Synergistic Flame-Retardant Properties Of Polyphosphazene And Halloysite Nanotubes On PET

Polyethylene terephthalate（PET）is a kind of cheap polymer with various applications.It is used in film sheets,packaging,engineering plastics and polyester fabrics.However,as a thermoplastic material,PET is flammable.Due to its straight-chain molecular structure,the oligomers produced by PET’s pyrolysis can form molten droplets,causing burns or secondary fires.The aromatic substances in its molecular chain can create smoke that harms human health.So it is of great importance to improve the fire-retardant properties of PET.Currently,polyphosphazene derivatives have attracted extensive research due to their rich fire-retardant elements such as P and N and the low toxicity and environmental protection.They are expected to improve the mechanical properties of matrix.In this paper,using hexachlorocyclotriphosphazene（HCCP）and 4,4′-（Hexafluoroisopropylidene）diphenol（BPAF）as monomers,4,4′-（Hexafluoroisopropylidene）diphenol-substituted cyclotriphosphazene（PZAF）nanospheres were prepared,which were melt-blended with PET to obtain PET nanocomposites,and the fire safety and mechanical properties of the composites were investigated.Natural clay halloysite（HNT）exhibits hollow nanotube-like structure and high aspect ratio,which can effectively adsorb the oligomers produced in the combustion and toughen the composites.Based on this,an organic-inorganic multifunctional flame retardant was prepared and melt-blended with PET to further reduce the release of smoke and heat and improve the mechanical properties.（1）Preparation of PZAF and its effect on the flame-retardant properties of PETBy adjusting the molar ratio of reacting monomers,PZAF microspheres were synthesized using an in-situ template method and then added to PET to improve fire-retardant properties.SEM images showed that the PZAF microspheres had a particle size of approximately 1600 nm and were uniformly dispersed with no obvious aggregation.Thermogravimetric analysis revealed that PZAF promoted the pyrolysis of PET in advance to generate a stable char layer that protects the matrix from heat,consequently increasing char residues.With addition of 10 wt%PZAF,the PET nanocomposites obtained a V-0 grade in vertical combustion test and its LOI value increased from 24.2 vol%to 32.1 vol%.Moreover,the peak heat release and CO production decreased by 46.6%and 50.6%,respectively.This was because the phosphonic acid fragments and pyridine ring compounds produced by the PZAF pyrolysis encouraged the development of a robust char layer.Meanwhile,the PZAF pyrolysis-generated·PO radicals can capture free radicals（·H/·OH）,ultimately ending the chain reaction of combustion.Notably,the addition of 3 wt%and 5 wt%PZAF improved the mechanical properties of the PET nanocomposites.（2）Synergistic flame retardant based on PZAF and HNT and its application in PETThe organic-inorganic composite flame retardant HNT@PZAF was prepared by adding HNT as a template to polymerize the reactive monomer on its surface during the synthesis of PZAF.The surface modification of HNT was expected to improve the dispersion and compatibility of HNT in PET.HNT@PZAF was then melt-blended with PET and the fire retardancy and mechanical properties of PET nanocomposites were investigated.As a comparison,a mixture of the same content of HNT and PZAF（HNT+PZAF）was blended with PET to obtain PET/HNT+PZAF composites.The fire-retardant properties of the composites were analyzed by LOI tests,UL-94 tests and cone calorimetry experiments.It was found that PET/HNT+PZAF showed the best performance,with a LOI of 28.4 vol%,passing V-0 grade in UL-94 test and reducing peak heat release,total heat release and smoke emission by 67.5%,68.0%and 37.8%respectively.The products of PET/HNT+PZAF composite during combustion were further analyzed.It showed that the release of volatile products was reduced,and a robust and dense char layer was formed.HNT+PZAF can play P-Si synergistic effect in the combustion:phosphonic acid derivatives produced by PZAF pyrolysis promote the dehydration and carbonization of PET,forming a stable char layer,and the phosphate-containing free radicals generated by the pyrolysis can trap active free radicals.HNT can effectively reduce the oligomers produced by PET,and ultimately remain in the condensed phase in the form of Si O₂ and C-Si,protecting the char layer from thermal degradation.The mechanical properties of PET/HNT+PZAF composites were enhanced.