Abstract:

Objective Poly(ethylene terephthalate) (PET) is a semi-aromatic polyester known for its high performance and low cost. When considering fire safety requirements,the high flammability and serious melt-dripping behaviors during PET combustion restrict its application in many fields. In order to overcome these shortcomings, various types of flame retardants are added to PET. In recent years, various non-halogen flame retardants and high-temperature resistant materials have been developed using hexachlorocyclotriphosphazene(HCCP). The proposed method is put forward for addressing the issues of flammability and droplet melting of PET.

Methods HCCP and phloroglucinol as monomers were selected to synthesize highly crosslinked poly(cyclotriphosphazene-phloroglucinol) (PCTP) microspheres through the precipitation polymerization method. PCTP microspheres containing flame retardant elements P and N were incorporated into the PET matrix through melt blending. The influences on flame retardancy, mechanical properties, and the flame retardancy mechanism of PCTP/PET composite were investigated.

Results The synthesized PCTP microspheres was characterized. For PET, its temperature of initial decomposition (T_5%) is 402 ℃. After incorporating various components of PCTP microspheres into PET, the T_5% of the composites decreased to 390.9, 384.8, 379.2 and 365.6 ℃, respectively, indicating that PCTP microspheres can catalyze the thermal degradation of PET. PET has the lowest limit oxygen index (LOI value), and the UL-94 grade is the worst, with 24.4% and V-2, respectively. A significant phenomenon of melt dripping can be observed during combustion. After adding PCTP microsphere flame retardant, the flame retardant performance of the material was enhanced. Starting from PET/PCTP2.0(adding 2% of PCTP), all flame retardant composites with UL-94 grade achieved V-0, and the droplet phenomenon during combustion was minimized. By incorporating 2% PCTP microspheres into PET, the LOI value of the composite was rapidly increased to 31.1%, which is higher than that of PET (24.4%), and that of PET/PCTP5.0 (adding 5% of PCTP) increased to 33.9%. The peak heat release rate (PHRR) value of PET is very high at 775.24 kW/m², and the total heat release (THR) value is 125.47 MJ/m². Among these composites, PET/PCTP5.0 exhibited the best performance, showing a significant decrease in PHRR and THR values by 40% and 21.7%. Adding 2% of PCTP to PET, PCTP increased the release of CO₂, while its concentration increased from 9.1% in PET to 14.0%. PET/PCTP composites released fewer combustible gases during pyrolysis, and the combustion process was slowed down by reducing the fuel. In addition, limiting CO release significantly reduced the toxicity of pyrolysis. Results showed that reducing the release of aromatic compounds not only postponed the availability of combustion sources, but also delayed the generation of smoke. Images of char residuces showed that with the increased PCTP content, the surface pores of char residuces in the composites became smaller, and the char layer became denser. PET exhibited the lowest area ratio of the D-band to the G-band (I_D/I_G value), at only 1.54. Compared to PET, the I_D/I_G values of various types of flame retardants were enhanced. The PET/PCTP composites proved that flame retardant elements phosphorus and nitrogen remained in the char residue.

Conclusion The polyphosphazene derivative microspheres (PCTP) were synthesized by precipitation polymerization using HCCP and phloroglucinol as raw materials, which have excellent thermal stability. Subsequently, it was melted and blended with PET to improve fire retardancy. By adding 2% of PCTP microspheres, the LOI value of PET/PCTP2.0 composite was increased to 31.1%, which also passed the V-0 of UL-94 and had good fire resistance. The LOI value of PET/PCTP5.0 increased to 33.9%. The cone calorimeter (CCT) results indicate that during the combustion process, the smoke and heat release of PET/PCTP composites is suppressed, while the char residues increases. The addition of PCTP hinders the pyrolysis of PET, thereby reducing the release of combustible gases such as CO and aromatic compounds. The role of PCTP in the condensed and gaseous phases is the reason for improving fire safety. Most importantly, the mechanical properties of the PET/PCTP2.0 composite are damaged by 13.3%, which is within the acceptable range. In summary, the PET/PCTP composites displayed comprehensive performance, offering great application value for the investigation of green flame-retardant PET.

Key words: flame retardant agent, melt blending, poly(ethylene terephthalate), polyphosphazene, poly (cyclotriphosphazene-phloroglucinol) microsphere