Thermal, crystalline and mechanical properties of flame retarded poly(lactic acid) with a PBO-like small molecule - phenylphosphonic bis(2-aminobenzothiazole)

Abstract

Poly(lactic acid) (PLA) has received considerable attention due to its potential to replace the petrochemically derived plastics. However, its high flammability and the tendency to melt drip during combustion has restricted its applications in many areas. This manuscript reports the synthesis, characterization, and application of PBO-like small molecule additive flame retardant (FR) - phenylphosphonic bis(2-aminobenzothiazole) (P-ABZT) for bioplastic PLA. P-ABZT was incorporated into PLA by the combinatory solvent mixing and the compression molding approaches, and its FR, crystalline and mechanical properties were investigated. Fourier transform infrared (FTIR), 1H, 13C, and 31P NMR confirmed the success of the synthetic process while thermogravimetric analysis (TGA) showed improved thermal stability and higher char yield after phosphorous was introduced. Cone calorimeter test demonstrated substantial reductions in peak heat release rate (PHRR ∼ 50%), total heat released (THR ∼ 37%), average effective heat of combustion (AEHC ∼ 31%), peak CO and CO2 produced (PCOP ∼ 60.4%, PCO2P ∼ 31%) with just 3% P-ABZT loading. The limiting oxygen index (LOI) improved to 32.4% whilst a V-0 rating was attained in the UL-94 vertical burning test. Substantial reductions in pyrolysis gaseous products were obtained by P-ABZT/PLA composites compared to neat PLA in the TG-IR study. The Young's modulus, tensile strength, and elongation at break of PLA were not compromised with 1% P-ABZT loading. SEM and Raman spectroscopy showed improved char yield and quality due to the condensed and gas phase inhibition by P-ABZT. The low FR loading coupled with the improved FR performance indicates that P-ABZT is an excellent and economical FR for PLA.