**4. Conclusions**

This study used 29Si NMR to determine that the siloxane condensation degree of the PU/HBNPSi was 74.4%. High condensation density indicates superior material properties. P- and Si-mapping revealed that the additive HBNPSi was well dispersed and homogeneously distributed in the polymer matrix. When the HBNPSi concentration was increased, aggregation did not occur, showing that the materials and additives were compatible. The UL-94 standard was used to determine the flame retardancy of the hybrid material. Pristine PU failed to meet the standard. The 40% PU/HBNPSi, however, achieved the V-2 level, proving that the flame retardant effectively improved the flame retardancy of the pristine PU. Using XPS, char analysis was conducted on the hybrid material PU/HBNPSi to determine the correlation between char formation and the material's thermal property. Cls, Ols, and Si2p XPS spectra revealed that the phosphide and siloxane formed by burning had the effect of protecting the material. A higher concentration of HBNPSi resulted in a stronger effect, achieving high thermal stability and an antioxidation effect. The organic– inorganic hybrid material prepared by the sol–gel method has good compatibility and flame resistance, and has grea<sup>t</sup> potential for future applications in coating applications.

**Author Contributions:** Conceptualization, C.-L.C.; Methodology & Software, M.-Y.S.; Validation, C.-F.K., H.-C.K., and C.-L.C.; Formal Analysis, C.-L.C.; Investigation, C.-Y.K.; Data Curation, Writing— Original Draft Preparation, Writing—Review & Editing, C.-L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors would like to express their appreciation for the National Science Council of the Republic of China for the financial support of this study under gran<sup>t</sup> MOST-108-2221-E-241-007.

**Institutional Review Board Statement:** Not applicable for studies not involving humans.

**Informed Consent Statement:** Not applicable for studies not involving humans or animals.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
