Chemical modification of carbon fiber surfaces by nitric acid oxidation followed by reaction with tetraethylenepentamine

Amino groups react rapidly with both isocyanates and epoxides. Thus, to prepare carbon fibers which might exhibit enhanced adhesion to both polyurethanes and epoxy resin matrices, attempts were made to introduce a high surface amine concentration onto high-strength carbon fibers (derived from PAN) by nitric acid oxidation followed by reaction with excess tetraethylenepentamine (TEPA). Fibers were oxidized with concentrated (70%) nitric acid at 115 °C (20, 40, 60 and 90 minutes) to generate surface acidic functions, primarily carboxyl and phenolic hydroxyl groups. These oxidized fibers were then reacted with TEPA at 190–200 °C to introduce surface-bound amino functions onto the surface via amide functions. The amide functions formed at the surface to graft TEPA to the surface. TEPA does not react with hydroxyl functions so hydroxyls remain on the surface. Decarboxylation of surface carboxyl groups was minor during the TEPA grafting. The quantity of surface-bound acidic and basic functions on these modified fiber surfaces was measured by NaOH and HCl uptake experiments. Then, methylene blue and metanil yellow dye adsorption experiments were employed to provide a measure of the surface area and both the surface density and steric availability of surface acidic and basic functions. The dye adsorption gave lower values of acidic and basic functional group surface concentrations than did NaOH/HCl uptake measurements. The number of acidic groups increased continuously with increasing oxidation times but after an initial jump in acidic groups per 100 Å2 at the start of the oxidation, the density of the surface acidic was approximately constant. Instead, the surface area increased, which accounted for the increase in total acidic groups. Nitrogen BET measurements were also performed and the BET surface areas were compared to those derived from dye adsorption experiments. Similarly, both HCl and metanil yellow adsorption measurements led to a value of 5.6 basic (amine) groups per 100 Å2 after reaction with TEPA. The results were consistent with an increase in surface area during nitric acid oxidation. About 52% of the surface acidic functions, which were generated during nitric acid oxidations of 20–60 minutes, had reacted with TEPA. An average of 2.6 amino groups were introduced for each carboxyl group consumed in the reaction with TEPA. Thus substantial looping of TEPA occurs where more than one amino function of TEPA has formed an amide bond at the surface. The average ratio of the total amino groups introduced to the total amount of acidic groups present after nitric acid oxidation was about 1.35.