Ultralight. Highly stable. Freely formable. These properties make carbon fiber reinforced plastics (CFRP) a sought-after material for lightweight construction applications. Colloquially, such carbon fiber-reinforced components are often abbreviated as carbon, for example in the case of carbon wheels. In particular, the production of carbon fibers and their use in polymers to form carbon fiber-reinforced plastics (CFRP) is very energy- and cost-intensive. Therefore, in terms of sustainability, such materials should be consistently reused. However, there is still a lack of processes for recycling these composite materials, and aspects of occupational safety are also insufficiently researched throughout the cycle. In the "Carbon Fibre Cycle" (CFC) project, researchers from KIT, RTWH Aachen, and the Leibniz Institute for Composite Materials (IVW), together with industrial partners VITROCELL Systems GmbH and PALAS GmbH, are investigating the extent to which processing and, in particular, their recycling at the "end of life" (EoL) could have an impact on health. Findings on release behavior and toxicity provide important clues for future occupational health and safety measures, as well as in the area of the circular economy of CFRP.

Too valuable to throw away

"Carbon fibers and composite materials were originally developed for the aerospace sector so that they could deliver top performance in these applications as a highly stable lightweight material. Due to its advantageous material properties, this high-tech material has also conquered other industries in recent years, from automotive to sports to applications in medical technology," tells project collaborator Sonja Mülhopt from KIT's Institute of Technical Chemistry (ITC). According to the market reports of the "Composites United" network for fiber composites, the demand for carbon fibers has almost tripled worldwide since 2010 and will be around 100,000 tons per year in 2022. An immense amount for which end-of-life (EoL) recycling is still largely open.

"The problem: There are still no sufficient, satisfactory solutions of a high-quality recycling for such carbon components," reports Werner Baumann, group leader particle technology at the ITC. "On the part of industry and research, various methods have been developed for this purpose, but further efforts are needed for their implementation and, in particular, for the market acceptance of recycled carbon fibers (rCF). In general, information on the fate of EoL materials has been insufficient so far," Baumann added. There is no final recycling and thus disposal in incineration plants because the residence times of the fibers in both household waste and hazardous waste incineration are not sufficient to completely degrade the fibers.

Young material with open questions

"The increasing manufacture of carbon products and a resulting increase in production and processing steps, as well as the associated disposal, can lead to the release of respirable carbon dusts. How does the material change during mechanical processing or thermal stress? And how are humans exposed in the process? These questions have not yet been adequately answered," says Mülhopt. She and her team, in close cooperation with the working groups of biologist Carsten Weiss at the Institute of Biological and Chemical Systems (IBCS-BIP) and chemist Andrea Hartwig at the Institute of Applied Biosciences (IAB), have been researching for many years how airborne particulate pollutants can affect the lungs. Asking questions about unresolved issues early on and putting carbon under the microscope is the goal of the CFC collaborative partners in the interdisciplinary project. "With the help of all partners, we are identifying, analyzing and evaluating respirable fiber fragments of carbon fiber reinforced plastic (CFRP) after mechanical processing or thermal treatment to learn more about material behavior. This systematic investigation provides initial indications of how the properties and thus the behavior of carbon fibers change over the course of their life cycle. This can of course have an influence on the effect in humans and toxicity," reports Carsten Weiss.

Examples from the past show how important such discussions and fundamental materials research are: asbestos is a natural fiber building material that was very popular from the 1930s onwards because of its practical properties. Almost 60 years later, the production and use of asbestos was strictly banned due to its carcinogenic properties. It was only after the building material was introduced that the danger was recognized. The whitening agent titanium dioxide is currently suffering a similar fate, as it has since been classified as a suspected carcinogen and its use as a food additive is therefore prohibited in the future.

Adapted exposure system

This involved the use of an exposure system for bioassays that Mülhopt and her research group developed together with colleagues at the IBCS and Vitrocell in a long-term collaboration. Co-inventor Mülhopt explains how the system works: "During exposure, we apply gas-borne particles to different types of lung cells: In the project, these were specifically carbon fiber fragments deposited on bronchial epithelial cells and cells of the immune system. An aerosol loaded with carbon fiber fragments flows over the surface of the biological cultures and induces dose-dependent reactions, such as inflammatory processes, through the deposited particles. The biological effectiveness of the particles on the cells is thus quantified in a realistic manner." KIT toxicologists, at the IBCS and IAB institutes, were able to observe exactly which cellular responses occurred based on gene expression changes that could be of concern to the lungs.

The commercial exposure system has so far been used primarily to study emission sources and environmental aerosols at particle sizes from 0.02 to 2 micrometers. Adapting the device to larger particles was therefore an important part of the project. Mülhopt reports, "We had to work mainly on the aerosol generation side. It was not easy to provide realistic test dusts with which investigations could be carried out. Vitrocell contributed its expertise in developing a completely new aerosol source for emissions from CFRP machining. This includes a new method to generate a test aerosol – that is, to bring fibers from composite materials into the aerosol phase. In addition, a pre-separator was adapted to map the nasopharynx." Project coordinator Mülhopt emphasizes, "There were many different adjusting screws that were worked on the system so that the overall concept delivers plausible results."

From knowledge gain to consequence

The findings from the CFC project provide the first, valuable indications of the extent to which carbon aerosols pose a health hazard to humans. "We can prove that the processed carbon material is not as uncritical as had been hoped. As a consequence, however, this does not yet mean that a ban on this material is necessary," summarizes Mülhopt. All those involved in the project agree: "In addition to the initial findings, many new research questions have been raised. At this early stage, it is impossible to make a generally valid statement." Carbon Fibre Cycle is culminating in important results that allow conclusions to be drawn about the protection of people involved in the processing operation. "One practical recommendation for action is to avoid exposure of employees by preventing the formation of such fibers and direct contact with them, for example by using appropriate protective equipment," Mülhopt says.

As the past teaches, the assessment of materials initially classified as safe can change. Mülhopt makes it clear: "We have received just a hint with the first project of this kind and the first data. The results make us sit up and take notice, and at the same time highlight the need for precautionary research, which should be perceived."