The ubiquity of polymeric materials in daily life comes with an increased fire risk, and sustained research into efficient flame retardants is key to ensuring the safety of the populace and material goods from accidental fires. Phosphorus, a versatile and effective element for use in flame retardants, has the potential to supersede the halogenated variants that are still widely used today: current formulations employ a variety of modes of action and methods of implementation, as additives or as reactants, to solve the task of developing flame‐retarding polymeric materials. Phosphorus‐based flame retardants can act in both the gas and condensed phase during a fire. This Review investigates how current phosphorus chemistry helps in reducing the flammability of polymers, and addresses the future of sustainable, efficient, and safe phosphorus‐based flame‐retardants from renewable sources.

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. Further, nitrogen-containing additives were reported to induce synergistic effects with phosphorus-based FRs. However, no systematic study on 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 were identified, e.g. 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.

We successfully synthesized multifunctional P-based hyperbranched polymeric flame retardants (hb-FRs) with varying oxygen-to-nitrogen (O : N) content and characterized them via¹H and ³¹P NMR and GPC. Their miscibility in epoxy resins (EP) and impact on glass-transition temperatures (T_g) were determined via differential scanning calorimetry (DSC). Using thermogravimetric and evolved gas analysis (TGA, TG-FTIR), pyrolysis gas chromatography/mass spectrometry (Py-GC-MS), hot stage FTIR, flammability tests UL-94 and LOI, fire testing via cone calorimetry, residue analysis via scanning electron microscopy (SEM) and elemental analysis, detailed decomposition mechanisms and modes of action are proposed. hb-polymeric FRs have improved miscibility and thermal stability, leading to high FR performance even at low loadings. Polymeric, complex FRs increase flame retardancy, mitigate negative effects of low molecular weight variants, and can compete with commercial aromatic FRs. The results illustrate the role played by the chemical structure in flame retardancy and highlight the potential of hb-FRs as multifunctional additives.

A rigid aromatic phosphorus‐containing hyperbranched flame retardant structure is synthesized from 10‐(2,5‐dihydroxyphenyl)‐10H‐9‐oxa‐10‐phosphaphenanthrene‐10‐oxide (DOPO‐HQ), tris(4‐hydroxyphenyl)phosphine oxide (THPPO), and 1,4‐terephthaloyl chloride (TPC). The resulting poly‐(DOPO‐HQ/THPPO‐terephthalate) (PDTT) is implemented as a flame retardant into an epoxy resin (EP) at a 10 wt% loading. The effects on EP are compared with those of the monomer DOPO‐HQ and triphenylphosphine oxide (OPPh₃) as low molar mass flame retardants. The glass transition temperature, thermal decomposition, flammability (reaction to small flame), and burning behavior of the thermosets are investigated using differential scanning calorimetry, thermogravimetric analysis, pyrolysis combustion flow calorimetry, UL 94‐burning chamber testing, and cone calorimeter measurements. Although P‐contents are low at only 0.6 wt%, the study aims not at attaining V‐0, but at presenting a proof of principle: Epoxy resinswith PDTT show promising fire performance, exhibiting a 25% reduction in total heat evolved (THE), a 30% reduction in peak heat release rate (PHRR) due to flame inhibition (21% reduction in effective heat of combustion (EHC)), and an increase in T_g at the same time. This study indicates that rigid aromatic hyperbranched polymeric structures offer a promising route toward multifunctional flame retardancy.

Vitrimers are a promising alternative to conventional composite materials as they can be recycled and reshaped but still need additives. Herein, intrinsic flame-retardant phosphorus-containing vitrimers are presented, which were used in composites.