Phosphazenes Catalyzed Benzoxazine Resin System And Study Of Its Ablation Performance

Benzoxazine resin is a new type of phenolic resin,its has excellent thermal properties,mechanical properties and corrosion resistance,so it can be applied to automotive,electrical appliances,coatings,aerospace and other fields.However,the high curing temperature and relatively low char rate of common benzoxazine resins greatly limit their use.Therefore,it is important to modify the benzoxazine resin.Through synthesizing phosphazenes with catalytic groups to modify benzoxazine resin,and then to reduce its curing reaction temperature and increase its ablation resistance.Preparation of Hexa(4-aldehydephenoxy)cyclotriphosphazene(HAPCP)and Hexa(4-nitrophenoxy)cyclotriphosphazene(CPN)by means of a bimolecular nucleophilic reaction.Oxidation of HAPCP to hexa(4-carboxyphenoxy)cyclotriphosphazene(HCPCP)and CPN to hexa(4-aminopHenoxy)cyclotriphosphazene(CPA)by redox reaction and reduction.The synthetic substances were characterized by FTIR and NMR spectrometer,the results showed that HCPCP and CPA were successfully synthesized.The synthesized HCPCP and CPA were mixed with bisphenol A-aniline type benzoxazine resin,obtained BA-a/HCPCP and BA-a/CPA blends.Model compound hexachlorocyclotriphosphine(HCCP),benzoic acid and aniline were used.Using DSC and FTIR to study the curing mechanism of BA-a/HCPCP and BA-a/CPA systems.It is found that the catalytic effect of phosphrile ring in HCPCP is weaker than that of HCCP due to the spatial steric effect caused by the large molecular structure of HCPCP.The comparison between BA-a/CPA system and BA-a/HCPCP system shows that the effect of HCPCP on BA-a was stronger than that of CPA,HCPCP catalyzes the benzoxazine resin to rely on its carboxyl functional group and phosphazene ring,CPA catalyzes the benzoxazine ring opening is based on the N element in the phosphazene ring and aniline.Thermal properties of BA-a/HCPCP and BA-a/CPA blends were studied by TGA and DMA.It has been found that HCPCP gives polybenzoxazine excellent thermal stability in the range of 250 °C-400 °C and when the content of HCPCP was 3 %,the glass transition temperature of the cured product of the blend system reached a maximum of 222.2 °C.Compared with the pure BA-a curing system,HCPCP made the initial curing reaction temperature of BA-a reduce by 26 °C;the 5 % and 10 % weightlessness temperatures of the cured product of the blended system increased by 89.0 °C and 43.6 °C,respectively.The rate rose is to 41.3 %.The addition of CPA made the initial curing reaction temperature of BA-a decreased by 16.6 °C,and the 800 °C residual carbon ratio of the cured product of the blend system almost doubled and increased to 49.2 %.The Tg of the cured product of the blend system is also greatly increased,and the maximum up to 21.9 °C.The ablation resistance of BA-a/HCPCP and BA-a/CPA modified resins was tested by oxyacetylene degasser.The results show that the ablation performance of BA-a/HCPCP modified resin is not only lower than that of BA-a/CPA modified resin,but also lower than that of unmodified BA-a resin.Compared to the BA-a resin,the ablation performance of the BA-a/CPA modified resin was significantly improved,and when the CPA content of 1 %,it had the best modification effect.Through SEM testing of ablated surfaces of BA-a/HCPCP and BA-a/CPA modified resins.It is observed that the carbon layer of the ablated surface of the BA-a/HCPCP modified resin is severely cleaved and the carbon layer of the ablated surface of the BA-a/CPA modified resin is more complete and dense.The graphite on the ablated surface carbon layer of BA-a/HCPCP and BA-a/CPA modified resins was characterized by XRD and Raman.It was found that compared to BA-a resin,although the surface graphitization degree of BA-a/HCPCP system after ablation is high,the crystal integrity of graphite is not high,and the defects are more.However,the ablated surface of BA-a/CPA modified resin not only has a high degree of graphitization but also has a complete crystallinity of graphite.This is the reason why the BA-a/CPA system has the best ablation performance.