractiveness of organic sheets for the automotive sector. The possibility to recycle the CFRPC applied in cars will back this attractiveness. Finally, out of a scientific perspective the use of rCF staple fibers will initiate further research in the field of staple fiber structures in CFRPC.
The project „InTeKS“ (Innovative Textile Structures Made Of Carbon Staple Fibers; Development of an innovative, plastically deformable organic sheet) receives funding through the Bundesministerium für Wirtschaft und Energie. (Förderkennzeichen VP2088343TA4)
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