A phosphate covalent organic framework: Synthesis and applications in epoxy resin with outstanding fire performance and mechanical properties

• A phosphate covalent organic framework (PCOF) was successfully synthesized by a facile method. • The introduction of PCOF improved the fire performance of epoxy resin. • PCOF participated in the curing process of EP and thus enhanced the mechanical properties. To prepare epoxy resin (EP) with satisfied fire performance and mechanical properties, a novel phosphoric acid modified triazine covalent organic framework (PCOF) was successfully synthesized. The structure and microtopography of PCOF were fully characterized. By the presence of 12wt% PCOF, the limiting oxygen index value of EP composite was increased from 24.8% to 28.6%, and V-0 rating was achieved in UL-94 vertical burning test. The cone calorimeter test showed that the peak heat release rate and total heat release of the 12PCOF-EP sample were decreased sharply by 78.9% and 68.9% compared with that of the control EP. The flame retardant effect was maintained after soaked in 75 °C for 168 h, indicating an excellent water resistance. The thermogravimetric analysis of the composites indicated that more char residue was formed, especially at a high-temperature range. Moreover, both the tensile and impact properties of EP composites were improved by adding appropriate amount of PCOF. By analyzing the products formed during combustion, it was proposed that PCOF took effects in both gas and condensed phase.