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 re duce the adhesion of the matrix to the carbon fibre surface, various H T carbon fibres were coated with poly(dimethylsiloxane) and poly(methy lphenylsiloxane) using solutions of these materials with very differen t concentrations. The surface activity of the commercially surface-tre ated and coated carbon fibres was studied by contact-angle measurement s. One of the fibres studied contains sodium, which effected partial c onversion of the polysiloxanes to silicon dioxide already during curin g at 360 degrees C in humid air. In studies with unidirectional (UD) c omposites (fibre volume fraction 60 vol%) such a coating was found to be very effective up to 1000 degrees C, but the composites did not wit hstand the graphitization treatment. In all other cases, two-dimension al coatings, which just block active sites and surface functional grou ps of the carbon fibres, are shown to be extremely effective, as follo ws from the flexural strength data after curing, carbonization and gra phitization treatment (values obtained with commercially surface-treat ed carbon fibres and commercially surface-treated and sized carbon fib res are given in parentheses): after curing, 2300 MPa (1300/1600 MPa); after carbonization, 410 MPa (180/220 MPa); after graphitization trea tment, 800 MPa (480/400 MPa). Poly(dimethylsiloxane) and poly(methylph enylsiloxane) are equally effective.