Identification of mechanisms and experimental implementation for enhancing the interfacial force between oxygen functionalized waste carbon fibers and polyamide resin

To recycle waste carbon fibers (CFs) and utilize them as a composite material with polyamide–6 (PA–6), the oxidation of the carbon surface is crucial for enhancing its bonding with PA–6 without damaging the waste CFs. However, the most effective oxygen-containing functional group (O–group) was hitherto unknown. In this study, computational simulations demonstrated that incorporating O–groups enhanced the interfacial bonding force via hydrogen bonding between the oxygen of the O–groups and hydrogen (N–H) of PA–6. Among various O–groups, the bonding of lactone groups to PA–6 was energetically most favorable, corroborated by different oxidation treatments such as acid, heat, and plasma. As the reaction time or temperature increased, the amount of O–groups, such as lactones, increased. Regardless of the oxidation treatment type, an increase in the amount of O–groups increased the interfacial force, and this tendency was predominantly observed in lactone groups. However, excessive surface oxidation introduced defects on the surface of CFs, which reduced the interfacial force. The modification of waste CFs by identifying the proposed mechanisms lays the groundwork for synthesizing high-quality waste CF composites.