Halogen and phosphorus-free sustainable bio-silica reinforced imidazole-based Polybenzoxazine/epoxy hybrid composites for low-dielectric and flame-retardant applications

Abstract

In pursuit of environmentally friendly materials for next-generation micro-electronics insulation applications, thermosetting hybrid networks were developed by integrating imidazole-cored benzoxazine (IMP-BZ) with fossil-based bisphenol-F epoxy (BFE) and bio-derived card-bisphenol epoxy (CBE) resins, reinforced with GPTMS-functionalized bio-silica (BS). The IMP-BZ monomer, containing a rigid, nitrogen-rich heterocyclic core, enhanced thermal stability and reduced dielectric response by increasing cross-link density and restricting molecular mobility. The incorporation of 20 wt% bio-silica further improved thermal endurance, flame retardant behaviour, and dielectric characteristics. The BFE-based hybrid (IMP-BZ/BFE) exhibited a decomposition temperature above 370 °C and a limiting oxygen index (LOI) exceeding 40 %, reflecting the influence of its aromatic backbone, whereas the CBE-based system offered a sustainable alternative with greater flexibility and improved char retention. Dielectric measurements indicated a decrease in dielectric constant to 2.80 and 2.63 for the IMP-BZ/BFE and IMP-BZ/CBE hybrids, respectively, while dielectric losses decreased from 0.0524 to 0.0135 and from 0.0415 to 0.0069 upon incorporation of bio-silica. These changes were attributed to the synergistic effects of the imidazole–benzoxazine core, low-polar silica domains, and the formation of a Si–O–Si network that limits dipole orientation and segmental motion. This study demonstrates the development of phosphorus- and halogen-free hybrid composites combining flame resistance, thermal stability, and low dielectric behaviour, suitable for advanced microelectronics insulation, and structural applications emphasizing both performance and sustainability.