Modification of Carbon Fiber Surfaces Using Dopamine

Carbon fibers are surface-inert materials. Therefore, their surfaces must be activated if they are to be used as reinforcing agents in resin matrix composites. In this paper, the mechanism of dopamine (DA)-modifying carbon fiber surfaces and the influence of modification conditions on the modification effect were explored. The results showed that carbonyl and ester groups on the surfaces of carbon fibers can react with the primary amine in DA or polydopamine (PDA) by nucleophilic addition, and that is a Schiff base reaction, which introduces DA or PDA molecules onto the surfaces of the carbon fibers by chemical bonding. The existence of -C=O functional groups on the surfaces of the carbon fibers were beneficial to chemical bonding between DA or PDA molecules and the surfaces of the carbon fibers. The difference in the physical structures of the carbon fiber surfaces had no significant effect on the DA-modified carbon fibers. The modification effect was positively related to DA relative concentration, oxidant dosage, modification temperature, and modification time. The hydrophilicity and interfacial shear strength of the modified carbon fibers were obviously improved after modification. When the concentration of DA hydrochloride was fixed at 2 g/L, the optimized process using the single-variable method was as follows. The concentration of NaIO4 was 2 g/L, and the mass ratio of DA to CF was 0.2, and the modification temperature and time were 70 degrees C and 15 min, respectively. After applying the optimized modification conditions, the grafting rate of PDA on carbon fibers reached 1.38wt% and the contact angle of the carbon fibers with the water was reduced from 85.63 degrees to 60.13 degrees. The interfacial shear strength between the modified carbon fibers and epoxy resin E51 reached 63.97 MPa, which was 43.59% higher than that of untreated carbon fibers. The modification process had almost no effect on the tensile strength of the modified carbon fibers.