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

Abstract

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 ( G e ) and the molecular weights of the reagent being grafted fit the relationship G e = KM n a where K = 3.07 × 10 3 and a = −0.848 for isocyanates, whereas K = 2.57 × 10 4 and a = −1.25 for the epoxy compounds.