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, nitrogencontaining 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 (PO(OR)3), phosphoramidate (PO(OR)2(NHR)), phosphorodiamidate (PO(OR)(NHR)2), phosphoramide (PO(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.
Highlights
Polymers are omnipresent in our everyday life
All P-Flame retardants (FR) were of sufficient purity after the synthesis without the need for additional purification steps as proven by 1H and 31P NMR spectra (Figures S2−S13 and Figure 1b). 31P NMR spectroscopy is a precise technique to control the correct binding pattern and purity of the compounds: the phosphate exhibited a single resonance at −0.67 ppm, whereas the signal shifted downfield with increasing nitrogen content (Figure 1b)
The results show that the blended FRs achieved higher residue yields a lower peak or heat release rate (PHRR), decreased THE, and a lower effective heat of combustion (EHC) than resins with only 2 or 3
Summary
Polymers are omnipresent in our everyday life. their inherent risk of fire makes the use of flame retardants (FRs) inevitable. In stark contrast, during the decomposition of 1, almost no cis-elimination occurred, and only little amounts of phosphoric acid derivatives were observed, indicating the formation of nonvolatiles and underlining the condensed phase activity of the phosphoramide This was further supported by solid-state 31P NMR of the char residues, which exhibited distinct signals for P−N compounds (Figure S46). An exception was the epoxy resin with 4, where the phosphate still displayed characteristic bands during the main decomposition step, implying the presence of phosphate beyond the FR’s boiling point This phenomenon was caused by the reaction between matrix and phosphate (Scheme 1), as the phosphate was more likely to produce phosphoric acid than the nitrogen-containing counterparts due to the difference in bond dissociation energies, leading to incorporation of phosphates into the polymer matrix by transesterification.
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