A phosphorus-containing PET ionomer: from ionic aggregates to flame retardance and restricted melt-dripping

In this study, a novel phosphorus-containing ionic monomer, named sodium salt of 10H-phenoxaphosphine-2,8-dicarboxylic acid,10-hydroxy-,2,8-dihydroxyethyl ester,10-oxide (DHPPO-Na), was synthesized, characterized, and then copolymerized to prepare poly(ethylene terephthalate)based ionomers. The chemical structure of the resulting ionomers was confirmed by H-1, C-13, and P-31 NMR spectroscopy. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to investigate the thermal properties of the ionomers. Compared with that of neat PET, the initial decomposition temperature of PETIs decreased in a nitrogen atmosphere while it increased in air. The crystallinity of PETIs was enhanced firstly and then destroyed with the ionic group increase. The limiting oxygen index (LOI) test and cone calorimeter were used to characterize the flame-retardant properties of the ionomers. The results showed that the introduction of DHPPO-Na could endow an expected flame-retardant performance, meanwhile it considerably restricted the melt-dripping behavior and suppressed the smoke release. The rheology test confirmed that the ionic groups increased the melt viscosity via ionic aggregation during heating, which was a benefit for the flame-retardant property of the copolyester.