Thermal degradation behavior of 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide modified epoxy resin with liquid oxygen compatibility

Abstract

The phosphorus-containing epoxy resin, 10-(2,5-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (ODOPB) modified epoxy resin, has been proved compatible to liquid oxygen, and it is prospective to be used as the matrix in composites for liquid oxygen tanks. It is considered that impact energy would be partially converted into heat and concentrated in small regions to form “hot spots”, which cause the degradation of polymer molecular chains to produce large quantities of free radicals and combustible products, thus initiating sensitive reactions of polymer with liquid oxygen. In the present paper, the temperature of the hot spot was estimated and the thermal degradation behavior of ODOPB modified epoxy resin was investigated simultaneously in gas and solid phases, and then on the base of that, the mechanism of its compatibility with liquid oxygen was proposed. The calculated results showed that the theoretical temperature of hot spots could be instantaneously raised up to 810 K (537 °C) under the impact energy of 98 J, which is sufficient to result in the degradation of epoxy resin. In the investigation of degraded gas products from epoxy resin samples, thermogravimetric-Fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) results revealed that the incorporation of ODOPB effectively reduced the generation of free radicals and combustible volatiles. On the other hand, in the solid phase, the FTIR results demonstrated that the introduction of ODOPB could endow epoxy resin with preferable char-forming ability, which effectively hindered the diffusion of oxygen, flammable volatiles, and free radicals. The quenching effects of phosphorus-containing radicals and favorable char-forming ability of ODOPB played key roles in the transformation of epoxy resin from sensitive to compatible with liquid oxygen. This work provides a series of experimental evidence for revealing the mechanism of compatibility of ODOPB modified epoxy resin with liquid oxygen, which is important for the development of composites for liquid oxygen tanks.