Recyclable Flame Retardant Phosphonated Epoxy Based Thermosets Enabled Via a Reactive Approach

The development of reusable polymers with a prolonged lifetime heralds the switch for a transition towards more circular economy. In this work we report a novel phosphonated thermoset network which is composed of phosphonate ester bonds containing one P-C bond and two 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 finally cured with a cycloaliphatic hardener to obtain multifunctional thermosets with P content up to 8%. 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 °C – 140 °C) and good thermal stability. The thermoset containing 2.5% P was explored as a transparent fire safe coating on wood where it acted via an intumescent mechanism. Fire performance evaluation of this thermoset (2.5% P) alone via cone calorimetry showed effective reduction of the peak heat release rate (pHRR, 57.3%) and significant inhibition of total smoke production (TSP, 72.5%) which was attributed to the gas and condense phase actions of the phosphonate monomer. The phosphonated thermosets with higher P content (> 5%) demonstrated excellent damage reparability and thermomechanical reprocessability driven by transesterification mechanism. In addition, the phosphonated thermoset was used in flax fiber reinforced composite manufacturing, to reveal its future potential in composite application.