Investigating into the liquid oxygen compatibility of a modified epoxy resin containing silicon/phosphorus and its mechanical behavior at cryogenic temperature

Abstract

A 9,10-dihydro–9–oxa–10–phosphaphenanthrene–10–oxide (DOPO) derivative (DOPO–TVS) was synthesized through a reaction between DOPO and triethoxyvinylsilane (TVS). To modify the common epoxy resin molecular without consuming the epoxy group, the general bisphenol F epoxy resin was first treated with isocyanate propyl triethoxysilane (IPTS). In the next step, the pretreated epoxy resin and DOPO–TVS were mixed to initiate the sol–gel process to generate the organic–inorganic hybrid Si–O–Si network within the epoxy matrix. The characterization of each reaction product was confirmed by Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (¹H NMR, ³¹P NMR) spectroscopy. The liquid oxygen compatibility of the cured epoxy resin was evaluated through mechanical impact in accordance with ASTM D2512-95. The surface elemental composition of the specimen before and after mechanical impact was investigated by X-ray photoelectron spectroscopy (XPS). The results of liquid oxygen mechanical impact showed that the liquid oxygen compatibility of the silicon/phosphorus containing epoxy resin was obviously enhanced. Moreover, the surface element composition also confirms the migration of the silicon to the surface of the cured epoxy resin and the generation of phosphoric oxyacid, which belongs to the condensed phase flame retardant mechanism. The tensile test and fracture toughness test under cryogenic conditions (77 K) were also carried out. The results showed that the modified epoxy resin possesses an elevated toughness property, which was attributed to the flexible Si–O–Si network in the cured modified epoxy resin.