Solubility Parameters of Carbon Fiber for CFRP Recycling

Miscibility changes of carbon fiber (CF) through carbon fiber-reinforced plastic (CFRP) recycling by thermal treatment were elucidated using solubility parameter analysis. A comparison was made among recycled CF (ReCF), acetone-washed original CF (AwCF), and ozone-treated CF (OzCF). While the Hildebrand solubility parameters (δt) of the three types were almost identical, only the Hansen solubility parameters (HSPs; δd, δp, and δh) of ReCF differed. This finding is consistent with the results obtained from X-ray photoelectron spectroscopy analysis, confirming its validity. The δd, δp, δh, and δt for CFs were as follows, respectively: 21.6, 6.4, 9.1, and 23.3 MPa0.5 for ReCF and 20.5, 3.9, 10.6, and 23.4 MPa0.5 for both AwCF and OzCF. The proposed method identified key solvents crucial for HSP analysis of CFs, which are benzaldehyde, ethylbenzene, methyl benzoate, and 1-methylnaphthalene. The changes in the surface properties of CF due to recycling necessitate adjustments in epoxy formulations and sizing agents to ensure effective reuse of CF.