Design of functionalized ammonium polyphosphate for enhancing fire safety of unsaturated polyester resin

Design of functionalized ammonium polyphosphate for enhancing fire safety of unsaturated polyester resin

Unsaturated polyester resin (UPR) is a widely used thermoset polymer for engineering applications but suffers from high flammability and dense smoke emission when combusting, which substantially limits its practical applications in fields where demanding fire safety is required. Ammonium polyphosphate (APP), a halogen-free intumescent flame retardant, has attracted attention for improving the fire safety of UPR due to its multiple merits, including relatively high efficiency, low toxicity and low smoke. However, its application is restricted by its high loading requirements to meet target fire retardancy, and it often shows poor compatibility with the UPR matrix, thus resulting in compromised mechanical performance and limited smoke suppression. To address these limitations, this thesis systematically reviewed existing strategies to enhance the flame retardancy of APP/UPR systems and developed a novel trinity flame retardant (DO) by chemically modifying APP with ethylenediamine and ethanolamine via a cation exchange reaction. The work synthesized three different types of DO by tuning the molar ratio of ethylenediamine to ethanolamine, that is ̶ DO3-1, DO1-1, and DO1-3 refer to the ethylenediamine and ethanolamine molar ratio is 3:1,1:1, and 1:3, respectively. The as developed DOs were then incorporated into vinyl ester resin (VER, one type of UPR) as a model thermosetting system. Comprehensive characterizations including limiting oxygen index, UL-94 vertical burning test, cone calorimetry, thermogravimetric analysis, TG-FTIR, Raman spectroscopy, and SEM were employed to evaluate the fire performance and other material properties of the final composites. The optimal formulation, 26% DO3-1/VER, achieved a limiting oxygen index (LOI) value of 26.5% and a UL-94 V-0 rating, far outperforming the unmodified APP system. This required loading level is moderately smaller than conventional fire retardants which often need a loading level as high as 30% for APP in UPR to pass the UL-94 V-0 rating. Cone calorimetry results showed substantial reductions in peak heat release rate, total heat release, and peak smoke production rate, while TG-FTIR confirmed the release of inert gases and reduced toxic volatiles during decomposition. The DO also facilitated the formation of dense, graphitized char layers that effectively inhibited heat and mass transfer. This research presents an effective scalable strategy for improving the flame retardancy and smoke suppression of UPR materials, contributing to the development of halogen-free environmentally friendly flame retardants for high-performance polymer composites.

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