Simultaneous optical clarity and fire protection in Novolac resin via in situ amorphous silica and a liquid DOPO derivative

Silica–polymer composites are widely used to enhance mechanical performance in industries ranging from packaging to transportation. However, extending their use into high-demand sectors such as electronics and construction requires additional functionalities, particularly transparency and fire safety. Here, we demonstrate both by developing fully transparent, self-extinguishing silica–epoxy nanocomposites (SiEpo-NCs) via an in situ sol–gel process. Using a Novolac epoxy matrix cured with a cycloaliphatic hardener, we obtained uniformly dispersed amorphous silica nanoparticles (SNPs), as confirmed by microscopy and particle size analysis. An alternative masterbatch-dilution route produced silica-rich and silica-free domains, which further enhanced the thermo-mechanical performance of the composite materials. To achieve flame retardance while maintaining optical clarity, we incorporated the liquid phosphorus-based additive 6H-dibenz[c,e][1,2]oxaphosphorin-6-propanoic acid, butyl ester, 6-oxide (DOB) into the SiEpo network. This strategy yielded a UL94-V0 classification at only 3 wt.% phosphorus and 2 wt.% SNPs, delivering a rare combination of transparency, non-dripping behavior, and self-extinguishing performance. Cone calorimetry and gas analysis revealed a synergistic mechanism between SNP-induced char formation and DOB’s gas-phase inhibition, establishing a promising route toward multifunctional epoxy nanocomposites.