The Role of Hollow Glass Microspheres as Functional Fillers in Fiber-Reinforced Polymer Composites: A Review.

Fiber-reinforced polymer composites (FRPCs) have gained increasing attention as lightweight structural materials with tailored mechanical, thermal, and functional properties for diverse engineering applications. However, achieving optimal performance requires overcoming challenges such as poor interfacial bonding, high density of conventional fillers, and limitations in multifunctionality. Hollow Glass Microspheres (HGMs), owing to their unique spherical morphology, low density, high strength-to-weight ratio, and tunable physical-chemical characteristics, have emerged as promising functional fillers for FRPCs. This review provides a comprehensive overview of the structural features, chemical composition, and synthesis techniques of HGMs, followed by an outline of FRPCs systems with emphasis on matrix and fiber types, their functional requirements, and the critical role of fillers. The discussion highlights how HGMs influence the mechanical (tensile, flexural and compression strength) properties, thermal (conductivity and insulation) properties, acoustic (sound absorption and transmission) properties, and dielectric performance of FRPCs, enabling weight reduction, improved insulation, and multifunctional capabilities. Reported studies demonstrate that when properly dispersed with an optimal amount, HGMs significantly enhance mechanical properties, thermal stability, and acoustic damping, while maintaining processability. Despite these advantages, challenges remain regarding interfacial adhesion (agglomeration) and filler dispersion. The review concludes by emphasizing the need for advanced surface modification strategies, hybrid filler systems, and sustainable processing methods to fully exploit HGMs in next-generation high-performance FRPCs.