A flexible silicon-oxygen chain segment-containing phosphoramidite toward modification of epoxy resin: Flame retardancy, toughness and transparency

Epoxy resins (EP) with excellent mechanical properties, adhesive properties and heat resistance are favored in high-end fields. In this study, a silicone-containing phosphoramidite (DPATS) was synthesized through the nucleophilic substitution between diphenylphosphine chloride (DPPC) and 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (PATS). DPATS markedly improved the flame retardancy of EP due to the synergy of phosphorous, nitrogen and silicon elements. The results demonstrated that, with only 2 wt% of DPATS, the EP/DDM/DPATS-2 sample achieved a UL-94 V-0 classification along with a limiting oxygen index (LOI) of 31.4 %. Meanwhile, the peak heat release rate (PHRR) and total heat release (THR) of EP/DDM/DPATS-2 sample reduced by 16.4 % and 8.7 % when compared to virgin EP. The possible flame-retardant mechanism of DPATS on EP during combustion was studied. It was determined that the primary mechanism responsible for the flame-retardant behavior of DPATS may be achieved by the promoting char-formation effect from P and Si elements in the condensed phase, and quenching effect from P-containing radicals as well as the dilution effect of N-containing gases in the gas phase. Additionally, the flexible silicon-oxygen (Si-O) chain segments from DPATS contributed to the toughness of EP. Compared with neat EP, those containing 2 wt% DPATS showed an increment in tensile strength by 23.8 % and elongation at break by 21.9 %. Moreover, high transparency was maintained thanks to excellent compatibility between DPATS and EP. Hence, our work provides an effective approach for producing flame-retardant EP that is simultaneously tough and transparent.