Recyclable flame retardant phosphonated epoxy based thermosets enabled via a reactive approach

The development of reusable fire-safe polymers with a prolonged lifetime heralds the switch for a transition towards circular economy. In this framework, we report a novel phosphonated thermoset which is composed of networked phosphonate esters containing both P-C and P-O bonds. Employing a simple one-pot, two-step synthetic methodology, oxirane groups of the epoxy resin were partially reacted with various amounts of reactive bis H-phosphonate monomer, and cured with a cycloaliphatic hardener to obtain multifunctional thermosets with up to 8% phosphorus (P) content. Though 2.5% P was adequate to achieve good flame retardancy, a concentration > 5% P was necessary for accomplishing material reprocessability and recyclability. These phosphonated thermosets exhibited high Tg (94 degrees C - 140 degrees C) and good thermal stability. Fire performance of the thermoset with 2.5% P via cone calorimetry showed effective reduction in the peak heat release rate (pHRR, 75%) and significant inhibition of total smoke production (TSP, 72.5%), which was attributed to the gas and condense phase actions of the phosphonate moieties. This thermoset was explored as a transparent fire-safe coating on wood where an intumescent flame retardant mechanism was observed. The phosphonated thermoset with higher P content (6%) demonstrated excellent damage reparability and thermomechanical reprocessability driven by transesterification induced network rearrangement. This thermoset was used for manufacturing flax fiber reinforced composite, to demonstrate its future application as a polymer matrix for composite materials.