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, Volume 10, Issue 2, pp 16–19 | Cite as

Hybrid Composites with Recycled Carbon Fibres

  • Renate Lützkendorf
  • Thomas Reussmann
  • Martin Danzer
Materials Hybrid Composites


Carbon Fibre Natural Fibre Hybrid Composite Semifinished Product Natural Fibre Composite 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Hybrid composites made of natural fibres and recycled carbon fibres possess significantly better mechanical properties compared to conventional natural fibre composites. The potential of the new material class and its process-related challenges is part of the investigations at the Thuringian Institute of Textile and Plastics Research (TITK).

Natural fibre reinforced plastics (NFRP) have been used to manufacture components in the automotive industry for many years. The main reasons for this are the good specific material properties (high stiffness accompanied by low density), the favorable price but also the positive ecological footprint compared to other materials. Typical applications are door panels, trunk covers and instrument panels in the automobile interior. The usual grammages of such materials range from about 1600 to 2000 g/m2. With regard to the required weight optimisation in new vehicles, a reduction in the grammages of the semi-finished products is necessary. With the currently used natural fibre composites, this cannot be achieved without reducing the mechanical values (stiffness, impact strength). Hybrid composites with recycled carbon fibres (rCF), which allow a significant reduction in component weight with the same or better mechanical properties, are an alternative.

Manufacturing of Natural Fibre Semi-finished Products

The processing of NFRP is mainly carried out in the form of compression molding processes. Thereby, non-woven natural fibre mats made of 40 to 60 mm long natural fibres (flax, hemp, kenaf, etc.) are used. The non-woven mats are produced by carding or aerodynamic processes. Additionally, wood fibre composites are still being used to a great extent. Depending on the matrix material used, natural fibre composites can be subdivided into:
  • ▸ thermoset matrix (e.g. PUR, epoxy resin, acrylic resin, phenolic resin)

  • ▸ thermoplastic matrix (e.g. PP).

Thermosetting natural fibre composites are based on 100 % natural fibre mats which are impregnated with resin in a subsequent step and then pressed in hot tools into the component shape. Typical fibre contents in thermoset natural fibre composites range from 60 to 80 % by weight. In case of thermoplastic natural fibre composites, mixtures of thermoplastic fibres and natural fibres are used to produce nonwoven fabrics. The mixing ratio adjusts the fibre content of the composite and the mechanical properties. Thermoplastic natural fibre materials generally possess lower fibre contents from 40 to 60 % by weight. As a result, the mechanical properties of natural fibre reinforced thermoplastics are not as good compared to thermoset NFRP.
An optimisation of the composite properties can be achieved by:
  • ▸ variation of matrix material (thermoplastics or thermosets)

  • ▸ optimisation of fibre-matrix-adhesion (utilisation of coupling agents)

  • ▸ variation of type of natural fibre (fibres with different mechanical properties)

  • ▸ admixture of other fibre materials (e.g. glass or recycled carbon fibres).

Earlier investigations concerning the optimisation of impact behaviour of natural fibre composites [1] show that the admixture of fibres with high tensile strength results in a significant improvement in impact strength. High stiffness values, on the other hand, can be achieved by the admixture of fibres with high E-modulus, e.g. glass or carbon fibres. For cost reasons, recycled carbon fibres from cutting and preforming processes can be used. High-quality fibres can be obtained by a special textile process from carbon fibre cut-offs [2].

Manufacturing of Semi-finished Products with Recycled Carbon Fibres

The integration of recycled carbon fibres into natural fibre composites can be achieved in various ways. In the first option, Figure 1, the carbon fibres were processed directly as a fibre-fibre mixture together with the natural fibres and PP fibres in the carding process. The desired carbon fibre amount is mixed with the other components and fed into the nonwoven machine. The equipment technology (fibre opener, mixing and carding machine) is industrially available but has to be adapted with regard to the processing requirements of carbon fibres (encasing of electrical installations, adjustment of the pins on the carding machine). If these conditions are given the direct admixture of fibres can be implemented efficiently and cost-effectively.
Figure 1

Non woven natural fibre, rCF and PP fibre (© TITK)

A second option, Figure 2, is the application of thin cover layers with carbon fibres on top of the natural fibre semi-finished product (for example by needle punching or thermofixation). Thus, a layer structure can be realised which allows the subsequent optimisation of the properties of natural fibre reinforced plastics. This work step can also be carried out at the component manufacturer and is less demanding regarding the equipment technology than the processing of mixtures with carbon fibres. For this second option, thermoplastic carbon fibre semi-finished products for the outer layer have to be available. These products must be manufactured in a separate process line. The company SGL / ACF in Wackersdorf already produces such semi-finished products with recycled carbon fibres which can be used for the production of layer structures [3].
Figure 2

Layer construction of natural fibre, rCF and PP fibre (© TITK)

Composites with recycled carbon fibres possess significantly higher bending stiffness values than pure natural fibre composites.

Composite Manufacturing and mechanical Properties

The production of the natural fibre mats was carried out in the laboratory of the TITK on a carding machine. The recycled carbon fibres were integrated into natural fibre composites through homogeneous fibre mixtures as well as layer constructions. The admixture of the carbon fibres varied between 10 and 30 % by weight. The total content of reinforcing fibres in the composites was always 50 % by weight.

The textile semi-finished products were pressed to flat sheets having densities of 0.8 g/cm3 and 1.0 g/cm3. The composites were produced in a two-stage process:
  • ▸ Heating via contact plates, 210 °C, 2 min heating time

  • ▸ Consolidation within tool at 30 °C, 1 min pressing time

Samples for composite tests were milled out of the composite sheets and tested according to the following standards:
  • ▸ tensile test DIN EN ISO 527-4

  • ▸ bending test DIN EN ISO 178

  • ▸ determination of bending forces at constant bearing distance (30 mm) and 2 mm deflection, 10 mm/min testing speed

  • ▸ impact test DIN EN ISO 179-1.

Figure 3 and Figure 4 show the bending properties of NF/rCF/PP compounds from mixed nonwoven fabrics with different rCF fractions. Without optimised fibre matrix adhesion, the bending strength and the bending modulus increase only slightly with increasing carbon fibre content. A higher compaction of the composite material (1.0 g/cm3 compared to 0.8 g/cm3) results in significantly better mechanical properties. The use of modified PP fibres (including maleic anhydride) resulted in a significant increase in the mechanical properties, whereby the increasing carbon fibre content also showed clearer increasing effects on strength and modulus.
Figure 3

Bending strength depending on rCF — content (© TITK)

Figure 4

Bending modulus depending on rCF — content (© TITK)

Based on the bending properties of the composites with natural and carbon fibres, a high weight reduction of automotive interior components is possible. However, a reduction in the grammage results in a significant reduction in the material density at a constant tool gap (same component thickness) or leads to a reduction in the component thickness when the composite density is maintained. In order to investigate these interactions, additional bending tests with constant bearing distance were carried out and the mechanical properties were compared with the values of natural fibre composites made of nonwoven mats with approx. 1800 g/m2 grammage (reference material), Figure 5.
Figure 5

Bending strength of natural composites depending on grammage (© TITK)

Taking into account the findings from the testing of composites with different densities, a high density of 1.0 g/cm3 was set in the tests. In the case of lighter semifinished products, this leads to low sheet thicknesses in the range of 1.4 to 1.5 mm. The bending test results at a constant bearing distance show that the grammage of the semifinished products with carbon fibres can be reduced to approx. 1300 g/m2 without a reduction in the bending force at 2 mm deflection. This corresponds to a weight saving of approx. 30 %. The effects of different integration options (homogeneously mixed nonwovens versus layer structures) on the bending modulus can be seen in Figure 6.
Figure 6

Bending modulus of sheets made of different composite constructions (© TITK)

Composites with recycled carbon fibres possess significantly higher bending stiffness values than pure natural fibre composites. The highest mechanical properties can be achieved with layer constructions. The mechanical values are adjustable through the rCF content in the cover layers. The use of a coupling agent mostly leads to an improvement of the bending values. In the case of hybrid modifications, the bending properties could also be increased by optimising the fibre matrix adhesion.

Component Manufacturing and Testing

The production of demonstration components was carried out in cooperation with an industrial partner from the automotive supplier industry (Boshoku Automotive Europe). Door panels, Figure 7, consisting of different semi-finished products (layer construction and hybrid construction) were pressed and their mechanical properties tested. The component investigations confirm that the significantly lighter versions with added recycled carbon fibres (1250 to 1300 g/m2) possess equal or better component strength and stiffness compared to pure natural fibre composites (1800 g/m2).
Figure 7

Door panel made of NF/RCF/PP composite (© SGL ACF Wackersdorf)

Figure 8 shows a comparison of the component weights after the pressing process. Compared to the reference material — made of flax fibre reinforced PP — weight savings of 30 to 40 % can be achieved. The mechanical properties of the weight-optimised components meet all requirements according to the OEM’s delivery specification.
Figure 8

Component weight regarding different composite constructions (© TITK)

Summary and Prospect

Hybrid composites made of natural fibres and recycled carbon fibres possess significantly better mechanical properties compared to conventional natural fibre composites. The admixture of carbon fibres is possible by means of fibre-fibre mixtures (production of nonwovens) as well as with layer constructions (combination during nonwoven fabrication or further processing). Component manufacturing can be carried out by compression molding and if required in combination with injection molding.

Good fibre matrix adhesion and high composite densities are crucial for optimum mechanical values.

Based on the higher mechanical properties of the natural fibre / carbon fibre hybrids, it is possible to reduce the grammage of natural fibre reinforced semi-finished products and components by approx. 30 % without any loss in the mechanical properties. Good fibre matrix adhesion and high composite densities are crucial for optimum mechanical values to fully utilise the mechanical properties of the carbon fibres.

The natural fibre-carbon fibre hybrids can be used for interior components in the automotive industry. Typical applications are door panels, instrument panels and other components with higher mechanical requirements. The TITK is currently working on the optimisation and industrial implementation of the novel material together with industrial partners.



We would like to thank Josef Huber-Hesselberger for his professional and constructive support as well as Stephanie Cierpka for the practical implementation of the experiments.


  1. [1]
    Oberländer, E.; Reußmann, T.: Optimierung der Crashverhaltens von naturfaserverstärkten Kunststoffen. In: Lightweight Design 6 (2013), No. 4, pp. 20–25CrossRefGoogle Scholar
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    Reußmann, T.; Oberländer, E.; Danzer, M; Honderboom, A.: Verbundwerkstoffe aus Recyclingcarbonfasern. In: Lightweight Design 7 (2014), No. 6, pp. 18–24CrossRefGoogle Scholar
  3. [3]
    N.N.: Recycelte Carbonfaservliese. Firmenprospekt SGL Group 2016Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Renate Lützkendorf
    • 1
  • Thomas Reussmann
    • 1
  • Martin Danzer
    • 2
  1. 1.Thuringian Institute for Textile and Plastics Research (TITK)RudolstadtGermany
  2. 2.SGL ACF GmbH & Co. KGWackersdorfGermany

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