Kinetic study on the synergistic effect between molecular weight and phosphorus content of flame retardant copolyesters in solid-state polymerization

2-Carboxyethyl(phenyl phosphinic) acid has been proved to modify flame retardant properties of poly(ethylene terephthalate) (PET) through copolymerization; however, there is no industrial technological breakthroughs in polyester industrial yarns which requires high strength and modulus, because it is related to the coordinated control of molecular weight and flame retardancy. In this work, the influence of solid-state polymerization reaction kinetics and parameters on flame retardancy retention have been investigated, based on the prepared precursors optimized at 6,000 ppm phosphorus content and 0.64 dL/g intrinsic viscosity (I.V). Results showed that the copolyester polydispersity index increase was significant compared to PET with molecular weight increase. The I.V of optimized precursor could be increased to 1.31 dL/g at 220°C in 20 hr, but at higher phosphorus content loses up to 7%. Flame retardant component deteriorates with increasing reaction rate and temperature as a result of severe side reactions. Optimized precursor at the optimum temperature (210°C) reacted for 20 hr, copolyester with the phosphorus content of 5,910 ppm and the I.V of 1.12 dL/g (( was about 3.5 × 10⁴ g/mol) was prepared. The coordination between high molecular weight and flame retardant performance was realized.