Effects of Flame Retardant Additives on the Performance of Epoxy Composites

Abstract

Composites based on epoxy resin have been applied in many industry sectors, such as construction, aerospace and transportation. However, rather poor fire performance of epoxy composites can limit their applications, especially in the recent years when the strict fire safety regulations have become mandatory. Therefore, it is important to enhance the flame resistance of the composites and understand their behaviour after incorporation of flame retardant additives at room and elevated temperatures. Among all flame retardants, the intumescent additives have gained substantial attention due to their high efficacy and low toxicity. In this research, the effects of flame retardant additives based on intumescent ammonium polyphosphate (APP) and talc on the thermal and mechanical properties of epoxy resin and its glass fibre composites have been investigated by conducting a comprehensive set of fire and mechanical tests. In addition, the performance characteristics of two additive systems, APP/halloysite nanotube (HNT) and APP/layered double hydroxide (LDH), with regards to fire and mechanical properties of epoxy and epoxy/glass fibre composites have been compared. Furthermore, a customised method has been explored to determine the tensile properties of glass and flax fibre epoxy composites (with and without APP) at elevated temperatures. The effects of heat-induced damage and APP on the impact properties of glass and flax fibre reinforced composites have also been evaluated and compared. The cone calorimeter results have shown that the combination of APP and talc, compared to APP alone, can enhance the flame retardancy of epoxy/glass fibre composites. However, the combination of the additives may adversely affect the fire reaction properties of the epoxy resin. On the other hand, the decomposition of talc particles by the application of flame in the vertical burn test could not provide any rating for the glass fibre composites. In addition, the combined effects of APP and nano-clays, layered double hydroxide or halloysite nano-tube, significantly improve the flame retardant properties of epoxy and its glass fibre composites. ii The incorporation of the additives (APP, APP/talc and APP/nano-clays) generally tends to reduce the tensile and flexural strengths of the resin but enhances both tensile and flexural moduli; whereas the mechanical properties of epoxy/glass fibre composite are not significantly affected by the incorporation of the additives. Furthermore, the tensile tests of glass and flax fibre composites at elevated temperatures have demonstrated the significant reductions in the tensile properties of both composites at 100 ºC. However, the glass fibre composites could retain the tensile properties at the same reduced level when exposed to temperatures between 100 and 300 ºC before a slight reduction at 400 ºC. The flax fibre composites, on the other hand, lose most of their tensile properties during the temperature rise up to 250 ºC. An addition of APP further degrades the tensile properties of the composites in the temperature range of 250 to 400 ºC, even though it has been effective to improve the fire properties of the composites. The glass and flax fibre composites have also shown different drop-weight impact properties after being exposed to heat (at 300 ºC). The absorbed energy by the glass fibre composite as well as the maximum deflection increase but the maximum impact force of the composite reduces due to the heat exposure. However, the energy absorption, maximum deflection and impact force of the flax fibre composite decrease after the heat exposure. Furthermore, the impact energy absorptions of both heatexposed composites have improved in the presence of APP. Overall, it can be concluded that the incorporation of suitable flame retardant additives can improve the flame retardancy of fibre reinforced composites without significantly affecting their mechanical properties. Furthermore, in spite of higher flammability of natural fibre composites compared to that of the synthetic fibre composites, they show comparable flame retardant properties by incorporating the intumescent additives. Moreover, the differences between the mechanical performances of synthetic and natural fibre composites during and after heat exposure should be considered for the potential replacements of the synthetic fibre reinforced composites by their natural fibre counterparts.