The Study Of Cleaning And Oxidation Of Carbon Fiber At Critical Conditions

Carbon fibers combine exceptional mechanical properties and low weight, actingas ideal reinforcements for polymer matrix composite materials. The performance ofthese composites depends largely on the quality of the matrix–reinforcementinterface, which determines the way loads can be transferred from the polymer tothe fiber. It has long been recognized that carbon fiber is essentially graphitic innature, when untreated and unsized, and therefore possesses a chemically inertsurface.In this paper, PAN-baded carbon fibers were cleaned and oxidated at hightemperature and presure. The results of XPS indicate that residual layers on thesurfaces of carbon fibers can be thoroughly removed by supercritical water at693Kfor less than9min. The single filament strength of treated fibers decreases slightly.But the results of XRD and Raman measurement suggest that there was noexcessive destruction taken placed in this cleaning procedure.Experimental results also reveal that the method using supercritical acetone orsubcritical potassium hydroxide aqueous solution act as the processing mediumshows a better cleaning effect compared to the traditional method, Soxhletextraction with acetone. The method using supercritical acetone is more appropriateto wipe off the oxygenated contaminants on carbon fibers’ surfaces and causes arelatively smaller damage to the bulk strength of each carbon fiber. As far as treatingmethod using the subcritical alkali aqueous solution, it can thoroughly removesilicious contaminants on the surfaces of treated fibers.After oxidation by H₂O₂/subcrtical water systems, oxygen content of the treatedfibers enlarged about5%. The monofilament tensile strength decreased about2.6³.1%. The results of SEM observations indicate that oxidation reaction therewas no significant effect on the surface appearance of the treated carbon fibers.After oxidation by KMnO₄/subcrtical water systems, oxygen content of thetreated fibers increased to16.0%, meanwhile, the grooves on the surfaces of thecarbon fibers were deepened gradually after treatments. The results of dynamiccontact angle measurements suggest that an obvious increase of the carbon fiber’ssurface tension is mainly because of the enlargement of its polar component. Aftertreated with increasing amounts of oxidants, the monofilament tensile strength decreased about3.6%.To reduce the damages of the oxiadation treatments and control the generationgounps, Br₂was mixed into these systems. It shows that the oxidation can alsoincrease the oxygen content on fiber surfaces, but the generating group is hydroxyrather than carboxyl. Scanning electron microscope was used to follow the surfacetopography of the fibers. It reveals that the grooves on the surfaces of the oxidatedfibers were deepened gradually for both oxidation systems.After oxidation by Br₂/concentrated sulfuric acid, oxygen content of the treatedfibers increased about10%. DCA measurement show that carbon fiber’s surfacetension increased about14%, and The monofilament tensile strength decreasedabout3.9⁴.5%. And the results of SEM observations indicate that oxidationreaction has a significant effect on the surface appearance of treated carbon fibers.In order to follow how the effect of Cl2compared with Br₂in the oxidationsystems, the carbon fibers were oxidized with KClO3/H2SO4systems. The XPSresults show that oxygen functional groups on the surfaces of carbon fibers increaseby about11%after treated, and the main of them are hydroxys and carboxyls. Theresults of monofilament tensile tests verify that the strength loss of the treated fiberswere about1.2³.5%.A discusion of the mechanism of carbon fibers is given in this paper. It indicates:decarburizing step takes place before the formation of carbonyl and carboxyl groups,so mass loss in the highly oxidizing process of carbon fiber is inevitable; because ofsteric hindrance, the formation of the carbonyl and carboxyl groups leads togenerally warping graphite layers; the generation of lactone bond is preferred to thegeneration of carboxyl group in the oxidation process of carbon fibers.