A rigid-flexible integrated polyester with bis-phosphaphenanthrene pendants towards simultaneously flame-retarded and toughened epoxy resins with multifunctionalities

To ameliorate the inherent deficiencies of epoxy resins (EP) and to achieve flame-retardant EP with comprehensive performance for versatile applications, a phosphorus-containing polyester named PEDP with rigid-flexible spacers and bis-phosphaphenanthrene pendants was designed, prepared and successfully applied to EP as polymeric flame retardant with multifunctionalities. The obtained EP-PEDP thermosets did not show phase separation to achieve a high dispersion of PEDP and enhanced interaction between two components, which effectively improved the mechanical properties of the thermosets. EP-PEDP displayed excellent toughness, with impact toughness, fracture toughness KIC and GIC reaching 30.9 kJ·m−2, 1.27 MPa·m1/2 and 0.67 kJ·m−2, respectively, showing 166.4 %, 28.3 %, and 59.5 % greater than neat EP; yet tensile and flexural strength also increased rather than sacrificed. The combined flame-retardant action of PEDP in gaseous and condensed phases effectively improved the self-extinguishment and greatly suppressed the release of smoke and heat during burning to achieve high fire safety of EP-PEDP. In addition, EP-PEDP also possessed high glass transition temperature, satisfactory thermal stability, transparency with UV shielding capacity, as well as good dielectric and thermomechanical properties. The design and synthesis of this multifunctional phosphorus-containing polyester with a rigid-flexible integrated nature, which integrates mechanical enhancement, serves as a promising polymeric flame retardant for high-performance EP to meet diverse requirements.