To improve the flame retardancy of vinyl epoxy (VE) resins more effectively, a half-cage and cage structure polyhedral oligomeric silsesquioxanes (P/N-POSS) containing phosphorus and nitrogen was synthesized by a one-step Kabachnik-Fields reaction from paraformaldehyde, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and 3-aminopropyltriethoxysilane. It was then blended with tetrabutyl titanate (TBT) in VE resins to construct a phosphorus-nitrogen-silicon-titanium synergy flame retardant system. P/N-POSS was well dispersed in the VE matrix at low loadings (≤4 wt%). With the addition of only 4 wt% P/N-POSS and 0.2 wt% TBT, the LOI value reached 29.2 and the UL-94 grade passed V-1 rating. Moreover, compared with VE-0, the peak heat release rate, total heat release, smoke production rate and total smoke production decreased significantly. The investigations on pyrolysis volatiles and char residue of cured VE composites revealed that the excellent flame retardancy of the VE composites were due to the superior free radical capturing ability, the dilution effect of P/N-POSS in the gas phase, and the promoting the carbonation effect of P/N-POSS, the catalyzing char formation ability of TBT in the condensed phase. Meanwhile, VE composites exhibited improved mechanical properties, thermal stabilities and retained the low viscosity and good processability of VE resins.
Despite remarkable advances in developing flame retardants and smoke suppressants for epoxy resin (EP), engineering nitrogen-containing polyhedral oligomeric silsesquioxane (N-POSS) to impart superior fire safety properties to EP has remained an intractable challenge. In this work, two nitrogen-containing polyhedral oligomeric silsesquioxanes (N-POSSs), namely, aminoethyl-aminopropyl-hepta-phenyl polyhedral oligomeric silsesquioxane (AEAP-POSS) and aminopropyl-hepta-phenyl polyhedral oligomeric silsesquioxane (AP-POSS), were synthesized through the “corner-capping” reaction. The molecular structures of N-POSSs were fully characterized by FTIR, 1 H NMR, 29 Si NMR and MALDI-TOF MS, and the synthesized AEAP-POSS and AP-POSS were introduced into EP to solve the shortcomings of flammability. TGA results showed that the incorporation of 4 wt% N-POSS nanoparticles distinctly improved the char residue at 800 °C, which significantly enhanced the thermal stability of the EP composites. When 4 wt% of AP-POSS was introduced into EP, reductions in the peak of heat release rate (p-HRR), fire growth index (FGI), peak of smoke production rate (p-SPR), and the peak of CO production rate (p-COP) reached to 60.6%, 70.2%, 52.3% and 60.4%, respectively. Subsequently, TG-FTIR and XPS were utilized to investigate the flame retardancy and smoke suppression mechanism. Our work presented a considerable advancement for the facile fabrication of flame retardants based on nitrogen-containing polyhedral oligomeric silsesquioxane compounds.
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