Flame retardancy and mechanical effects of ZnB and ATH additives in glass fiber-reinforced polyester composites

In this study, the effects of halogen-free flame retardant additives aluminum hydroxide (ATH) and zinc borate (ZnB) on the mechanical and fire resistance properties of glass fiber-reinforced unsaturated polyester (GF/UP) composites were systematically investigated. ATH and ZnB additives (5%, 10% and 15%) were added to the polymer matrix both alone and as hybrids (1:1). The composites were produced by hand lay-up method and characterized by flexural strength, Charpy impact strength and UL-94 horizontal burning tests. In terms of mechanical performance, while addition of ATH and ZnB decreased the flexural strength, increased the impact strength of the composites. 5% ATH additive provided the highest flexural strength (479.98 MPa) among the fire retardant added composites. As the ATH additive increased, the impact strength also increased and a value of 241.03 kJ/m2 was reached with 15% ATH addition. While horizontal burning rate of the composite without additive was about 20.15 mm/min, it decreased with the use of flame retardant additives. Moreover, composite with hybrid additives exhibited the best burning rate performance as 10.23 mm/min. Post-combustion analyses revealed that ATH formed a porous alumina structure and ZnB formed a glassy and dense borate phase. The combination of these two structures formed a more compact and crack-resistant surface residue, limiting heat and oxygen transfer. Overall, the UL-94 horizontal burning results indicate a synergistic trend for the ATH/ZnB hybrid formulations, evidenced by the lowest burning rate and a more coherent post-combustion residue, offering significant potential for the design of sustainable and fire-resistant composite materials for automotive, structural and electrical applications.