Composition of Active Oxidizing Species in Electrolyzed Sulfuric Acid for Resin Degradation of Carbon Fiber Reinforced Plastics
Authors: Fukuoka Miwa , Okada Yuji , Kondo Takeshi 
Published: December 2024

Peroxodisulfate ion produced by electrolytic oxidation of sulfuric acid is a powerful oxidant that can decompose only the resin portion of carbon fiber reinforced plastics (CFRP), and is expected to be used in technology for reproducing carbon fiber from waste CFRP. Sulfuric acid electrolysis using a boron-doped diamond (BDD) electrode produces mainly three active oxidizing species: peroxodisulfuric acid, hydrogen peroxide, and peroxomonosulfuric acid. The purpose of this study is to investigate the effect of electrolysis condition on the composition of active oxidizing species in the electrolyzed sulfuric acid. In addition, we investigated the relationship between composition of active oxidizing species and resin decomposition properties.

30% and 50% sulfuric acid was electrolyzed by constant current electrolysis at BDD electrode. Concentration of the active oxidizing species in the electrolyzed sulfuric acid was estimated by a titration method. CFRP samples were immersed in the electrolyzed sulfuric acid with air bubbling to conduct a resin decomposition test. Concentration of active oxidizing species were investigated before and after the resin decomposition. In addition, changes in the concentration of active oxidizing species before and after resin decomposition of CFRP samples was examined for electrolyzed sulfuric acid with addition of hydrogen peroxide.

10 mL of 30% and 50% sulfuric acid were electrolyzed at a constant current of 200 mA for 1 h to obtain electrolyzed sulfuric acid. Concentrations of peroxodisulfate, peroxomonosulfate and hydrogen peroxide were estimated to be 0.191, 0.073, and 0 mol/L for the electrolyzed 30% sulfuric acid, while those were 0.0213, 0.201, and 0.0117 mol/L for the electrolyzed 50% sulfuric acid, respectively. The composition of the active oxidizing species also varied with constant current. Thus, the result indicates that the composition of electrolyzed sulfuric acid can be controlled by the electrolysis condition. After the resin decomposition test, concentrations of peroxodisulfate and peroxomonosulfate decreased, indicating that these species contributed to the resin decomposition.

Next, the CFRP samples before and after the resin decomposition test were observed by SEM. As a result, the electrolyzed 30% sulfuric acid was not so effective for resin decomposition, showing partial decomposition of resin in the CFRP. On the other hand, in the case of electrolyzed 50% sulfuric acid, almost no resin was found to remain leaving intact carbon fibers after the decomposition test. Since the total concentration of the active oxidizing species was comparable between these electrolyzed sulfuric acids, the composition of the active oxidizing species is considered to be important for effective resin decomposition. In addition, addition of hydrogen peroxide into the electrolyzed 30% sulfuric acid was found to activate the resin decomposition rate, which is considered to be based on the significant increase of peroxodisulfate and peroxomonosulfate concentrations even after the resin decomposition.