​Basic research on rods made of recycled carbon fibers

Table of contents

1. 1. 1. Project data
      2. Report in the annual report 2024/25
      3. Report in the annual report 2023
      4. Report in the annual report 2022

Project data

Titel | Title TP Bewehrungsentwicklung und Charakterisierung stabförmiger Halbzeuge im Verbundvorhaben EDISON-rCF: Energieeffizienter und werkstoffgerechter Recyclingprozess von carbonfaserverstärkten Kunststoffen von der Wertstoffaufbereitung bis hin zum neuen Bauteil unter Anwendung eines innovativen Solvolyseprozesses sowie der Entwicklung und Herstellung neuartiger standardisierter quasi-unidirektionaler Halbzeuge | Subproject Reinforcement development and characterization of bar-shaped semi-finished products in the joint project EDISON-rCF: Energy-efficient and material-appropriate recycling process of carbon fiber-reinforced plastics from the preparation of recyclables to the new component using an innovative solvolysis process and the development and production of novel standardized quasi-unidirectional semi-finished products Förderer | Funding Bundesministerium für Wirtschaft und Klimaschutz (BMWK) Zeitraum | Period 01/2021 – 04/2024 Teilprojektleiter | Subproject leader Prof. Dr.-Ing. Steffen Marx Bearbeiter | Contributor Dipl.-Ing. Enrico Baumgärtel Projektpartner | Project partners Zeisberg Carbon GmbH, Hannover | DLR Institut für Faserverbundleichtbau und Adaptronik, Braunschweig | Institut für Aufbereitungsmaschinen, TU Bergakademie Freiberg Assoziierte Partner | Associated partners Extrathive SAS, Sorgues (FRA) | Fraunhofer IGCV, Augsburg | Veolia Umweltservice GmbH, Aubervilliers (FRA)

Report in the annual report 2024/25

Rebars made of rCF

The reuse of carbon fibers is becoming increasingly important in view of the growing scarcity of resources and climate protection efforts. Particularly in the construction industry, the holistic consideration of closed material cycles is becoming increasingly important. The open cycle is closed by processing recycled carbon fibers into semi-finished products.

Due to various technological hurdles, the continuous production of rebars from recycled carbon fibers (rCF) with consistent quality is still subject to large variations. Although the use of carbon materials has long been established in other branches of industry such as aircraft construction, automotive engineering, etc., recycled carbon is rarely used. In contrast, the construction industry in particular offers an extraordinary opportunity to use recycled carbon as a reinforcement material in concrete. This makes it possible to close the material cycle on the one hand and to use CO₂-reduced concrete matrices due to the corrosion resistance of carbon on the other.

The objective of the research, founded by the Federal Ministry for Economic Affairs and Climate Action, is to investigate, characterize and optimize carbon rebars made from recycled fibers for the use as reinforcement in concrete to achieve equivalent tensile and bond strength compared to semi-finished products made from virgin carbon fibers (vCF). In experimental tests, 600 mm long carbon fiber rebars with a bond length of 80 mm were embedded in concrete cubes in order to obtain a first indication of the bond behavior of rCF rebars (Zeisberg rCF Rebar®). Measurement data were recorded using displacement transducers and via load cell. In order to achieve higher bond between the concrete and the rebar, the geometric dimensions of the bars and the structural design of the semi-finished tool were modified and further developed. A straight fiber orientation as well as the reduction of pores or defects along the rebar axis are the most important apects for increasing composite strength. To determine this, the rebar was examined using computed tomography and microscopy.

Report in the annual report 2023

From recycled filament to the carbon fiber rebar

In the EDISON-rCF project, we conduct extensive experiments with semi-finished products made from recycled carbon fibers, including rods and fabrics. The tests aim to determine the products’ properties, such as tensile strength, bond behavior, and performance at varying temperatures. Fiber orientation, fiber volume content, fiber length, and impregnation material significantly affect the load-bearing behavior.

To investigate the influence of the recycling process (pyrolysis or solvolysis) on the material properties of carbon fibers, virgin fibers were also subjected to pyrolysis or solvolysis. After the recycling process, the tensile strengths of the virgin, pyrolyzed, and solvolysed fibers were determined. Compared to the virgin fibers, a tensile strength loss of around 10 % was observed for recycled fibers.

Afterwards, yarns were spun from the recycled fibers. Two impregnations, based on either a thermoplastic or a thermoset, were utilized to generate the bond within the roving. All yarns  analyzed in tensile tests failed at the point where the cross-section was smallest and within the free length between the load introductions. Regardless of the impregnation material used, the tensile strength of the recycled fiber rovings was clearly lower than that of rovings made of virgin fibers. It is crucial to optimize the manufacturing process.

To determine the tensile strength of recycled carbon fiber rods with an outer diameter of 10 mm and 1.20 m length, they were clamped between load introduction plates specifically designed for this purpose and loaded until failure. Two series of five rods were tested. The rebar’s elongation was measured using inductive displacement transducers and photogrammetry. Through the modification of the bar composition from series 1 to series 2 with regard to fiber orientation and fiber volume content, an increase in tensile strength was achieved. The tensile strength of the rods of the second series was roughly 25 % of that of the rods created from new carbon fibers.

Report in the annual report 2022

Reinforcements made from recycled carbon fibers

Within the EDISON-rCF project, funded by the German Federal Ministry of Economics and Climate Protection (BMWK), the recycling process of carbon fiber reinforced plastics is being investigated from the preparation of recyclable materials to new components. The semi-finished products, like scrims and rods, made from recycled carbon fibers produced in this project will be extensively analyzed in regard to their most important properties, e.g. bond behavior, tensile strength, durability and behavior under different temperatures.

By preparing a working paper, it was possible to establish the requirements demanded of the semi-finished products to be manufactured. In particular, the fiber length and the fiber orientation in the reinforcements are considered to be important properties for the load-bearing behavior.

The impregnation of the carbon fibers is also essential for the manufacturing of bars and fabrics. In order to make an initial statement about the suitability of an impregnating agent, yarns made from recycled carbon fibers were impregnated with polymer, duromer, and geopolymer and investigated for the respective tensile behavior. The yarn impregnated with epoxy resin prevailed due to the highest tensile strength and the highest stiffness. All yarns failed at the point of lowest cross-section and without advance notice. Using the thin section method, it was also possible to assess the impregnation quality.

The profiled carbon fiber rods made from recycled carbon fibers by one of our project partners had an outer diameter of 10 mm and a relative short length of 60 cm due to the current manufacturing process. The bars were embedded in 20 × 20 × 20 cm concrete cubes with 5 cm bond length. In order to be able to make a statement about the bond behavior of the bars, they were loaded to failure in pull-out tests. All bars tested failed before the onset of pull-out. Therefore, in the further course of the project, the increase of the inner bond of the bar is to be advanced.