Novel phosphorus–nitrogen–silicon flame retardants and their application in cycloaliphatic epoxy systems

To solve the incompatibility of flame retardants with epoxy resin, two novel phosphorus–nitrogen–silicon flame retardants (DPP-Si and PSAP) containing active hydroxyl groups were obtained via a simple and efficient synthesis. Both of the obtained flame retardants were characterized via FTIR, 1H NMR, and 31P NMR. Flame retardant epoxy composites (FR-Ep) were prepared with different contents of DPP-Si and PSAP in 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate (ERL4221), and cross-linked by UV-radiation and thermal curing. The influence of DPP-Si and PSAP on the flame-retardant properties of FR-Ep was investigated through limiting oxygen index (LOI) and vertical burning tests (UL-94) respectively, the results show that the LOI of the FR-Ep containing 15 wt% of either DPP-Si or PSAP was about 27.6%, and attained a V-1 rating, whereas pure epoxy resin has an LOI of 20.5%. The microstructures of the char were investigated by scanning electron microscopy (SEM), the SEM results indicated that the residual char of the FR-Eps after burning have compact and coherent appearance, which plays an important role in the excellent thermal stability and improves the flame retardancy of FR-Ep by means of isolating fire sources and cutting off from oxygen.