Abstract
Carbon fibre-reinforced composites (CFRCs) are increasing in popularity due to their high strength-to-weight ratio and resistance to corrosion. However, when exposed to temperatures above 300°C, the polymer matrix within CFRCs decomposes and then starts burning, exposing carbon fibres to the surroundings. The residual carbon fibres being electrically conductive, may pose a hazard to the surrounding electronics. Moreover, at over 550°C the carbon fibres begin to oxidise. This can lead to fibre defibrillation which also poses significant harm to human health as broken fibres can be sharp enough to cut through human skin, and under 7µm these particles are considered respirable where on inhalation they can causes damage to the trachea and lungs.
While considerable work has been carried out on assessing the effect of heat/fire on degradation of the composite resin (matrix) and CFRCs themselves, there are limited studies on identifying the damage to carbon fibres within CFRCs and the hazards posed by the exposed damaged carbon fibres. This study examined the damage caused by high temperatures, radiant heat and flames on carbon fibres and CFRCs, and the effects on their physical properties. A methodology was developed to study and quantify the structural damage to carbon fibres and CFRCs after exposure to a range of heat/fire conditions. These included thermogravimetric analysis (up to 900oC in nitrogen and air atmospheres), the tube furnace (450oC–900oC), cone calorimeter (35kWm-2 to 75kWm-2 ) and a propane burner (116kWm-2 ) to simulate jet fuel fire conditions.
Residual fibres were removed from different parts of the CFRCs and the physical properties were studied, such as fibre diameter reduction, change in electrical conductivity and decrease in tensile strength. It was found that at heat fluxes ≥60kWm-2 oxidation of the carbon fibres occurred. After 10min exposure to the propane flame, fibres in direct contact with the flame showed signs of internal oxidation.The aim of this PhD project was to also improve the structural retention of CFRCs on exposure to heat/fire so that the structural integrity is maintained and the carbon fibres are not exposed to the environment. To address this, the following approaches were undertaken:
• Modification of the resin by adding flame retardants and nanoparticles in order to reduce the flammability of CFRCs, improve the mechanical integrity of the char and its adherence to the fibre. Flame retardants included ammonium polyphosphate, resorcinol bis-(diphenyl phosphate), 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide, and the nano-additives, nano-clay, layered double hydroxide and carbon nano-tubes. Cone calorimeter testing at 75kWm-2 showed that the addition of 15wt% ammonium polyphosphate resulted in large char formation and adherence to fibres in the underlying plies, which resulted in less oxidation to these carbon fibres. The addition of layered double hydroxides and carbon nano-tubes on the other hand caused pitting on fibres.
• Provide heat protection to carbon fibres within CFRCs by the inclusion of high performance fibrous veils/woven fabrics of aramid, basalt, E-glass, polyphenylene sulphide and Kevlar.
The inclusion of the woven E-glass resulted in a notable reduction in the percentage of carbon fibre oxidised. However, the volatiles produced during the decomposition of Kevlar and PPS sensitised the carbon fibre to oxidation, causing it to occur more rapidly and at a lower temperature.
• Using high temperature chemical coatings to individually coat carbon fibres prior to making the CFRCs. Ceramic compounds (silica, alumina and zirconia), chosen as coating materials because of their high thermal stability, were applied by different processes. The most promising coatings included alumina and silica formed via sol-gel process and polysiloxane deposited during plasma exposure. Tows coated in these chemicals underwent heat testing in a tube furnace where those coated with alumina maintained the largest fibre diameters. While polysiloxane coating provided oxidation protection up to 600°C, after which cracks in the coating were observed. This was attributed to the mechanical mismatch of the polysiloxane coating and the carbon fibre.