In Situ P-Modified Hybrid Silica–Epoxy Nanocomposites via a Green Hydrolytic Sol–Gel Route for Flame-Retardant Applications

Flame retardance of epoxy resins is usually imparted using suitable additives and/or properly modified curing agents. Herein, via a two-step green synthetic procedure, the chemical modification of the epoxy matrix with reactive silicon and phosphorus precursors is explored to obtain nanocomposites with intrinsic flame-retardant features. Nanoscale phase separation occurs in the first step, forming an inverse micelle system in which polar nanodomains act as nanoreactors for the hydrolysis of silanes (Si precursors), giving rise to silica lamellar nanocrystals (SLNCs). In the second step, inside the silica nanodomains, the formation of stable Si−O−P bonds occurs because the reactivity of phosphoric acid (P precursor) with the oxirane rings of the polymer chain is balanced by its tendency to diffuse into polar nanodomains. Intriguingly, the use of phosphoric acid alone in epoxy composite manufacturing leads to a wormlike morphology of the network, whereas its addition in the presence of silanes results in the formation of SLNCs with a thinner interlayer distance. The morphology of the hybrid Si/P−epoxy nanocomposites, comprising organic and inorganic co-continuous phases, can confer, through a prevalent mechanism in the condensed phase, interesting flame retardant performances, namely, the absence of dripping during vertical burning tests, the formation of a large amount of coherent char after combustion, and a remarkable reduction (up to 27.7%) in the peak of heat release rate. The above characteristics make these nanostructured hybrid materials very promising for the manufacturing of epoxy systems with enhanced fire behavior (e.g., coatings, sealants, matrices for reinforced composites), even containing a low amount of specific flame retardants and thus keeping good viscoelastic properties.