Synthesis and application of a benzoxazine-type phosphorus-containing monomer on epoxy/benzoxazine copolymer: Thermal stability and compatibility with liquid oxygen

Abstract Alkene-terminated monofunctional benzoxazine monomer (A-BA) was synthesized using allylamine, phenol and paraformaldehyde. Subsequently, the phosphorous-containing benzoxazine monomer (PBA) was obtained through the addition reaction between 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and A-BA in toluene. Finally, epoxy/benzoxazine copolymer with chemically-bonded DOPO was prepared using the synthesized PBA as hardener of DGEBA. DMA results showed that polymer with the chemically-bonded DOPO possessed higher modulus and crosslink density than that with physical-blended DOPO. Thermal gravimetric analysis (TGA) showed that the epoxy/benzoxazine copolymer possessed much better thermal stability around 500 °C and the DOPO-containing epoxy/benzoxazine copolymer showed promoted behavior compared to the neat one. The char residue of the epoxy/benzoxazine copolymer revealed dense surface layer and unbroken original dimension which was in accordance with the TGA results and the phosphorous-rich cover was observed. The liquid oxygen (LOX) compatibility of the copolymers was evaluated through mechanical impact in accordance with ASTM D2512-95 which showed that the LOX compatibility of the epoxy/benzoxazine copolymers were better than the epoxy/amine ones and the epoxy/benzoxazine copolymer including PBA exhibited promoted stability in the impact test. The surface elemental composition of the specimen before and after mechanical impact was investigated by X-ray photoelectron spectroscopy (XPS) and increased oxidation state of phosphorus was observed which explained the mechanism of phosphorus on promoting the LOX compatibility of the copolymer.