Reactivities of amine functions grafted to carbon fiber surfaces by tetraethylenepentamine. Designing interfacial bonding Original Research Article

PAN-base carbon fibers were oxidized with 70% nitric acid at 115 °C to introduce surface carboxyl, hydroxyl and other oxygenated functions. Subsequent reactions of these surfaces at 190–200 °C with tetraethylenepentamine (TEPA) generated amide bonds at carboxyl and ester sites thereby grafting TEPA and introducing primary and secondary amine groups onto the fiber surfaces. The reactivity of these surface-bound amine groups was evaluated in reactions with the small model reagents: phenyl isocyanates, 1,6-diisocyanatohexane, acetic anhydride and styrene oxide. Larger model reagents including isocyanate-terminated prepolymers and a series of epoxy resin prepolymers (MW 378–1000) were also used. The reactivity of the surface carboxyl and hydroxyl functions (introduced by nitric acid oxidation) was also examined in reactions with phenyl isocyanate. About 30% of these acidic functions were able to react with phenyl isocyanate. After grafting TEPA to the fibers, acetic anhydride and phenyl isocyanate reacted with 60% and 52%, respectively, of the surface-grafted amine groups. The larger isocyanate prepolymers reacted with only 2.5 to 7.8% of the amino groups and three epoxy resins, with molecular weights of 378, 500 and 1000, reacted with 15, 7.8 and 5.3% of the surface amino groups, respectively. The grafting efficiencies to surface amino groups (Ge) and the molecular weights of the reagent being grafted fit the relationship Ge = KMna where K = 3.07 × 103 and a = −0.848 for isocyanates, whereas K = 2.57 × 104 and a = −1.25 for the epoxy compounds.