Electrochemical coating for prevention of carbon fibre release from polymer composites: Thermogravimetric analysis of organophosphorus coatings on carbon fibres

Abstract

Fire retardant coatings of phosphorus compounds were formed on graphite fibres by a new electrochemical technique. Thus, tetrakis (hydroxymethyl)-phosphonium sulphate, ammonium polyphosphate, titanium di(dioctylpyrophosphate) oxyacetate, di(dioctylphosphato)ethylene titanate, and propargyltriphenyl-phosphonium bromide, were electrodeposited or electropolymerized on commercial graphite fibres used for polymer reinforcement. The effect of these coatings on the thermal oxidative behaviour of the coated carbon fibres, epoxy resin, and composites prepared from them was studied by thermogravimetric analysis, and compared with that of polyimide coatings. Generally, the coated fibres showed higher decomposition temperature than the untreated carbon fibres. The fire retardant phosphorus compounds promoted the formation of char from the matrix resin, and accelerated the decomposition of char. Organophosphorus titanate coatings left an incombustible, white residual layer of titanium dioxide. The polyphosphate coating caused the decomposition of the fibres in the epoxy composite to occur at a reduced temperature compared to that in the absence of the matrix resin. A synergistic interaction between the polyphosphate and the amine-cured, epoxy resin to catalyse the decomposition of carbon fibres is inferred from this. Polyimide precursor coatings lowered the oxidation temperature of the carbon fibres, both as neat coatings and in the presence of epoxy matrix resin, thus reducing the temperature of survival of the fibres under combustion conditions. The results confirm the potential of this novel approach of forming precursor coatings on carbon fibres to minimize the release of conductive fibre fragments from carbon fibre-reinforced polymer composites exposed to fire.