Bo Xu,Meng Jia,Dong Wang,Siheng Zhao,Shouao Zhu,Lijun Qian
Four polyphosphate/polyphosphonate flame retardants (PAP, POP, PAP-UB and POP-UB) have been designed and synthesized to study the effects of phosphorus valence states and the presence of unsaturated carbon-carbon double bonds on self-cross-linking impacting flame-retardant modes of action. It was found that phosphorus valence states affect the proportion of flame-retardant activity between the gas phase and condensed phase. A phosphonate containing +3-valence phosphorus is primarily effective as a gas-phase flame-retardant, while the one containing +5-valence phosphorus mainly exerted an impact in the condensed phase. The introduction of an unsaturated bond enhances the carbon-forming effect promoted by polyphosphates/polyphosphonates in a degrading polymer matrix and increases the melt viscosity for blends at high temperature. The phosphorus-containing fragments which might have been released into the gas phase are locked in the condensed phase due to C C bonds cross-linking, thus forming more high-quality phosphorus-rich char layers. This effect balances the distribution of gas-phase and condense-phase flame-retardant effects and is especially obvious in PET/polyphosphonate blends containing +3-valence phosphorus, which originally plays mainly the role of a gas-phase flame retardant . PET blends containing unsaturated carbon-carbon double bonds display better carbon formation, flame-retardant activity and anti-dripping behavior than these do without this feature.
A high-phosphorus-content polyphosphonate (PBDA), containing two phosphorus-based structures: phosphaphenanthrene (DOPO) and phenyl phosphonate groups, was synthesized and used in flame retardant polyethylene terephthalate (PET). Good self-extinguishing property (high UL 94 grade and LOI value), superior flame retardancy (lower heat/smoke release), and high quality retention (high carbon residue) were endowed to PET by PBDA. When 10 wt% PDBA was added, the peak heat release rate (pHRR), total heat release (THR), and total smoke rate (TSR) of PDBA/PET were found to be significantly reduced by 80%, 60.5%, and 21%, respectively, compared to the pure PET, and the LOI value jumped from 20.5% for pure PET to 28.7% with a UL-94 V-0 rating. The flame-retardant mode of action in PET was verified by thermogravimetric analysis-Fourier transform infrared (TGA-FTIR), pyrolysis gas chromatography/mass spectrometry (Py-GC/MS), real-time FTIR, and scanning electron microscopy (SEM). Phosphaphenanthrene and phosphonate moieties in PDBA decomposed in sequence during heating, continuously releasing and keeping high-content PO· and PO2· radicals with a quenching effect and simultaneously promoting the formation of viscous crosslinked char layers causing a high barrier effect. PDBA mainly acted in the gas phase but the condensed-phase flame retardant function was also considerable.Keywords:flame retardant;polyphosphonate;poly(ethylene terephthalate);high phosphorus content
Superior flame retardant polylactic acid (PLA) composites were prepared using bio-based polyphosphonate (BPPT) and polyethyleneimine-modified graphene oxide (M-GO) to be used as a flame retardant, the total amount of which is only 3 wt%. With 2.4 wt% BPPT and 0.6 wt% M-GO in the polymer matrix, the as-prepared PLA composites can achieve the UL94 V0 grade and a LOI value of 36.0. Analysis of the residual char and pyrolytic products revealed that the conjunction of a gas-phase and condensed-phase mechanism contributes to the good flame retardant performance. Moreover, the tensile toughness of PLA was also enhanced. The PLA composite with 2.1 wt% BPPT and 0.9 wt% M-GO displayed an elongation at break of 13.1% and maintained a tensile strength of 39.1 MPa. The debonding between the M-GO and PLA matrix and the plastic deformation around the M-GO particles were responsible for the improved tensile toughness.
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