Oxygen plasma and isobutylene plasma treatments of carbon fibers: Determination of surface functionality and effects on composite properties

Abstract

High strength, PAN-based carbon fibers were treated with oxygen plasmas and isobutylene plasmas. The effects of exposure time, plasma power and gas pressure on the quantity of acidic functional groups introduced onto the fiber surfaces were examined. NaOH uptake measurements provided a quantitative determination of the surface acidic functions. Plasma treatments were able to generate a three-to-five-fold increase in the number of acidic functional groups per 100 Å 2. Methylene blue (MB) adsorption measurements, when used with NaOH uptake values, provided an estimate of the surface density of the acidic functions (functions/100 Å 2) that was independent of direct surface area measurements. This method was compared to the direct use of NaOH uptake values with nitrogen BET measurements of the surface area to give the number of acidic functions per 100 Å 2. These methods were compared for both oxygen plasma-oxidized and nitric acid-oxidized carbon fibers. The largest quantity of acid functions (16 μeq/g fiber) was obtained after 4 minutes in an oxygen plasma at 50 W. MB adsorption decreased while NaOH uptake increased as exposure time to oxygen plasma increased implying some decrease in surface area occurred. Surface area measurements (BET), however, showed no change in surface area over a 10 minute exposure to a 200 W oxygen plasma (all surface areas were in the range of 0.62 to 0.75 m 2 g −1). These results are discussed in terms of possible effects of surface roughness, dye configuration or multiple dye layers. The rate of depositing polyisobutylene polymer residues on a flat glass surface was greater than that achieved on carbon fiber surfaces where shadowing effects could have existed within a tow. Oxygen plasma treatment improved the interfacial shear strength (determined by single filament fragmentation tests using an epoxy resin). The interlaminar shear strength (three-point bending) of carbon fiber/epoxy composites ( V f = 0.57) increased 28–29% using oxygen plasma-treated fibers while the Izod impact strength was unchanged. Plasma-polyisobutylene coated fibers (~ 650 Å thick coating) exhibited a 37% increase in Izod impact strength when incorporated into epoxy matrix composites but the interlaminar shear strength decreased by 21% versus composites prepared with as-received fibers.