recovering carbon fibers from composite materials that are discarded

Researchers show how the mechanical qualities of recovered carbon fibers are preserved through thermal degradation in superheated steam.

Our journey towards sustainable societies must include minimizing energy consumption, and advanced materials are essential to this goal. Composite materials, such as carbon-fiber-reinforced plastics (CFRPs) and carbon-fiber-reinforced thermoplastics (CFRTPs), have the potential to greatly increase energy efficiency across a range of applications.

Carbon fibers are incorporated into a polymer matrix, like epoxy resin, to create these composites. The fuel consumption of cars, spacecraft, and airplanes can be significantly decreased by using CFRPs and CFRTPs because of their exceptional mechanical strength and low weight. They are also resistant to corrosion and long-lasting, which makes them ideal for renewable energy applications like wind turbines.

The need for CFRPs and CFRTPs has skyrocketed in the last several years. But this also implies that there is a sharp rise in the quantity of CFRP/CFRTP waste. Because the process of producing carbon fibers requires a lot of energy, scientists have been searching for ways to recover them from CFRP/CFRTP waste through a process called "reclamation" that can be done economically. The most successful method to date appears to be thermal decomposition or pyrolysis; however, maintaining the mechanical qualities of the recovered fibers has proven difficult.

Amidst this context, scientists at Doshisha University in Japan endeavored to examine the advantages of performing CFRP/CFRTP pyrolysis in a superheated steam (SHS) environment rather than the conventional atmosphere. Associate Professor Kiyotaka Obunai and Professor Kazuya Okubo disseminated their research findings to illuminate this novel strategy in a paper that was recently published in Composites Part A: Applied Science and Manufacturing. Volume 176 of the journal will publish the study on January 01, 2024, after it was made available online on October 17, 2023.

It is rather simple to understand why pyrolysis should be carried out in a SHS atmosphere. "SHS not only creates a low-oxygen environment that prevents carbon fibers from oxidizing, but it also removes polymer residues from the surface of the reclaimed fibers," says Dr. Obunai. The performance of real CFRP composites manufactured with reclaimed carbon fibers was assessed by the researchers in addition to testing the fibers' mechanical properties. To achieve this, they performed Izod impact strength tests and bending strength tests, which evaluate a material's resistance to abrupt blows and its capacity to bear applied loads, respectively.

Their experiments' findings demonstrated several appealing features of pyrolysis reclamation in SHS. Using sophisticated microscopy methods, the researchers first discovered that the SHS atmosphere prevented the formation of "pitting," or dimple-like defects, in the recovered fibers, resulting in a smooth surface. Furthermore, compared to "virgin" fibers, fibers recovered in an air atmosphere showed significantly lower tensile strength and fracture toughness when pyrolysis was carried out at high temperatures (≥ 873 K). The advantage of the SHS atmosphere in maintaining the fracture toughness and tensile strength of reclaimed fibers is highlighted by the fact that these mechanical properties stayed largely unchanged in fibers recovered in an SHS atmosphere.

Furthermore, the mechanical properties of fibers recovered in an SHS environment showed less variation, improving their consistency and suitability for real-world uses. Furthermore, during pyrolysis, the SHS atmosphere lessened the deterioration in the Izod impact strength and bending strength, bringing them closer to virgin fiber composites.

When considered collectively, these results demonstrate the possibility of recovering carbon fibers from composites through pyrolysis reclamation within an SHS environment. This approach may hold the key to successfully integrating CFRPs/CFRTPs into a circular economy by offering a practical means of recycling. The effectiveness of using a SHS atmosphere rather than inert gases for the mass-scale pyrolysis reclamation of waste CFRP should be investigated in future work says Dr. Obunai, "as this work potentially provides an effective method for the reclamation of waste CFRP and contributes to the feasibility of achieving Sustainable Development Goals."

The utilization of CFRPS in the automotive, aerospace, and renewable energy industries is remarkable due to its lightweight and remarkable strength. However, this means that to ensure sustainability over the long term, it is imperative to find efficient ways to recycle the waste generated by CFRPS.

In the hopes of securing CFRPs/CFRTPs' status as sustainable and energy-efficient materials for a range of applications, more research will help create a more environmentally friendly future.

In 2018, Dr. Kiyotaka Obunai became an Associate Professor in the Department of Mechanical and Systems Engineering within the Faculty of Science and Engineering at Doshisha University. His primary areas of research are materials science and mechanics, as well as manufacturing technology, which includes mechanical, electrical/electronic, and chemical engineering. In addition to winning other honors, such as the JSMS Kansai Branch President Award in 2022 and the JSMS Award for Promising Researchers, he has written over 75 papers on these subjects.