THE SIGNIFICANCE OF THE FIBER COATING IN THE PRODUCTION OF CARBON-FIBER-REINFORCED CARBONS FROM HT CARBON-FIBERS .1. POLY(DIMETHYLSILOXANE) AND POLY(METHYLPHENYLSILOXANE) COATINGS

This paper is concerned with the role of carbon fibre coatings in the production of carbon fibre-reinforced carbons from high tenacity (HT) carbon fibres and phenolic resin as a matrix precursor. In order to reduce the adhesion of the matrix to the carbon fibre surface, various HT carbon fibres were coated with poly(dimethylsiloxane) and poly(methylphenylsiloxane) using solutions of these materials with very different concentrations. The surface activity of the commercially surface-treated and coated carbon fibres was studied by contact-angle measurements. One of the fibres studied contains sodium, which effected partial conversion of the polysiloxanes to silicon dioxide already during curing at 360 degrees C in humid air. In studies with unidirectional (UD) composites (fibre volume fraction 60 vol%) such a coating was found to be very effective up to 1000 degrees C, but the composites did not withstand the graphitization treatment. In all other cases, two-dimensional coatings, which just block active sites and surface functional groups of the carbon fibres, are shown to be extremely effective, as follows from the flexural strength data after curing, carbonization and graphitization treatment (values obtained with commercially surface-treated carbon fibres and commercially surface-treated and sized carbon fibres are given in parentheses): after curing, 2300 MPa (1300/1600 MPa); after carbonization, 410 MPa (180/220 MPa); after graphitization treatment, 800 MPa (480/400 MPa). Poly(dimethylsiloxane) and poly(methylphenylsiloxane) are equally effective.