Thermal Degradation and Carbonization Mechanism of Fe-Based Metal-Organic Frameworks onto Flame-Retardant Polyethylene Terephthalate

Abstract

Currently, the metal-organic framework (MOF) is a promising candidate for flame−retardant polymers. In this study, a Fe−based MOF, MIL-88B(Fe), was introduced to polyethylene terephthalate (PET) and 3−hydroxyphenylphosphinyl-propanoic acid copolymer (P−PET) to reduce the fire hazard involved in using PET. The limiting oxygen indexes (LOIs) of MIL−PET and MIL−P−PET improved by 27% and 30%, respectively. The UL−94 level achieved for MIL−P−PET was V−0 rating. The thermal degradation and carbonization mechanisms of MIL−PET and MIL−P−PET were systematically investigated through thermogravimetric analysis coupled with a Fourier transform infrared spectroscopy (TG−IR), pyrolysis-gas chromatography−mass spectrometry (Py−GC−MS), x−ray photoelectron spectroscopy (XPS), and Raman spectrum combined with quantum chemical molecular dynamics simulation. With the addition of MIL−88B(Fe), high graphitization and a hard flammability char residual were generated. Compared with neat PET, the ferric ions efficiently catalyzed the homolytic cleavage and dehydrogenation of PET to produce a large amount of CO2 and terephthalic acid for MIL−PET in gas phase. Rough and hierarchical char residual with ferric oxide was also generated when temperatures exceeded 600 °C. However, the carbonization process was inhibited due to the coordinated complex between phosphorus and ferric ions in MIL−P−PET, invaliding the decarboxylation and generating more benzoic acid and its precursor, which led to heavy smoke.