Abstract
This thesis concerns the tensile strengths and Young's moduli of carbon fibres as they are affected by the conditions used to spin the polyacrylonitrile (PAN) precursor fibres. Particular attention has been given to factors affecting the strengths of the fibres since higher strengths would lead to increased impact resistance in carbon fibre reinforced plastics. Impurity particles in the PAN fibres were the main source of flaws in carbon fibres, with samples derived from PAF fibres spun under clean-room conditions having significantly higher strengths. For example clean PAN fibres stretched x 14 in steam gave carbon fibres with average strengths of 3.0 GN/m2 after heat-treatments to both 1400+/-C and 2500+/-C. When the PAN fibres were spun under normal laboratory conditions however the strength of the fibres after heat-treatment to 1400+/-C was only 2,1 GN/m2 and this decreased to 1.7 GN/m2 after treatment to 2500+/-G. Thus it was found that fls.ws due to impurities caused the average strengths of carbon fibres to decrease after heat-treatments to 2500+/-C and also to vary with the length of fibre tested. Following the analysis of samples of airborne dust, and experiments involving the deliberate contamination of PAN fibres before conversion to carbon fibres, it was concluded that the flaws were mainly caused by inorganic impurities such as silica, silicates and iron oxide. The flaws were thought to form in two stages (1) by reactions with the fibres leading to the reduction of the oxides and (2) because the impurities promoted the formation of localized randomly-oriented three-dimensional graphite in the fibres. The mechanical properties of the-carbon fibres were also found to be influenced by the amount of hot-stretch applied during the spinning of the PAN fibres and the hot-stretching temperature used.