Systematically controlled decomposition mechanism in phosphorus flame retardants by precise molecular architecture

Abstract

Flame retardants (FR) are inevitable additives to many plastics. Halogenated organics are effective FRs but are controversially discussed due to the release of toxic gases during a fire or their persistence if landfilled. Phosphorus-containing compounds are effective alternatives to halogenated FRs and have potential lower toxicity and degradability. In addition, nitrogen-containing additives were reported to induce synergistic effects with phosphorus-based FRs. However, no systematic study of the gradual variation on a single phosphorus FR containing both P–O and P–N moieties and their comparison to the respective blends of phosphates and phosphoramides was reported. This study developed general design principles for P–O- and P–N-based FRs and will help to design effective FRs for various polymers. We synthesized a library of phosphorus FRs that only differ in their P-binding pattern from each other and studied their decomposition mechanism in epoxy resins. Systematic control over the decomposition pathways of phosphate (P═O(OR)3), phosphoramidate (P═O(OR)2(NHR)), phosphorodiamidate (P═O(OR)(NHR)2), phosphoramide (P═O(NHR)3), and their blends was identified, for example, by reducing cis-elimination and the formation of P–N-rich char with increasing nitrogen content in the P-binding sphere. Our FR epoxy resins can compete with commercial FRs in most cases, but we proved that the blending of esters and amides outperformed the single-molecule amidates/diamidates due to distinctively different decomposition mechanisms acting synergistically when blended.