Effects of Fluorinated Side Groups on the Insulation Performances of Polyphosphazenes with Organic Fiber Reinforcement

Poly(diaryloxy)phosphazene (PDAP) possesses good ablative resistance properties, owing to unique charred layers formed during ablation, showing promising applications in aerospace thermal protection. However, the high glass-transition temperature (Tg) of PDAP insulations limits the application in elastomeric heat-shielding materials (EHSMs) that is a need for meeting the growing requirements of solid rocket motors such as low-temperature storage and application. In this work, we focused on improving the low-temperature resistance and ablation resistance of polyphosphazene composites. A class of ablative-resistant poly(octafluoropentyloxy/butoxy/aryloxy)phosphazene (PFBAP) with low Tg was prepared by introducing flexible octafluoropentyloxy side groups into poly(butoxy/aryloxy)phosphazene (PBPP). The results showed that the octafluoropentyloxy side groups could improve the thermal oxidative stability, flame retardancy, and interfacial interaction of polyphosphazene. Compared with PBPP composites, the values of linear ablation rate, mass ablation rate, and charring ablation rate of PFBAP 2 (containing 11.86% octafluoropentyloxy side groups) composites were reduced by 65, 34, and 25%, respectively. The study on the ablation mechanism of PFBAP composites shows that the introduction of fluorinated side groups can not only improve the charring degree of insulations during ablation but also exert a flame-retardant effect in the gaseous phase, resulting in a denser and more coherent charred layer. This work demonstrates a way to develop low-temperature and ablation-resistant polyphosphazene materials for potential thermal protection in the aerospace industry.