Study On The Synthesis Of Phosphazenes And Their Flame Retartant Properties

The preparation of flame resistant materials has been an important issue in scientific research for a long time. The traditional halogen flame retardants produce toxic gas and smoke as burning. It needs to develop non-halogenated flame retardants to meet the requirements of environmental protection. Over the past decade, there is a sustained research interest on the phosphorus-containing flame retardants which have excellent flame retardant properties. Cyclotriphosphazene is a kind of ring material which contains alternating phosphorus and nitrogen atoms. There are three phosphorus atoms in a cyclophosphazene molecule which are active to be substituted by different nucleophiles. Multifunctional compounds can be obtained by replacing the chlorine groups with various functional substituents. Phosphazenes exhibit good thermal stability and flame retardancy for the synergistic flame retardant effect of phosphorus and nitrogen. Due to less toxic gas and smoke as burning, the phosphazene-based materials are able to fulfill the requirements of flame retardant which have low toxicity, pollution and corrosive. Poly (ethylene terephthalate)(PET) and polyurethane (PU) are two important polymers in modern industry. For the outstanding properties, PET and PU are widely used in various areas such as surface coatings, fibers, electronic industry, automobile industry, petroleum engineering and barrier materials. However, the flammability of PET and PU polymers limit their applications in some special fields. Therefore, it is important to develop flame retardancy of PET and PU. Phosphorus-containing flame retardants are effective to PET and PU, containing phosphate, phosphorus oxide, organic phosphate and so on. In this paper, two kinds of cyclotriphosphazene flame retardants are synthesized via a simple method, and the structure of the flame retardants are investigated. The effects of flame retardants on the thermal and flame retardancy of PET and PU are mainly investigated. And then, the synergic effect of hexakis (4-nitrophenoxy) cyclotriphosphazene (HNCP) and maleic anhydride grafted ethylene-octene copolymer (POE-g-MA) on the anti-dripping of PET is also studied.(1) The synthesis and characteristic of cyclotriphosphazene derivativesCyclotriphosphazene derivatives were synthesized in a new system. Hexakis (4-nitrophenoxy) cyclotriphosphazene (HNCP) and hexakis (4-formacylphenoxy) cyclotriphosphazene (HAPCP) were prepared by reacting hexachlorocyclotriphospha zene with4-nitrophenol, p-hydroxybenzaldehyde respectively, using potassium carbonate as acid binding agent, acetone or tetrahydrofuran as solvent. It was simplified the reaction process and shortened the reaction time. It was restudied the reduction of hexakis (4-nitrophenoxy) cyclotriphosphazene to hexakis (4-aminophenoxy) cyclotriphosphazene (HACP). The reduction was catalyzed by Pd/C in tetrahydrofuran, using hydrazine hydrate as reducing agent. This method eliminated the influence of dark insoluble matters and improved the yield. The structures of products were characterized by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR) and element analysis (EA).(2) Application of cyclotriphosphazene flame retardants in PET and study of the properties of flame retarded PETThe cyclotriphosphazene flame retardants HNCP and HAPCP were added into PET to improve its flame retardancy by melt blending. The effects of HNCP and HAPCP on the thermal stability of PET were investigated by TGA. The results revealed that the addition of flame retardants could reduce the initial degradation temperature and the maximum mass loss rate while the char formation of PET was improved by HNCP and HAPCP both in nitrogen and in air atmosphere. The flame retardancy of the flame retarded PET was studied by LOI and vertical burning test (UL-94test). It was found that the LOI value of PET/HNCP was up to35.1%and could pass UL-94V-0. For the PET/HAPCP, the value of LOI was up to34.3%. Although the UL-94grade was V-2except the sample which the content of HAPCP was10wt.%, the afterflame time and the afterglow time were reduced. SEM graphs showed the outer of residue of flame retarded PET after burning appeared smooth and compact but the inner was frothy and swollen. This structure of char layer was an ideal insulation structure. It was helpful to hinder the exchange between the substrate and external environment. The degradation behavior of flame retarded PET was studied by TGA. The results declared that HNCP and HAPCP could reduce the degradation activation energies (E) of flame retarded PET and promote the char formation in the early degradation. While the E of flame retarded PET was higher than that of PET at high degradation conversion degree. It meant that flame retarded PET had a better thermal stability of char formed than PET.(3) The synergistic effect of HNCP and POE-g-MA on the improvement of the dripping resistance of PETPOE-g-MA was added to PET/10wt.%HNCP composite by melt blending. The synergic effect to improve the dripping resistance of PET was investigated. TGA results suggested POE-g-MA had no improvement on the thermal stability and the final char yield of PET while the presence of HNCP would promote charring processes. The thermal stability of PET/10wt.%HNCP/POE-g-MA was better than PET meanwhile the LOI value was up to28.3%. When the loading of POE-g-MA increased to3wt.%, the flame retarded PET achieved a V-0rating with no dripping. DSC illustrated POE-g-MA improved the compatibility between PET and HNCP. It was found that the inner side of the char layer of PET/10wt.%HNCP/POE-g-MA was a frothy internal structure with many uniform microporous cells. That was to say the presence of POE-g-MA increased the compatibility and dispersion between PET and HNCP. It was in favor of forming a more continuous char layer by HNCP which was effective to improve the flame retardancy and anti-dripping of PET in fire. The flame retarded PET with anti-dipping property was obtained by the synergic effect of HNCP and POE-g-MA.(4) Application of cyclotriphosphazene flame retardants in PU and study of the properties of flame retarded PUThe flame retarded PU was prepared by incorporating HNCP and HAPCP into the PU coating solution through wet film forming process, respectively. TGA results showed that the thermal stability and the char formation of PU were enhanced with HNCP and HAPCP both in nitrogen and in air atmosphere. HAPCP was more effective than HNCP on promoting the char formation of PU. Both HNCP and HAPCP could improve the LOI value of PU though they did not pass the vertical burning test and UL-94grade was VTM-2for the heavy dripping. The afterflame time and afterglow time were reduced greatly which meant self-extinguishing property of PU was improved. The FTIR spectra provided evidence that phosphorus and nitrogen elements still remained in the residue of PU after burning. SEM graphs revealed the outer of residue of flame retarded PU after burning was not smooth. It was not an ideal char structure for insulation. So, the flame retardancy properties of PU were not significantly enhanced. In the study of the degradation behavior of flame retarded PU, it was found the degradation activation energies (E) of flame retarded PU were reduced by HNCP and HAPCP at the early degradation stage. E of flame retarded PU increased at high degradation conversion degree in comparison with pure PET due to the formation of char with better thermal stability.