Curing mechanism, thermal and ablative properties of hexa-(4-amino-phenoxy) cyclotriphosphazene/benzoxazine blends

The demand for ablative material with high performance is eager in the aerospace industry. In this paper, hexa-(4-amino-phenoxy)-cyclotriphosphazene (CPA) was synthesized and then blended with bisphenol-A aniline benzoxazine (BA-a) firstly. The curing mechanism, thermal and ablative properties of cured blends were studied. The addition of CPA not only can decrease the curing temperature but can also increase the crosslink density of polybenzoxazine which is further beneficial for the improvement of thermal properties of cured blends. When the content of CPA is 5%, the T-g of the cured blend increases by 20 degrees C, and the char yield at 800 degrees C increases from 25.2% for pure polybenzoxazine to 49.2%. The oxyacetylene ablation tests showed that the ablation rate of cured blends could reach 0.047 mm s(-1) while that of polybenzoxazine is 0.128 mm s(-1). Further discussion based on SEM, XRD, Raman spectra showed that the CPA can effectively improve the graphitization degree of ablated samples and improve the integrity of graphite layer crystals. Moreover, the density functional theory (DFT) calculations and bond order analysis predict the bonding breakage sequence which is the C-N bonds between BA-a and CPA (N3-C55) > P-O bonds (P2-O12) > phosphazene ring, which means the phosphazene ring shows an effect after benzoxazine partly decomposed. The preparation method by adding CPA to benzoxazine to improve the ablative performance is facial and easy, which provides a new way of making novel ablative materials.

Automated summary

In this study, the new material hexa-(4-amino-phenoxy)-cyclotriphosphazene (CPA) was synthesized and blended with bisphenol-A aniline benzoxazine (BA-a) to improve the thermal and ablative performance. It was found that the addition of CPA could enhance the crosslink density and char yield of the cured blends, leading to a decrease in curing temperature and an increase in ablation rate. The study also showed that CPA effectively improved the graphitization degree of ablated samples and enhanced the integrity of graphite layer crystals, suggesting a promising method for making novel ablative materials.