Sol–gel complements conventional strategies for the synthesis of self-extinguishing hybrid silica–epoxy nanocomposites.

Introduction. Growing industry demand, together with rising pollution and the depletion of phosphorus, is moving the scientific community towards the development of flame-retardant (FR) epoxy nanocomposites (ENCs) containing low P contents and more sustainable additives.

Methods. High-resolution transmission electron microscopy (HRTEM) analysis was carried out to study the morphology of ENCs. Cone calorimetry (CC) and UL-94 vertical flame spread tests were performed to investigate the fire response of all ENCs.

Results. The reaction of DGEBA (Bisphenol A diglycidyl ether)- or Novolac-based resins with APTES (3-aminopropyltriethoxysilane) allows the production of organic–inorganic silanized epoxy moieties. The hybrid moieties can condense with tetraethyl orthosilicate (TEOS), a silica precursor, to form an in situ silica phase through sol–gel reactions. HRTEM analysis revealed that in the case of DGEBA, the silica phase was composed of well-ordered multi-lamellar nanoparticles (NPs). In contrast, the investigation of Novolac highlighted that fully amorphous silica NPs were embedded in the hybrid co-continuous polymer network. The incorporation of DOPO-based FRs into silica–epoxy systems based on DGEBA/Novolac resin produces aliphatic nanocomposites with high transparency, no-dripping UL-94-V0 rating, and a strong decrease (up to 80%) in the peak of the heat release rate in CC tests, with up to 3 wt.% of P loading. Regarding Novolac, the transparency is remains even at loadings of silica NPs beyond 4 wt.%, thanks to their amorphous nature. More waste-to-wealth approaches involve the use of humic acids or biochar from spent coffee grounds, together with ammonium polyphosphate and urea, in APTES-modified DGEBA-based epoxy systems to obtain no-dripping self-extinguishing systems, even with only 1 wt.% of P content.

Conclusions. The sol–gel in situ generation of inorganic phases has been explored in combination with DOPO-based FRs, bio-wastes, and other synergists to prepare no-dripping self-extinguishing (V-0 rating in UL-94 flammability tests) aliphatic ENCs, even keeping P at low loadings (1-3 wt.%).