Two phosphorus-containing cage-like silsesquioxane derivatives were synthesized as reactive or additive flame retardants for epoxy resin. The silsesquioxanes were obtained via an epoxide ring-opening reaction using a 10-hydroxy-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPA). In one derivative containing in its structure 4 glycidoxypropyl and 4 phosphate groups, denoted as 4P4GS, only half of the epoxy rings was reacted with phosphate to obtain a reactive additive, while in the second derivative containing 8 phosphate groups, denoted as 8PS, all epoxy groups were converted, thus an additive modifier was obtained. The silsesquioxanes containing phosphorus atoms and the reactive phosphorus-free silsesquioxane derivative (octakis[(3-glycidoxypropyl)dimethylsiloxy]octasilsesquioxane (8GS)) were used to prepare hybrid materials based on epoxy resin. To compare the impact of the structure of silsesquioxane derivatives on the properties of hybrid materials, a number of samples containing 1, 5, and 10% of the modifiers making a series of epoxy materials containing additive or reactive modifiers, were obtained. The modified epoxies were studied using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), nanoindentation, water contact angle, and cone calorimetry tests to assess the effects of the modifier structure on the physicochemical properties of the investigated materials.
Keywords: epoxy resin; flammability; phosphate; silsesquioxane.