Cage–ladder‐structure, phosphorus‐containing polyhedral oligomeric silsesquinoxanes as promising reactive‐type flame retardants for epoxy resin

We synthesized a novel cage–ladder‐structure, phosphorus‐containing polyhedral oligomeric silsesquinoxane (CLEP–DOPO–POSS) via the hydrolytic condensation of 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO)–vinyl trimethoxysilane (VTMS) with 2‐(3,4‐epoxycyclohexyl) ethyl trimethoxysilane and then incorporated it into epoxy resins (EPs) in different ratios with thermal curing technology. The structure of CLEP–DOPO–POSS was confirmed by Fourier transform infrared spectroscopy, NMR spectroscopy (1H‐NMR and 29Si‐NMR), matrix‐assisted laser desorption ionization time‐of‐flight, and X‐ray diffraction. The thermal stability, mechanical properties and flame retardancy effect of CLEP–DOPO–POSS on EP were comprehensively evaluated via thermogravimetric analysis (TGA), dynamic mechanical analysis, universal tensile testing, limiting oxygen index (LOI) measurement, UL‐94 testing, and cone calorimetry. The flame‐retardant mechanism of the EP modified with CLEP–DOPO–POSS was investigated by TGA–IR, TGA–mass spectrometry, and scanning electron microscopy. The experimental results show that CLEP–DOPO–POSS was homogeneously dispersed in the EP matrix. The LOI value reached 31.9, and the UL‐94 grade passed V‐0 with the presence of only 0.28% P (2.91 phr CLEP–DOPO–POSS). In addition, the EP composite containing CLEP–DOPO–POSS exhibited a better thermal stability and mechanical properties. The flame‐retardant mechanism was attributed to the quenching effect of the phosphorus‐containing free radicals and the formation of phosphorus‐ and silicon‐containing char layers in the condensed phase.