Fire behavior of novel imidized norbornene-containing poly(ethylene terephthalate) copolymers: Influence of retro-Diels-Alder reaction at high temperature

Retro-Diels-Alder reaction (rDA) has been widely used as a common modification approach for expanding functional applications of polymers. Especially, it has become an effective method of current cross-linking chemistry. With the goal of designing a cross-linkable PET-based copolyester toward flame retardancy and anti-dripping during combustion, dimethyl 5-(1,3-dioxo-3a,4,7,7a-tetrahydro-1H-4,7-methanoisoindol-2(3H)-yl) isophthalate (DMTMI) was incorporated as the functional co-monomer into PET chains via random transesterification polycondensation. Thermogravimetry-differential scanning calorimetry (TG-DSC) and rheological studies demonstrated the existence of cross-linking behavior at high temperature. By controlling the co-monomer content, expected flame retardancy was obtained, as illustrated by the results from the limiting oxygen index (LOI) and cone calorimetry. Furthermore, LOI values increased firstly and then decreased with the content of DMTMI increased, which was related to the existence of rDA reaction, and further proposed that a competitive relationship between the release of inflammable cyclopentadiene during rDA reaction and cross-linking behavior occurred in combustion. The hypothesis was verified through pyrolysis-gas chromatograph-mass spectrometry (Py-GC-MS).