Char, gas, and action: Transfer of the flame-retardant modes of action in epoxy resins and their fiber-reinforced composites

Flame retardants are often developed for epoxy resins and then transferred into their fiber-reinforced composites with uncertain results. Understanding this transfer in detail represents a critical scientific challenge. This study systematically compares epoxy resins with their glass-fiber reinforced composites, focusing on bisphenol A diglycidyl ether with the hardener dicyandiamide, the flame retardants melamine polyphosphate, ammonium polyphosphate, and silane ammonium polyphosphate, along with inorganic silicate. The research investigates changes in pyrolysis (thermogravimetry), flammability (UL 94, limiting oxygen index), and fire behavior (cone calorimeter) while also examining the flame-retardant modes of action and overall fire performance. The findings reveal that alterations in the amount of fuel, thermal properties, melt flow, and protective layer significantly impact ignition, flammability, and fire load, with a critical reduction in carbonaceous char within the composites preventing intumescence. This study quantifies the effects and provides a fundamental scientific understanding of the complex transfer process of flame retardants from resins to composites, offering essential insights that are of major importance for developing more effective flame-retardant materials.