A phosphorous-based bi-functional compound HPDAl was used as a reactive-type flame retardant (FR) in an epoxy thermoset (EP) aiming to improve the flame retardant efficiency of phosphorus-based compounds. HPDAl, consisting of two different P-groups of aluminum phosphinate (AHP) and phosphophenanthrene (DOPO) with different phosphorous chemical environments and thus exerting different FR actions, exhibited an intramolecular P-P groups synergy and possessed superior flame-retardant efficiency compared with DOPO or AHP alone or the physical combination of DOPO/AHP in EP. Adding 2 wt.% HPDAl made EP composites acquire a LOI value of 32.3%, pass a UL94 V-0 rating with a blowing-out effect, and exhibit a decrease in the heat/smoke release. The flame retardant modes of action of HPDAl were confirmed by the experiments of the scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and thermogravimetry–Fourier transform infrared spectroscopy–gas chromatograph/mass spectrometer (TG-FTIR-GC/MS). The results indicate that the phosphorous-based FRs show different influences on the flame retardancy of composites, mainly depending on their chemical structures. HPDAl had a flame inhibition effect in the gas phase and a charring effect in the condensed phase, with a well-balanced distribution of P content in the gas/condensed phase. Furthermore, the addition of HPDAl hardly impaired the mechanical properties of the matrix due to the link by chemical bonds between them.

The aim of this work was to improve the smoke-suppression performance and flame-retardant effect of a phosphorus-based bifunctional flame retardant (PDEP)/expandable graphite (EG) binary flame-retardant system by incorporating two boroncontaining inorganic compounds, zinc borate and boron phosphate (BP), into rigid polyurethane foams (RPUFs). The results revealed that both the boron-containing inorganic compounds inhibited smoke/heat release of retardant RPUF by accelerating the crosslinking and char forming of the material to promote phosphorus retention in the solid phase. In particular, when 2% BP replaced the same amount of PDEP, the total smoke release value of the RPUF/2%BP/10%PDEP/8%EG (2B10P8E) sample decreased by 30.0% compared with the RPUF/12%PDEP/8%EG (12P8E) sample. This lifted smoke-suppressive performance mainly due to the existence of Brønsted and Lewis acid sites on BP, which can accelerate the dehydration and crosslinking of hydroxyl in compounds, thus resulting in more phosphorus-oxygen components retained in the condensed phase, effectively inhibiting the smoke/heat release of the composites. The addition of BP balanced the bi-phase flame-retardant effects of RPUF systems containing phosphorus-based flame retardant due to a strong catalytically charring effect. Thus, the flame-retardant efficiency was also greatly improved. The smoke-suppressive and flame-retardant modes of action of the ternary system was evaluated by experiments using TGA Fourier transform infrared spectroscopy, SEM, energy dispersive spectrometry, pyrolysis-gas chromatography/mass spectrometry and Fourier transform infrared spectroscopy.

This work investigated the effect of different valence states of phosphorus-containing compounds on thermal decomposition and flame retardancy of polyethylene terephthalate (PET). Three polyphosphates—PBPP with +3-valence P, PBDP with +5-valence P and PBPDP with both +3/+5-valence P—were synthesized. The combustion behaviors of flame-retardant PET were studied and the structure–property relationships between the phosphorus-based structures with different valence states and flame-retardant properties were further explored. It was found that phosphorus valence states significantly affected the flame-retardant modes of action of polyphosphate in PET. For the phosphorus structures with +3-valence, more phosphorus-containing fragments were released in the gas phase, inhibiting polymer chain decomposition reactions; by contrast, those with +5-valence phosphorus retained more P in the condensed phase, promoting the formation of more P-rich char layers. It is worth noting that the polyphosphate containing both +3/+5-valence phosphorous tended to combine the advantage of phosphorus structures with two valence states and balance the flame-retardant effect in the gas phase and condensed phase. These results contribute to guiding the design of specified phosphorus-based structures of flame-retardant compounds in polymer materials.