Investigation On Flame Retardant Poly(Butylene Terephthalate) With Aluminium Hypophosphite And Cyclotriphosphazene Derivative

Poly(butylene terephthalate) (PBT) with excellent performance has been widely used in automobiles, electronics, electrical appliances and other fields. However, the flammability of PBT material limits its extensive application. Therefore it is of great significance to prepare flame retardant PBT materials. Halogenated flame retardants used in PBT have good flame retardant efficiency, but its application is limited due to the release of toxic gas in the process of combustion. Phosphorous-containing flame retardants, such as diethyl aluminum phosphate, aluminum hypophosphite demonstrates good flame retardant efficiency when used in PBT. In order to improve the flame retardant efficiency of aluminum phosphate in PBT material, it is an effective way to select an appropriate synergist for flame retarding synergism on aluminum phosphate.In this paper, hexa-(4-aldehyde-phenoxy)-cyclotriphosphazene (HAPCP) combined with aluminum phosphate (AHP) with different mass ratio were incorporated into PBT to prepare flame retardant PBT composites. The fire retardancy of materials was investigated by limiting oxygen index (LOI) and vertical burning test (UL-94); the combustion behavior of materials was investigated by cone calorimeter (CONE) test; the mechanical properties of flame retardant PBT materials were measured by the tensile, flexural and impact strength test; the water resistant properties of materials were evaluated by putting the samples into distilled water at 70℃ for 168 h. The influences of mass ratio of HAPCP and AHP, total amount of the flame retardant on flame retardancy, mechanical properties and the water resistant properties of PBT material were studied comprehensively. The results indicated that the flame retardant PBT material passed UL-94 V-0 rating and LOI value reached to 23.8%, when the loading amount of AHP was 18 wt% alone. However, the flame retardant PBT material could pass the UL-94 V-0 rating and LOI value could reach to 25.3%, when the mass ratio of HAPCP to AHP was 15:1, the loading amount of flame retardant was 16 wt%.. It showed that there was a synergism flame resistant effect between AHP and HAPCP used in PBT material. The smoke produce rate (SPR) and total smoke produce (TSP) of HAPCP/AHP/PBT system decreased significantly compared to that of AHP/PBT system. The tensile and flexural strength of HAPCP/AHP/PBT system increased in different degrees compared with that of the AHP/PBT system. The HAPCP/AHP/PBT composite after water resistance test still successfully passed UL-94 V-0 rating when the loading amount of flame retardant was 16 wt%, LOI value fell by 0.8%, and the weight loss rate was only 0.064%. Under the same experimental conditions, the AHP/PBT composite after water resistance test only passed UL-94 V-1 rating, LOI value fell by 1.0%, and the weight loss rate was 0.079%. It can be deduced that combining AHP with HAPCP could improve the water resistance of the flame retardant PBT material. At the same time, the tensile and flexural strength of HAPCP/AHP/PBT composite increased compared with that of HAPCP/AHP/PBT before water resistance test.In addition, chemical structure of the reaction products at different temperature from AHP and HAPCP were characterized by Fourier transform infrared spectroscopy (FT-IR) to infer the reaction mechanism of AHP and HAPCP. Then we could predict the synergistic mechanism of HAPCP and AHP in HAPCP/AHP/PBT system. The results showed that HAPCP and AHP reacted to form a cross-linked structure containing P-C bond with relatively high stability at about 200℃. Therefore, we predicted cross-linking reaction also occurred between HAPCP and AHP during the processing of HAPCP/AHP/PBT system. It improved the thermal stability of materials, and the interfacial compatibility between the flame retardant and PBT matrix, which resulting in the increase of the mechanics properties, water resistance and flame retardant of the PBT composites.The thermal degradation behavior of AHP, HAPCP, AHP/HAPCP and pure PBT, AHP/PBT, HAPCP/AHP/PBT system were tested respectively by thermo gravimetric analysis (TGA). The results showed that the maximum decomposition rate of AHP/HAPCP reduced from 35.9 %-min-1 to 16.5 %-min-1compared with that of AHP; the initial decomposition temperature of HAPCP/AHP/PBT system increased from 317℃ to 324℃ compared with that of AHP/PBT, and its char residues at 700℃ increased from 15.7% to 15.9%. The morphologies of the char layer of AHP/PBT and HAPCP/AHP/PBT system after burning were measured by scanning electron microscopy (SEM). The results revealed that the char layer of HAPCP/AHP/PBT system after combustion was more uniform, compact and continuous, which protected the underlying matrix. So the flame retardant efficiency of PBT composite was improved.