Recyclable inherently flame-retardant thermosets: Chemistry, properties and applications

Thermosets are polymeric materials that contain permanent networks and thus are difficult to recycle. They are not reprocessable once cured and often do not degrade under mild conditions. Over the past decades, the use of polymeric materials in fire safety applications has increased, and so is the need for them to be more sustainable. From this standpoint, recently two major challenges in designing next-generation thermosets have attracted much attention in the scientific community: embedded fire safety and reprocessability/recyclability. In this review, a detailed report on research progress in design of fire-safe and thermomechanical reprocessable/recyclable thermosets is presented. Such thermosets are designed not only to enable the reuse and recycling of the polymer material but also recover valuable components (carbon fibers or rare additives) that are encapsulated in the matrix. The flame retardant recyclable thermoset materials are categorized based on the chemistry of labile bonds (covalent adaptable networks): i.e. (i) esters (carboxylic and phosphate esters), (ii) sulfur-containing linkages, (iii) nitrogen-containing structures, and (iv) other phosphorus-containing structures. In addition, the use of bio-based raw materials in constructing these thermosets is also highlighted. The synthetic route, fire performance, recycling methods, degradation mechanisms, and progress in various approaches being developed by researchers towards recyclable and fire-safe thermosets are summarized in detail in this review.