Hybrids of aluminum hypophosphite and ammonium polyphosphate: Highly effective flame retardant system for unsaturated polyester resin

A novel flame‐retardant unsaturated polyester (UP) was obtained by introducing ammonium polyphosphate (APP) and aluminum hypophosphite (AHP) into UP matrix. The UP/APP/AHP composites obtained were characterized by Fourier Transform Infrared Spectra (FTIR). From limiting oxygen index (LOI), microscale combustion calorimetry (MCC), and UL‐94 tests, the UP/APP/AHP composites present significantly enhanced flame retardancy when compared to pure UP, such as passed UL‐94 V ‐ 0 classification, decreased peak heat release rate (PHRR) maximally by 50.0% and total heat release (THR) maximally by 22.0%, and increased LOI by 34.8%. Thermal degradation behaviors of composites studied by thermogravimetric analysis (TGA) suggest that APP/AHP hybrids have a higher catalytic charring efficiency than APP alone. Char morphology investigated by scanning electron microscopy (SEM) indicates that the UP/APP/AHP composites form a compact and intumescent char on the sample surface during combustion, which could retard the transfer of heat and oxygen. Thermogravimetric analysis/infrared spectrometry (TGIR) results reveal that the APP/AHP hybrids reduce the release amount of combustible gas from polymer matrix during combustion. The UP/APP/AHP system acts in gas phase through flame inhibition and in condensed phase through charring, resulting in significantly enhanced flame retardancy. This work combining condensed‐phase mechanisms with gas‐phase mechanisms in flame retardance of UP would stimulate more efforts to explore highly effective flame‐retardant system of polymers. POLYM. COMPOS., 39:1763–1770, 2018. © 2016 Society of Plastics Engineers