Flame-retardation of thermoplastic polyesters via cyclotetramerization from phthalonitrile to phthalocyanine: Pyrolysis processes and fire behaviour

Abstract

Flame retardancy is a way to enhance the fire safety performance of polymers, which can slow down the flame spread and reduce the severity of fire events. Phthalocyanines (Pcs) skeletons have unleashed high flame-retardant efficiency in thermosetting resins. However, limited by the high stability and bulky structure of Pcs, how to make this classic macrocyclic structure play its flame retardant advantages in thermoplastic polymers is still a significant challenge. We synthesized flame-retardant copolyesters (CxPET) via chemically incorporating the phenoxyphthalonitrile structures into the polymer chains of poly(ethylene terephthalate) (PET) by melt polymerization. PY-GC–MS tests at 400, 500, and 700 °C are performed to detailedly investigate the pyrolysis processes. The TGA, LOI, and UL-94 tests are carried out to reveal the burning behavior. The LOI, vertical UL-94, and cone calorimeter tests, are used to characterize the flame-retardant properties and the fire behavior. The copolyesters, containing phenoxyphthalonitrile structures, exhibit good flame retardance (LOI 30.0, UL-94V-0 level with no dripping producing). The peak heat release rate (337 ± 32 kW m−2) and TSR (1377 ± 74 m2 m−2) are also reduced by 55.4% and 20.2%, compared with PET. Combining the pyrolysis processes and fire behavior results, the phthalonitrile structures can be quickly transformed into phthalocyanine structures at high temperatures. Further, the formed phthalocyanines play a flame-retardant role via promoting the charring of copolyesters. Therefore, this work supplies an available way to play the flame retardant advantages of phthalocyanine macrocyclic in thermoplastic polymers, and provides key guidance for the design of safer polymers.