Novel crosslinkable epoxy resins containing phenylacetylene and azobenzene groups: From thermal crosslinking to flame retardance

Several kinds of novel flame-retardant-free and thermo-crosslinkable epoxy resins (EPs) containing azobenzene or/and phenylacetylene groups have been synthesized and characterized, and the flame retardant properties as a result of thermal crosslinking during combustion were investigated in detail. Crosslinking behaviors were tested by simultaneous thermogravimetry-differential scanning calorimetry (TGA-DSC). Thermal stabilities were investigated by thermogravimetric analysis (TGA). Flame retardance of the resulting EPs was evaluated through LOI tests, and combustion behaviors were studied via cone calorimetry and micro-combustion calorimetry (MCC), which further confirmed that flame retardance of these EPs was significantly improved, despite the absence of conventional flame retardant. Py-GC/MS analysis was used to investigate the degradation mechanism of these epoxy resins, and the results confirmed that the flame-retardant activity of epoxy resins mainly took effect in the condensed phase. The chemical constitution of the char layers were investigated by XPS and Raman spectrum. The co-crosslinking behavior between azobenzene and phenylacetylene groups was predicted and confirmed, which led to the most compact char layer, therefore resulted in the best flame retardance of these EPs.