Effect of Ammonium Polyphosphate/Silicate Content on the Postfire Mechanics of Epoxy Glass‐Fiber Composites Using Facile Chocolate Bar‐Inspired Structures

ABSTRACT This study investigates the postfire mechanical properties of epoxy glass‐fiber reinforced composites (EP GFRCs) using increasing concentrations of ammonium polyphosphate (APP) and inorganic silicate (InSi) to modify the char and fire residue. A facile chocolate bar‐inspired structure was introduced for fire exposure and subsequent flexural testing of the GFRCs. The resin matrix used here was a diglycidyl ether of bisphenol‐A (DGEBA) resin, cured with dicyandiamide (DICY), and accelerated by Urone. The microstructures of the degraded composites after three‐point bending tests, were evaluated using scanning electron microscopy (SEM) and x‐ray computed tomography (XCT) imaging. A previous study showed that increasing the APP and InSi content significantly enhanced flame retardancy, via improved char formation under fire conditions. However, flexural properties and fire resistance were adversely affected after fire exposure, highlighting a trade‐off effect. Fiber breakage and delamination of the composites increased upon failure with increasing APP + InSi content in the composite due to unconsolidated char. The experimental values for the postfire flexural mechanics were in good agreement with the two‐layer model proposed in literature. This paper presents a preliminary basis for postfire mechanical testing of epoxy composites for use in fire‐safe structures, using a combination of standardized testing norms.