Reducing energy consumption is an essential element in the journey towards sustainable societies, and advanced materials play a key role in this regard.
Thanks to their light weight yet remarkable strength, the use of CFRPS in the automobile, aerospace, and renewable energy industries is soaring. However, this means finding effective ways to recycle the waste generated by CFRPS is essential to ensure sustainability in the long term. Image Credit: Lunchboxlarry at Openverse
Carbon-fiber-reinforced plastics (CFRPs) and carbon-fiber-reinforced thermoplastics (CFRTPs) are two prominent examples of composite materials that can considerably improve energy efficiency in various fields of application.
Constructed with carbon fibers integrated into a polymer matrix, typically epoxy resin, composites like CFRPs and CFRTPs offer a combination of low weight and impressive mechanical strength.
They are highly effective in diminishing fuel consumption in various modes of transportation, including aircraft, spacecraft, and automobiles. Beyond their lightweight properties, these composites boast durability and corrosion resistance, rendering them well-suited for applications in renewable energy, such as in the construction of wind turbines.
Though the need for CFRPs and CFRTPs has increased in recent years, their amount of waste is also accumulating. Scientists are finding ways to recover CFRP/CFRTP waste through a process known as “reclamation” at low cost as the manufacturing of carbon fibers is rigorous.
Thus, conserving the mechanical properties of the reclaimed fibers is challenging, and the thermal decomposition (pyrolysis) technique seems to be the most effective.
Scientists from Doshisha University, Japan, decided to examine the benefits of conducting the pyrolysis of CFRPs/CFRTPs in a superheated steam (SHS) atmosphere as opposed to the standard atmosphere. In a recent paper published in Composites Part A: Applied Science and Manufacturing, Associate Professor Kiyotaka Obunai and Professor Kazuya Okubo revealed their findings to shed light on this innovative approach. The study was made available online on October 17th, 2023, and will be published in the journal on January 01st, 2024.
SHS not only prevents the oxidation of carbon fibers by creating a low-oxygen environment but also removes polymer residues from the surface of the reclaimed fibers.
Kiyotaka Obunai, Associate Professor, Department of Mechanical and Systems Engineering, Doshisha University
The foundation behind conducting pyrolysis in an SHS atmosphere is relatively straightforward. The scientists test the mechanical characteristics of the reclaimed carbon fibers to evaluate the performance of actual CFRP composites made using these fibers. Scientists also perform bending strength tests and Izod impact strength tests, which make the materials withstand applied loads by bending and assessing their resistance toward sudden blows, respectively.
The results of their research revealed numerous attractive aspects of pyrolysis reclamation in SHS. The researchers established that the SHS atmosphere crushed the formation of dimple-like defects called “pitting” in the recovered fibers, realizing a smooth surface using advanced microscopy techniques.
During the process of pyrolysis at high temperatures (≥873 K), fibers reclaimed in an air atmosphere exhibited greatly reduced tensile strength and fracture toughness compared to those of “virgin” fibers.
These mechanical properties remained the same in fibers reclaimed in an SHS atmosphere. This highlights the benefit of the SHS atmosphere in protecting the fracture toughness and tensile strength of reclaimed fibers.
Fibers reclaimed in an SHS atmosphere also exhibited less variation in their mechanical properties, making their performance more reliable and appropriate for practical applications. The SHS atmosphere during pyrolysis alleviated the deprivation in the bending strength and Izod impact strength, making them akin to composites made with virgin fibers.
In conclusion, these discoveries highlight the potential of pyrolysis reclamation in an SHS atmosphere to develop carbon fibers from composites. This approach is the key to successfully introducing CFRPs/CFRTPs into a circular economy by providing an effective way to recycle. “This work potentially provides an effective method for the reclamation of waste CFRP and contributes to the feasibility of achieving Sustainable Development Goals. The effectiveness of adopting a SHS atmosphere instead of inert gases for the mass-scale pyrolysis reclamation of waste CFRP should be investigated in future work.
Kiyotaka Obunai, Associate Professor, Department of Mechanical and Systems Engineering, Doshisha University
Journal Reference
Obunai, K. & Okubo, K. Mechanical characteristics of reclaimed carbon fiber under superheated steam atmosphere and its feasibility for remanufacturing CFRP/CFRTP. ScienceDirect. doi.org/10.1016/j.compositesa.2023.107843.