# Recycled Carbon Fibers with Improved Physical Properties Recovered from CFRP by Nitric Acid
## 4 Discussion  
### Mechanism of the Improvement of Interfacial Shear Strength between Carbon Fiber and Resin  
The mechanism of why the interfacial shear strength between the carbon fibers and resin was increased in the rCF was considered. Usually, a carbon fiber surface is inactive, and it cannot directly create bonds with the matrix resin. Therefore, a surface treatment is applied to form polar functional groups on the virgin carbon fiber surface to bond it to the resin [24]. It is known that since functional groups that contain oxygen atoms, nitro groups, and amino groups can form hydrogen bonds with hydroxy groups in the principal chain of the epoxy resin, a larger abundance of these functional groups will increase the interfacial shear strength between the carbon fibers and resin [25; 26]. Therefore, the reason why the interfacial shear strength of the rCF was higher than that of the virgin carbon fibers is presumed to be due to the new polar functional groups formed by the nitric acid on the carbon fiber surface, and the abundance of these polar functional groups increased as confirmed by the XPS, resulting in higher-strength bonds with the resin. However, the peak of the interfacial shear strength between the carbon fibers and resin was around rCF 24 h and declined slightly with longer immersion. This was considered to be due to the fact that at longer immersion times, the oxidation of the functional groups progressed, increasing the abundance of carboxy groups that have weak bonds with the resin [27]. The interfacial shear strength of vCF with the treatment in nitric acid for 8 h slightly increased, however, showed lower interfacial shear strength compared to rCF. To consider the reason, the total amount of polar functional groups in vCF 8 h was calculated at 58.6% while rCF 24 h was 62.7%, which was calculated from the XPS results shown in Table 1. The amount of polar function groups was less in vCF 8 h compared to rCF 24 h so the interfacial shear strength in vCF 8 h showed lower interfacial shear strength than that of rCF. This result also suggested that not only new polar functional groups were immobilized on the carbon fiber surface along with immersing into nitric acid, but also a slight resin residue was remaining on  
Figure 9: \(I_{\text{D1}}/I_{\text{G1}}\) ratio, \(I_{\text{D2}}/I_{\text{G1}}\) ratio, and \(I_{\text{G2}}/I_{\text{G1}}\) ratio of vCF and rCF obtained from Raman spectroscopy analysis.  
rCF 24 h surface, and both of them contributed to increasing the interfacial shear strength to resin.  
The surface roughness calculated from the AFM shape image shown in Figure 6 decreased more in rCF than in vCF and further declined in rCF 120 h. When surface roughness increases, the anchor effect affects the bonds with the resin, contributing to increasing the bond strength with the resin [28]. However, the interfacial shear strength between the carbon fibers and resin in rCF 120 h was approximately the same value as that in vCF. Therefore, although surface roughness can assist in the bonding with the resin, it was considered that a larger contribution was made by the chemical bonds via the polar functional groups.