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, Volume 11, Issue 5, pp 12–19 | Cite as

Sustainable Use of Carbon Fibers through CFRP Recycling

  • Tim Rademacker
  • Marc Fette
  • Günter Jüptner
Cover Story
  • 133 Downloads

CFK Valley Stade Recycling uses the pyrolysis process for recycling CFRP waste. Composites made from recovered carbon fibers have comparable and in some cases even better mechanical properties than reference materials. This enables the sustainable and economical use of carbon fibers in the aviation and automotive industry.

Recycling Inevitable

The Carbon Fiber (CF) market is rapidly growing. Due to new applications and an increasing interest in lifestyle products made of Carbon Fiber-Reinforced Plastics (CFRP), experts anticipate an increase in global CFRP production waste to 20,000 to 30,000 t per year by 2025.

Terms such as innovation, resource conservation and sustainability are increasingly present in our way of thinking. Therefore, CFK Valley Stade Recycling does not only focus on the disposal of CFRP waste but on the recycling and recovery of high-quality carbon fibers in order to re-introduce them to the market as CarboNXT products. Besides a short overview of the recycling process two possible applications using recycled Carbon Fiber (rCF) material are presented.

Carbon fibers are produced in a complex and energy-intensive process. From an economic and ecological point of view, recycling is therefore inevitable. The fiber is characterized by thermal and electrical conductivity, a low density of approx. 1.7 g/cm³ and high strength and rigidity. These features are perfect for lightweight design applications which should not get lost by disposing of the material. The reuse of recycled carbon fibers is the only way to achieve a faster break-even point in regard to the life of lightweight designs, considering the high CO2 equivalent used for new fiber fabrication.

Pyrolysis Process

In general, CFRP waste can be divided into three types of waste:

  • dry fibers from production waste, blends, cut-off waste, or scraps

  • prepreg material in the form of pre-impregnated fibers from production waste

  • end-of-life- or damaged CFRP parts with hardened matrix.

At present, CFK Valley Stade Recycling accepts waste from all branches throughout Europe: aviation, automotive, wind energy and sporting goods. When CFRP components reach the end of their life cycle - in the automotive and wind energy sector this can be after 10 to 15 years, in aviation after 20 to 30 years - they are recycled by CFK Valley Stade Recyling. Preparatory works of the delivered CFRP material are conducted before the pyrolysis process.

For presorting, Figure 1 (1), CRFP waste material can be accepted up to a maximum size of 6 m. If larger, the material must be pre- shredded at the customer's site in order to prepare for transport. On the plant premises the waste material is then sorted according to waste origin, processing state and fiber quality - differentiated according to CF residues, prepreg materials and end-of-life components - and shredded up to a size of a maximum of 1 m. The prepared material is then steadily transported through the pyrolysis oven using a special transport system.

Figure 1 CFRP recycling process of CFK Valley Stade Recycling in Wischhafen (Germany) (© CFK Valley Stade Recycling)

The pyrolysis process, Figure 1 (2), is used for energy production and resource recovery. In 2011, CFK Valley Stade Recycling started operating its pyrolysis plant with a capacity of approximately 1000 t/a and has since been running one of only three industrial plants worldwide. At temperatures above 350 °C, pyrolytic anaerobic decomposition of the plastic matrix takes place. Organic compounds are broken down thermochemically. The bonding resin becomes gaseous. The pyrolysis gases can be burned and thus supply the energy required for the process. Pure carbon fibers are exposed. Depending on the material, the retention time in the pyrolysis chamber is between 5 and 20 min. Due to the absence of oxygen the recovered exposed short fibers do not show any negative material changes or damage and have no residual adhesions such as pyrolysis coke on the fiber surface. The resulting pyrolysis gas is purified by thermal afterburning and subsequent filter systems and thus supplies heat energy for maintaining the pyrolysis process.

After being freed from any matrix the fiber has a clean surface. Within the refinement process, Figure 1 (3), the so-called resizing, recoating of the fiber surface, it is converted into a commercial product. Depending on individual customer requirements this includes cutting, Figure 1 (4) to a defined length or processing it into semi-finished textile products using various fleece laying processes which produce the products CarboNXT non-woven and CarboNXT veil, Figure 1 (5). These are non-woven textiles, Figure 2, which can be available as quasi-isotropic mats with grammages of 10 to 600 g/m2. Among other things, these non-woven products form an excellent basis for the production of Sheet Molding Compounds (SMC).

Figure 2 CarboNXT non-woven fleece pressed in a component (© CarboNXT)

SMC Fleece in Aviation Industry

Demand for and sales of passenger aircraft have grown by around 5 % annually in recent years. According to forecasts, growth will continue in the coming years [1]. However, the growing number of produced aircraft means an increasing number of fiber composites components. Against this background, work is increasingly being done on innovative material combinations and efficient production technologies for the reuse of carbon fiber recyclates in the aviation industry. Re-used high-quality CF recyclates could significantly improve material utilization, energy consumption and pollutant emissions throughout the entire life cycle, which would lead to increased environmental sustainability of future commercial aircraft.

A promising solution to bring recycled carbon fibers into aircraft applications can be SMC and also Bulk Molding Compounds (BMC) in combination with advanced compression molding technologies [2], Figure 3. Thus, for example, by impregnating rCF veils in a modified SMC impregnation process appropriate semi-finished products for compression molding can be produced in an industrial way. Figure 4 shows a process chain that is already in use for aerospace developments to produce complex components consisting of special SMC materials reinforced with rCF fleeces. Possible SMC formulations are based on thermoset matrix systems, such as unsaturated polyester, vinyl ester or epoxy resins.

Figure 3 Application example of a window frame covering as SMC component (© CarboNXT)

Figure 4 Schematic diagram of the impregnation and compression molding of SMC from recycled carbon fiber fleeces (© M. Fette [3])

Due to the compression molding process, these modified SMC compounds generate similar potentials as standard SMC materials from virgin endless carbon fibers. Consequently, geometrically complex components can be produced with the possibility of functional integration, for example by direct implementation of metallic load introduction elements. However, high cost and time efficiency cannot be generated solely by the possibility of functional integration. Similarly, short curing cycles of about 180 s, excellent material usage, high reproducibility and less rework add significant economic benefits compared to the established processing techniques in aviation industry. [2, 3]

However, component complexity is disadvantageously limited due to reduced flowability of the veils which can be compensated by a reduction in viscosity. This aspect must be taken into account in component development. Nevertheless, the SMC reinforcement by carbon fiber veils can cause an improvement of the mechanical properties and a reduction in density of the entire semi-finished product compared to glass fiber-reinforced SMC materials already used in the aviation industry. [3]

Comparing the properties of standard SMC reinforced by new chopped carbon fibers with SMC fabrics reinforced by recycled carbon fiber veils, similar or significantly better tensile as well as bending properties can be found. Table 1 shows a comparison of various SMC based on vinyl ester or epoxy resin systems and reinforced by different types of carbon fiber fabrics. Standard carbon fiber SMC materials are compared with SMC fabrics reinforced by airlaid veils consisting of pyrolized as well as non-pyrolized carbon fibers. To realize a reliable comparison all SMC fabrics with a similar fiber volume fraction and density are illustrated. The differences in the results of the SMC fabrics with different recycled carbon fiber veils are especially caused by the selected SMC formulation, the presence of or the type of carbon fiber sizing, the average fiber length and the fiber bundling as well as spreading during preparation for veil production. However, as already mentioned, flat and simple components are not made out of SMC materials in most cases. Consequently, the limited flow behavior of SMC fabrics reinforced by recycled carbon fiber veil as well as the lower part complexity must be considered. Furthermore, the values listed in Table 1 are based on standardized tests and appropriate specimens according to DIN EN ISO 527, DIN EN ISO 14125 and DIN EN ISO 179.

Table 1 Comparison of properties of different SMC materials (FVR: Fiber Volume Ratio) (© M. Fette)

SMC Type

Density

[g/cm3]

Tensile modulus

[MPa]

Tensile strength

[MPa]

Flexural modulus

[MPa]

Flexural strength

[MPa]

Impact Charpy

[kJ/m2]

Vinylester-based SMC materials

Polynt-SMCarbon 24-12K, reinforced by new chopped carbon fibers

FVR: 40 %

1.37

20,300

80

17,600

257

46

FVR: 50 %

1.40

25,350

108

22,650

285

46

Polynt-Recarbon 24, reinforced by recycled carbon fiber veils

Pyrolized carbon fibers

FVR: 40 % (400 g/m2)

1.36

26,000

195

24,300

260

15

Non-pyrolized carbon fibers

FVR: 40 % (400 g/m2)

1.35

20,000

183

19,800

340

35

Epoxy-based SMC materials

Polynt-SMCarbon 90-12K, reinforced by new chopped carbon fibers

FVR: 40 %

1.44

24,500

160

18,500

280

35

FVR: 50 %

1.46

29,400

195

23,750

322

52

Polynt-Recarbon 90, reinforced by recycled carbon fiber veils

Pyrolized carbon fibers

FVR: 40 % (400 g/m2)

1.47

30,100

210

33,000

335

12

Non-pyrolized carbon fibers

FVR: 40 % (400 g/m2)

1.47

32,800

233

30,100

140

32

There are currently promising research activities in the area of the development of new SMC formulations for applications in the aircraft cabin and in the cargo area. Those materials have to fulfill a high level of lightweight potential as well as the strict fire protection and flame retardancy requirements. For the fundamental material development and the fulfillment of the general requirements for aircraft interiors, special attention was given to further important aspects, such as manufacturing feasibility, surface quality, impregnation behavior, fiber-matrix adhesion and reduction of the density due to the use of alternative flame retardancy mechanisms.

In addition to the use of longer recycled carbon fibers for the production of semi- finished non-woven textile products for SMC, the production of thermoplastic compounds using a recycled reinforcing short carbon fiber proves to be extremely efficient and promising.

PC Compounds with Recycled Short Fiber

Hydroxyl, carbonyl, and carboxyl groups are located on the carbon fiber surface giving it a polar character. As polycarbonate (PC) is a relatively polar polymer, carbon fiber is an ideal partner to create high performance compounds by strong adhesion of the polymer matrix on the fiber. Hence, when bringing together PC and carbon fiber, no binder or adhesion promoter is required to achieve excellent interaction between the PC matrix and CF. This effect is supported by using CF, recycled by pyrolysis, showing a significantly higher concentration of these polar groups than that of virgin CF. As a consequence, PC compounds made from pyrolytic recycled CF are potentially of a higher mechanical strength than those based on virgin fibers.

The CF content and the CF length in a PC-CF compound are the main parameters with respect to the mechanical strength of the material whereas the influence of the molar mass of commonly used PC may be neglected. Figure 5 (left) shows a minimum weight average fiber length of 100 µm required to contribute to an increasing tensile strength. As the longest carbon fiber in this compound was found to be about 200 µm this seems to be the minimum fiber length required to increase the tensile strength. As demonstrated by Figure 5 (right) the Young's modulus is a linear function of the weight average fiber length.

Figure 5 Tensile strength (left) and E-Modulus (DIN EN ISO 180) (right) of PC compounds with 20 wt% CF as a function of the weight average fiber length (© G. Jüptner)

As shown in Figure 6, there is a linear relationship between flexural strength and tensile strength up to 40 wt% CF content, which approximately applies: Flexural strength = 1.5 × tensile strength.

Figure 6 Flexural strength (DIN EN ISO 178) of PC compounds as a function of the tensile strength up to 40 wt% CF content (© G. Jüptner)

Carbon fibers are sensitive against high shear, causing strong reduction of the fiber length. Hence PC-CF compounds have to be manufactured by mild extrusion conditions. For this purpose, the extruder screw must not contain heavy mixing or shearing elements. The extruder screw speed should be moderate too. In addition to that, CF must not be fed to the extruder at a point where still unmolten PC is present. Furthermore, to operate at relatively low viscosity of the PC melt, and thus run at low shear forces, carbon fibers should be added at a point where the PC melt has already reached the final processing temperature of the extruder.

The notched impact strength according to DIN EN ISO 180 of PC already drops to a value of 4 to 10 kJ/m² by adding low amounts of carbon fibers of less than 5 wt%. This is about a tenth of the impact strength of the pure matrix polymer. The impact strength slightly decreases with increasing CF content and decreasing CF length.

Good knowledge of the rheology as a main parameter of the flow properties of a PC-CF-compound melt is essential for optimum extrusion or injection molding processing. At a very low shear rate the viscosity of composites of linear PC and carbon fibers is significantly higher than that of the pure PC matrix, Figure 7 (left). This effect is stronger, the higher the melt flow rate (MFR) of the PC is (ISO 1133; MFR at 300°C; 1.2 kg stamp load). In the case of a MFR 3 g/10 min PC grade, the viscosity at very low shear of a composite with 20 wt% CF at 300 °C is about two times higher than that of the pure PC melt. For a MFR 60 g/10 min PC-grade, this factor was found to be about 30. Linear PC melts are showing a behavior close to a Newtonian fluid, i.e. there is only minor decrease of the viscosity with increasing shear in the range of low shear rates. By addition of carbon fibers, the melt shows shear thinning flow characteristics already at low shear rates, i.e. a decrease of the viscosity with increasing shear can be observed. As a consequence, the viscosity vs. shear curves of linear PC melts and those of the respective CF compound are approaching at higher shear rates. Hence at higher shear rates the melts of pure linear PC and the respective CF compounds are of similar viscosity.

Figure 7 Viscosity versus shear curves of linear MFR 3 g/10 min PC (left) and of branched MFR 2 g/10 min PC and the respective compound with 20 wt% CF (© G. Jüptner)

PC-CF-compounds of branched, i.e. structure viscous PC grades are showing a different behavior than those with a matrix of linear PC-grades. By addition of 20 wt% CF at lower shear rate there is no increase of the viscosity of the compound melt. Furthermore, at a higher shear rate these compounds by their more pronounced structure viscosity showed a significantly lower viscosity than the melt of the branched PC matrix itself, Figure 7 (right). This behavior is a potential benefit in processing a PC-CF-composite by extrusion or injection molding or blow molding.

By their high tensile and flexural strength and the reasonable impact strength, as well as their elevated electrical and thermal conductivity, PC-CF compounds are ideal lightweight materials in lightweight applications. Examples for the use of these composites are automotive components, electronic housings, antistatic sheets, or electromagnetic shielding.

Brief Outlook

Recycling and the associated recovery of high-quality carbon fibers under industrial conditions is a sustainable solution for all CFRP waste. While there are reservations from potential users and OEMs about the use of this high-quality secondary raw material, these can only be explained by a lack of confidence in quality and material availability. On the other hand, appropriate specifications are not provided. These obstacles can only be overcome by a strategic product development process that takes into account the reuse of secondary carbon fiber. Uncertainties and misinformation regarding availability of an industrial CFRP recycling process lead to the erroneous assumption that this is not solved. Economic interests often take precedence over sustainable solutions. It is therefore important that applications for high-quality secondary raw materials continue to be developed as a priority, accompanied by political framework conditions, so that industries producing CFRP waste take responsibility and consider reusing the material recovered from these wastes in their own product portfolio. Together with its sister company CarboNXT, CFK Valley Stade Recycling understands its innovative corporate philosophy to cooperate in developing process steps, starting with material preparation through semi-finished product manufacture to the final compound development, in order to close this CFRP material cycle jointly and sustainably. This is partly successful today. In many areas, however, there is a lack of further and greater willingness for such cooperation. |

References

  1. [1]

    Bundesverband der Deutschen Luft- und Raumfahrtindustrie e. V. (BDLI): Branchendaten der Deutschen Luft- und Raumfahrtindustrie 2017, Berlin, Germany, 2018, p. 2

     
  2. [2]

    Fette, M.; Wulfsberg, J.; Herrmann, A.; Stoess, N.; Rademacker, T.; Witte, T.: Hybride Faserverbundwerkstoffe mit Kohlenstofffaserrezyklate für Luftfahrtanwendungen. In: Zeitschrift für wirtschaftlichen Fabrikbetrieb ZWF, 109 (2014), pp. 663-668

     
  3. [3]

    Fette, M.; Rademacker, T.; Wulfsberg, J.; Herrmann, A.; Stoess, N.: Resource efficient and sustainable production of secondary structure aircraft components by using recycled carbon fibers for sheet molding compounds. In: Proceedings of the 10th Sampe China Conference, 2015, pp. 124-129

     

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Tim Rademacker
    • 1
  • Marc Fette
  • Günter Jüptner
  1. 1.CFK Valley Stade Recycling GmbHWischhafenGermany

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