Synthesis and characterization of inherently flame-retardant and anti-dripping thermoplastic poly(imides-urethane)s

A series of thermoplastic poly(imide-urethane)s (TPIUs), based on 4,4′-diphenylmethane diisocyanate (MDI) and pyromellitic anhydride (PMDA) as hard segments and poly(tetrahydrofuran) (PTMG) as soft segments, has been prepared by a two-step polymerization process. The objective of this study is to prepare a type of intrinsically flame-retardant polyurethane by incorporating PMDA as a flame retardant in the main chains. The thermal behavior and flame retardancies of the TPIUs have been characterized by thermal gravimetric (TG) analysis and limiting oxygen index (LOI), UL-94 vertical burning, cone calorimeter tests. The results indicate that the TPIUs display outstanding performance. The temperature at 5% mass loss (T_5%) and LOI value increase with the hard-segment contents, while the total heat released (THR) and peak heat release rate (p-HRR) show the opposite trend. Furthermore, the T _5% of TPIU211 (molar ratio: MDI: PTMG: PMDA = 2: 1: 1) is 33.2C higher than that of the conventional thermoplastic polyurethane TPU211 (molar ratio: MDI: PTMG: 1,4-butanediol = 2: 1: 1), and the THR and p-HRR of TPIU211 are 14.62% and 64.02% lower than the respective parameters of TPU211. In addition, UL-94 vertical burning tests show that the TPIUs exhibit excellent antidripping effects. The ultimate tensile strengths of the TPIUs reached 23.1-37.6 MPa with increasing hard segment contents, which meets the requirement of mechanical properties with regard to practical use.