CHEMICAL INTERACTIONS AT THE INTERFACE BETWEEN A CARBON-FIBER AND A BORON TRIFLUORIDE-CATALYZED EPOXY MATRIX

The cure of a tetrafunctional epoxy resin (largely tetraglycidyldiaminodiphenylmethane, TGDDM), cross-linked with diaminodiphenylsulphone (DDS) and boron trifluoride/ethylamine (BF3EA) catalyst, is affected in a complex fashion by the presence of an oxidized carbon fiber surface. If the fiber is aged in a humid environment (25-degrees-C, 95% humidity) before coating with the matrix, the affinity of the oxidized fiber surface for water leads to destruction of the catalyst and retardation of the cure. This retardation is noticeable at lower humidities if the polymer content of the composite is reduced to < 20%. For oxidized fibers that are stored under ambient conditions (40% humidity), the retarding effect of water is overwhelmed by the catalytic effect of the acidic fiber surface. This activates the latent catalyst, increasing the rate of epoxy consumption and the proportion of the epoxy-epoxy reaction relative to the epoxy-amine reaction. For the low-humidity case, even though the rate of chemical reaction is increased, vitrification of the matrix is retarded, implying that there is less cross-linking and more extended epoxy-epoxy sequences in the network. The proposed chemical changes in epoxy/carbon fiber composites were confirmed by analysis of model reactions in solution.