A novel DOPO-containing polymer derived from a main-chain benzoxazine: synthesis, characterization and its effect on thermal stability and flame retardancy of epoxy resin

Abstract

The application of reactive macromolecular flame retardants in polymers is gaining increasing attention. In this study, the polymer PBOZ-DOPO was readily synthesized via nucleophilic addition between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and a main-chain benzoxazine polymer (PBOZ). It was utilized as both a co-curing agent and polymeric flame retardant for epoxy resin, yielding epoxy thermosets designated as P_n, where n represented the phosphorus content in the cured polymers (n wt%). While all epoxy thermosets retained good transparency, the addition of PBOZ-DOPO reduced thermal stability, as indicated by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Fortunately, the macromolecular structure and the presence of active OH groups in PBOZ-DOPO helped alleviate the decline in glass transition temperature (T_g). When the phosphorus content was below 0.5 wt%, the T_g loss was less than 10 °C. PBOZ-DOPO imparted highly efficient flame retardancy to the epoxy resin. With a phosphorus content of only 0.3 wt%, the epoxy thermoset P3 achieved a limiting oxygen index (LOI) of 34.1% and met the vertical burning (UL-94) V-0 rating. Cone calorimetry tests demonstrated a progressive decrease in heat release rate (HRR), total heat release (THR), and total smoke production (TSP) from P0 to P5. Scanning electron microscopy (SEM) and Raman spectroscopy further revealed that the amount of graphitized carbon layers formed during combustion increased consistently from P0 to P5, supporting the enhanced flame-retardant performance. Tensile tests indicated that the incorporation of PBOZ-DOPO did not significantly compromise tensile strength or elongation at break, particularly in the case of P3. Overall, the results confirmed that PBOZ-DOPO was an excellent flame retardant with well-balanced properties for epoxy resin.