2.2. Chemical Recycling Method

Developing chemical recycling methods has already motivated some researchers to produce rCFs with essentially unchanged mechanical and physical properties [44,51,69,70]. Different types of reactive solvents can decompose the polymer matrix into soluble low molecular weight products, the end products of which will be fibers, inorganic fillers, and dissolved depolymerized resins and monomers. To increase the specific surface area in contact with the solution and promote matrix dissolution, the CFRPs are first mechanically crushed. The derived rCFs are washed at the end of the recycling process to remove residues from their surfaces [1,71], resulting in the rCFs with longer lengths and preserved mechanical properties. However, the hazards and toxicity of chemical solvents could have adverse effects on the environment [1,17].

Many studies have used subcritical or supercritical solvents to reduce these environmental issues to replace these chemicals at different temperatures and pressures, such as water, alcohol, ammonia, and organic solvents. The chemical recycling method of supercritical fluids is also known as solvent decomposition. Supercritical fluids have unique physical properties, including low viscosity, high density, good flow, mass transfer, heat transfer, and special solubility parameters [45,69,72]. In addition, they could be sensitive to changes in temperature and pressure. Removal of resin by solvent decomposition involves several steps [44]: (1) solvent diffusion; (2) reaction of the fluid with the fiber surface; (3) matrix dissolves into the fluid, and (4) mass transfer by convection. This chemical recycling method allows rapid and selective decomposition of CFRPs to obtain clean rCFs without significantly losing their mechanical properties. However, the operating conditions for this technology are harsh and costly. In addition, the theoretical research on supercritical fluid technology is not in-depth, so it has not been commercialized.

The chemical recycling method’s important parameters include solvent type, reaction time, processing temperature, and catalyst concentration. The processing efficiency of water and alcohols has been widely studied in different fluids. Due to the decrease in the dielectric constant of water molecules under supercritical conditions, water can be used as a non-polar solvent for dissolving organic compounds [45]. The critical point conditions for alcohols (200–300 °C, 2.0–6.0 MPa) are lower than those for water (374 °C, 22.1 MPa). Therefore, it is easier to recycle the process using alcohol. Piñero-Hernanz et al. [43] applied chemical recycling methods to recycle rCFs from CF-reinforced epoxy resins using various supercritical alcohols. Compared to vCFs, the rCFs with no matrix residues on their surfaces retained 85–99% mechanical strength. However, considering the cost-efficiency and environmental impact, water was mainly chosen as the fluid. Kim et al. [45] used supercritical water to remove 99.5% of the resin without a catalyst. The rCFs obtained were blended with cyclic butyl terephthalate (CBT) to fabricate new thermoplastic composites. Okajima et al. [46] decomposed CFRP wastes using supercritical water and potassium carbonate as a catalyst at a temperature of 400 °C, a pressure of 20 MPa, and a reaction time of 45 min. The rCFs obtained retained 85% of their mechanical strength. Khalil et al. [73] studied 17 supercritical fluids used to decompose thermoset resins. They found that a supercritical mixture of solvent and water could be more efficient in recycling rCFs. Henry et al. [74] showed that a water/ethanol mixture (50/50 vol.%) could obtain cleaner rCFs and higher resin removal efficiency. This enhanced efficiency appeared because the faster decomposition rate reduces the contact time between the resin and CFs, reducing the formation of residues on the surfaces of the rCFs. In addition, Henry and Kim reported that shortening the treatment time could make the surface of rCFs cleaner. They also found that after 120 min of treatment with supercritical water, the resin could be completely removed, and rCFs with a cleaner surface could be obtained [45,74].

Introducing acidic or basic compounds, catalysts, and oxygen to the chemical recycling system could accelerate and improve the removal efficiency of the resin effectively. Liu et al. [44] investigated the decomposition process of CFRPs using water as the reaction medium. The results showed that adding 1 M of sulphuric acid solution accelerated the degradation of the epoxy resin. The rCFs obtained were clean with no cracks or defects, and the average tensile strength was 98.2% of vCFs. Bai et al. [47] used chemical recycling by introducing oxygen into supercritical water. The results showed that the rCFs had comparable strength to vCFs when the epoxy resin decomposition rate was 94–97 wt%. Liu and colleagues also demonstrated that using supercritical fluid water and adding phenol and KOH to the system could improve the dissolution rate of epoxy resin with a removal rate greater than 95%. The recovery of rCFs could reach 95.2% and 100% at 315 °C and 325 °C for recycling for 30 min, respectively, with mechanical and surface properties comparable to those of vCFs [44]. Wang et al. [25] used acetic acid and AlCl₃ as catalysts at 180 °C to recycle rCFs from cured epoxy composites, with a tensile strength of 97% of vCFs. Das et al. [48] mixed acetic acid and hydrogen peroxide to form peroxyacetic acid as an oxidation method to decompose CFRPs and obtained rCFs with a clean surface and similar tensile strength to vCFs. In addition, this method does not require high temperatures and pressure, reducing the environmental impact.