Synergistic Enhancementof Flame Retardancy and CryogenicToughness: Achieving Liquid Oxygen Compatibility in Epoxy Resin viaa Phosphorus–Silicon Clustered Flame Retardant

Abstract To address the flammability and cryogenic brittleness of resin-based composites for liquid oxygen storage tanks, an organosilicon-based cluster-type phosphonate flame retardant, nMVCP, was prepared and composited with a DOPO derivative (TAD) to improve the flame-retardant performance of the biscitraconimide-reinforced epoxy resin (EP) material (BCIEP). When only 1 wt % of nMVCP and TAD were added, the BCIEP composite matrix achieved a V-0 rating in vertical burning tests. When 2.25 wt % of nMVCP and TAD were added, the limiting oxygen index (LOI) reached 31.0%. Concurrently, the peak heat release rate (PHRR) of 2.25(nMVCP/TAD)/BCIEP (1:2) was 891 kW/m2, representing a 30.8% decrease compared with that of the pure BCIEP matrix. A char layer with a stable structure containing P and Si was formed on the surface, contributing to condensed-phase flame retardancy. Meanwhile, the P-containing groups decomposed upon heating to quench active free radicals, resulting in gas-phase flame retardancy. Under cryogenic conditions (77 K), the impact strength was 115.82% higher than that of the pure BCIEP. Furthermore, the 2.25(nMVCP/TAD)/BCIEP (1:2) composite exhibited liquid oxygen compatibility with a liquid oxygen impact sensitivity coefficient (IRS) of 0%. This work led to the development of a modified EP with flame retardancy, cryogenic toughness, and liquid-oxygen compatibility.