Thermal degradation and pyrolysis behavior of wholly aromatic phosphorus-containing thermotropic liquid crystal copolyesters

Abstract

In this study, the thermal degradation and pyrolysis behavior of three wholly aromatic phosphorus-containing thermotropic liquid crystal copolyesters (TLCP) were investigated. The non-isothermal degradation kinetic parameters were determined using the Kissinger and Friedman method. The copolyesters exhibited two degradation stages in air but only one in N2, and better thermal stability was observed for those containing more 10-(2,5-Dihydroxypheny)-10 h-9-oxa-10-phospha-phe-nanthrene-10-oxide (DOPO-HQ) units, which could be reflected by the thermogravimetric analyzer (TGA) data and E values. The TGA coupled with Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis-gas chromatograph-mass spectrometer (Py-GC-MS) results showed that DOPO-HQ decomposed first in the early stage of degradation, generating gaseous pyrolysis products and forming phosphorus-containing char residue in the later stages of degradation. However, the copolyesters produced more CO2 and less organic volatiles during thermal oxidative degradation. The morphology and FTIR spectra of the char residue obtained after TGA experiments showed a continuous and dense structure due to the presence of phosphorus moieties. The microscale combustion calorimeter (MCC) and limiting oxygen index (LOI) results showed that the copolyesters have excellent flame retardant properties, and the LOI value increases monotonically with increasing of DOPO-HQ content, having a maximum value up to 73.8%, due to its excellent gas-phase and condensed-phase fire inhibition mechanisms. Overall, the resulted phosphorus-containing thermotropic liquid crystal copolyesters exhibited excellent thermal stability and flame retardancy, the thermal degradation and pyrolysis research would be a good guidance for its processing and application.