A cross-linked charring strategy for mitigating the hazards of smoke and heat of aluminum diethylphosphonate/polyamide 6 by caged octaphenyl polyhedral oligomeric silsesquioxanes

Aluminum diethylphosphonate (ADP) is a highly efficient phosphorus-based flame retardant, widely used in polyamide 6 (PA6). However, ADP/PA6 releases large amounts of heat and smoke under high heat flux, which commonly means serious hazards to life and property. Octaphenyl polyhedral oligomeric silsesquioxanes (OPS) is an organic-inorganic hybrid silicon compound, playing flame retardant role in condensed phase. In this work, combustion behaviors of OPS/ADP/PA6 were investigated by limited oxygen index (LOI), UL94 and cone calorimeter (CONE) tests. The LOI and UL94 rating results did not change obviously, while the CONE data and smoke density data showed the synergistic effect of OPS and ADP in PA6. For 2.5%OPS/7.5%ADP/PA6, the peak values of heat, smoke and CO release rate (pk-HRR, pk-RSR, D_s, max with/without pilot flame and pk-COP) decreased by 60.2%, 82.1%, 45.9%/38.3% and 80.4% respectively, compared with 10%ADP/PA6. Moreover, 2.5%OPS/7.5%ADP/PA6 produced 337.5% more residue than 10%ADP/PA6. TGA, TG-IR, SEM-EDS, XPS and py-GC/MS were used to further explore the synergistic mechanism of OPS and ADP. It was verified that the cross-linked charring strategy apparently has weakened the hazards of smoke and heat of PA6. This work proposed a possible technical approach to solve both fire risk and heat/smoke hazards of PA6.