# Hydrothermal Treatment with Different Solvents for Composite Recycling and Valorization Under Subcritical and Supercritical Conditions

## TL;DR

This paper explores hydrothermal treatments for recycling carbon fiber composites, showing high resin degradation rates and fiber recovery potential.

## Contribution

The study introduces a novel hydrothermal method using solvents and catalysts to efficiently recycle carbon fiber composites.

## Key findings

- Degradation rates of up to 92% were achieved under specific hydrothermal conditions.
- Recovered fibers retained their mechanical properties and morphology.
- Using water–isopropanol solutions reduced reactor fouling compared to water alone.

## Abstract

Worldwide, carbon fiber (CF) demand has been rising over the last decade, which contrasts with the fact that up to 30–50% of composite materials in aircraft production are scrapped. This situation highlights the increasing need for recycling methods to reduce fabrication costs and global warming potential. Emerging technologies focus on recovering long CFs, as they represent the most valuable form but are also the most difficult to reclaim using conventional recycling methods. Hydrothermal treatments offer a promising alternative to valorize this waste by decomposing the polymer matrix under subcritical and supercritical conditions without significantly damaging the fibers. Water, isopropanol, and mixtures of water/isopropanol or water/acetone were tested as solvents, with and without the addition of zinc chloride (ZnCl2) as a homogeneous catalyst. The influence of temperature, pressure, and solvent composition on resin degradation was evaluated. In this work, degradation rates of up to 92% were achieved at 415 °C, 233 bar, 120 min, 5 wt.% IPA, and ZnCl2 0.1 M. It should be noted that ZnCl2 caused reactor corrosion. Furthermore, the recovered fibers retained their morphology, including the sizing layer, and showed mechanical properties similar to the original material, while a small H2-rich gaseous fraction was generated as a byproduct of the hydrothermal degradation. Using water–isopropanol solutions resulted in the reactor being significantly cleaner than when using water alone, which can be advantageous for future scale-up and for reducing maintenance requirements. These results confirm the potential of hydrothermal processing as an efficient and selective method for the recycling and valorization of carbon-fiber-reinforced composites from the aeronautical industry.