Phosphorus-based flame retardant poly (butylene terephthalate): Synthesis, flame retardancy and thermal behavior

Abstract

The synthesis of a flame retardant formulation based on poly (butylene terephthalate) (FRPBT) and 3- (hydroxyphenyl phosphinyl) propanoic acid (HPPA) as phosphorous-based flame retardant, was performed in a laboratory reactor. The synergistic effect of antimony trioxide (Sb2O3) and titanium dioxide (TiO2), as co-additives in combination with HPPA in FRPBT samples, was studied concerning the intrinsic viscosity (IV) and flame retardancy. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy were used to characterize the produced polymers. The synthesized samples were analyzed by gel permeation chromatography (GPC) to elucidate the molecular weights and their distributions. Thermal properties, thermal stability, and mechanical properties were evaluated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and tensile testing, respectively. The flame retardancy of FRPBT was examined through cone calorimeter measurements, limiting oxygen index (LOI), and UL-941 tests. The characterization results for PBT and FRPBT showed that the modification of PBT with low levels of HPPA comonomer (3 mol%) increased LOI to 31% and improved UL-94 to V-1 rating. By insertion of HPPA into the copolyester chains, the crystallinity and melting point temperature (Tm) of the polymers plus their mechanical properties were lowered. The results suggest that presence of TiO2 and Sb2O3 along with HPPA exhibits flame retardancy synergistic effects in FRPBT and increase LOI to 33% and 34.8% for PBHT and PBHS, respectively, in comparison to neat PBT (22.5%). TGA and cone calorimeter results also confirmed the same synergistic effects. A reduction was observed in the maximum mass loss rate (Rmax) and maximum decompositions temperature (Tmax) in TGA thermograms for FRPBTs in presence of 3 mol % HPPA alone and along with 3–5 mol% of co-additives, compared to PBT. Also, the results of cone calorimeter test showed that the heat release rate (HRR), peak heat release rate (PHRR) and total heat release rate (THR) of the FRPBT samples were clearly reduced. These showed that using HPPA and HPPA/co-additive compounds promoted and improved the thermal degrading and flame retardation behavior of PBT. Also, co-additives in the range of 3–5 mol% provided excellent flame retardancy in FRPBT using minimum amounts of HPPA led to high LOI values and a V-0 rating in UL-94.